A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: a reassessment of dinosaur origin and phylogeny

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1 Journal of Systematic Palaeontology ISSN: (Print) (Online) Journal homepage: A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: a reassessment of dinosaur origin and phylogeny Martin D. Ezcurra To cite this article: Martin D. Ezcurra (2010) A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: a reassessment of dinosaur origin and phylogeny, Journal of Systematic Palaeontology, 8:3, , DOI: / To link to this article: Published online: 30 Jul Submit your article to this journal Article views: 1532 View related articles Citing articles: 58 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 11 January 2018, At: 09:26

2 Journal of Systematic Palaeontology, Vol. 8, Issue 3, September 2010, A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: a reassessment of dinosaur origin and phylogeny Martin D. Ezcurra Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Av. Angel Gallardo 470 (C1405DJR), Buenos Aires, Argentina (Received 8 December 2008; accepted 1 April 2010) It was traditionally thought that the oldest known dinosaur assemblages were not diverse, and that their early diversification and numerical dominance over other tetrapods occurred during the latest Triassic. However, new evidence gathered from the lower levels of the Ischigualasto Fm. of Argentina challenges this view. New dinosaur remains are described from this stratigraphical unit, including the new species Chromogisaurus novasi. This taxon is distinguished from other basal dinosauriforms by the presence of proximal caudals without median notch separating the postzygapophyses, femoral lateral surface with deep and large fossa immediately below the trochanteric shelf, and metatarsal II with strongly dorsoventrally asymmetric distal condyles. A phylogenetic analysis found Chromogisaurus to lie at the base of Sauropodomorpha, as a member of Guaibasauridae, an early branch of basal sauropodomorphs composed of Guaibasaurus, Agnosphitys, Panphagia, Saturnalia and Chromogisaurus. Such an affinity is for the first time suggested for Guaibasaurus, whereas Panphagia is not recovered as the most basal sauropodomorph. Furthermore, Chromogisaurus is consistently located as more closely related to Saturnalia than to any other dinosaur. Thus, the Saturnalia + Chromogisaurus clade is named here as the new subfamily Saturnaliinae. In addition, Eoraptor is found to be the sister-taxon of Neotheropoda, and herrerasaurids to be non-eusaurischian saurischians. The new evidence presented here demonstrates that dinosaurs first appeared in the fossil record as a diverse group, although they were a numerically minor component of faunas in which they occur. Accordingly, the early increase of dinosaur diversity and their numerical dominance over other terrestrial tetrapods were diachronous processes, with the latter preceded by a period of low abundance but high diversity. Keywords: dinosaur origin; Saurischia; Sauropodomorpha; Late Triassic; Ischigualasto Formation; Argentina Introduction Dinosaurs are among the most important group of terrestrial tetrapods, and their origin is a widely discussed topic. They originated in the Middle or early Late Triassic, with the oldest representatives collected from Carnian beds of Argentina, Brazil and India (Reig 1963; Colbert 1970; Chatterjee 1987; Sereno & Novas 1992; Sereno et al. 1993; Langer et al. 1999; Langer 2005a; Martinez & Alcober 2009), dated to about 231 Ma (Rogers et al. 1993; Furin et al. 2006). At the moment, the few assemblages containing the oldest known dinosaurs depicted low dinosaur abundance and diversity; and their diversification and numerical dominance over other tetrapod groups is first seen in younger beds of Norian age (latest Late Triassic) (Benton 1988, 2006; Bonaparte 1982; Novas 1997). The traditional view is that the early diversification and numerical increase of dinosaurs occurred almost simultaneously during the latest Late Triassic (post-carnian times) (Benton 1988, 1991, 1993, 2006; Bonaparte 1982; Charig 1984; Novas 1997; Brusatte et al. 2008a). One model postulates that the replacement of archaic tetrapods by dinosaurs and other modern groups was a long, drawn-out process involving competition (Bonaparte 1982; Charig 1984; Novas 1997). On the other hand, it has been claimed that the early radiation of dinosaurs occurred opportunistically in an empty ecospace, cleared during the end-carnian extinction event (Benton 1988, 1991, 1993). In recent years, new discoveries have changed our understanding of the evolution of the immediate precursors of dinosaurs. It has been shown that non-dinosaurian dinosauromorphs survived well into the Late Triassic, coexisting with early dinosaurs (Ezcurra 2006; Irmis et al. 2007). This suggests that the transition between assemblages with dinosaur precursors and those composed exclusively of dinosaur dinosauromorphs was gradual, and models of rapid competitive or fortuitous replacement are incorrect (Irmis et al. 2007), at least in North America. However, our understanding of the origin of dinosaurs is mostly based on a few discoveries made during recent decades (Sereno & Novas 1992; Sereno et al. 1993; Langer et al. 1999; Martinez & Alcober 2007, 2009; Ezcurra 2008). Here, martindezcurra@yahoo.com.ar ISSN print / online Copyright C 2010 The Natural History Museum DOI: /

3 372 M. D. Ezcurra I describe new dinosaur remains from the Ischigualasto Formation (late Carnian middle Norian; Rogers et al. 1993; Furin et al. 2006) of Argentina. This new evidence demonstrates that the numerical increase and diversification of early dinosaurs were diachronous processes, with dinosaur dominance (high abundance) preceded by a period of low abundance but high diversity prior or close to the Carnian Norian boundary. The Ischigualasto Fm. contains a diverse tetrapod fauna, but this stratigraphical unit is best known because it is one of the oldest known dinosaur-bearing assemblages (Sereno & Novas 1992; Benton 1993; Rogers et al. 1993; Langer 2005a, b). The lower third of the Ischigualasto Fm. (late Carnian) preserves the remains of the basal saurischian dinosaurs Herrerasaurus, Eoraptor and Panphagia (Reig 1963; Sereno & Novas 1992; Sereno et al. 1993; Martinez & Alcober 2009), and two or three new dinosaur species (Martinez & Alcober 2007; Ezcurra & Novas 2007a, 2008; Ezcurra 2008), one of them named here as Chromogisaurus novasi nov. gen. et nov. sp. Geological, palaeontological and chronostratigraphical settings The holotype of Chromogisaurus novasi (PVSJ 845) was found in the autumn of 1988 during a joint field trip to the Ischigualasto Fm. carried out by the Museo Argentino de Ciencias Naturales Bernardino Rivadavia, the Universidad Nacional y Museo de San Juan and the University of Chicago. This stratigraphical unit crops out in the San Juan and La Rioja provinces of NW Argentina, as part of the Agua de la Peña Group of the Ischigualasto-Villa Unión Basin (Bossi 1971). It lies between the Los Rastros Fm. (Ladinian-early Carnian) below and the Los Colorados Fm. (Norian) above. The Ischigualasto Fm. is composed of abundant fluvial-channel sandstones, overbank mudstones and paleosols (Currie et al. 2009). Deposition occurred in a north-west-trending continental rift basin on an upland alluvial plain dominated by low sinuosity, shallow streams and occasional lakes, within a seasonal climatic regime (Rogers et al. 1993; Currie et al. 2009). At the south-eastern end of the Ischigualasto Basin, the Ischiguaslto Fm. progressively thins from approximately 700 m in the west to 400 m in the east, over a distance of 7 km (Currie et al. 2009). Currie et al. (2009) reviewed the stratigraphy of the Ischigualasto Fm. in detail, recognizing four different members for the south-eastern end of the Ischigualasto Basin (Ischigualasto Provincial Park), from the lowermost to the uppermost: the La Peña, Cancha de Bochas, Valle de la Luna and Quebrada de la Sal members. Good chronostratigraphical control is currently available for the Ischigualasto Fm. Sanidine crystals from a bentonite sampled approximately 80 m above the base of the formation (Cancha de Bochas Member) (Currie et al. 2009) yielded an 40 Ar/ 39 Ar date of ± 0.3 Ma (Rogers et al. 1993). This dating was performed in Section 2 of the formation sensu Currie et al. (2009), which exceeds 400 m in thickness, thus the age dating of Rogers et al. (1993) belongs to the lower sector of the unit. However, there is a discrepancy between U-Pb and 40 Ar/ 39 Ar dates, the latter being 0.5 1% younger (Schoene et al. 2006). Following this discrepancy, Furin et al. (2006) recalculated the age dating obtained by Rogers et al. (1993) to be about ± 0.3 Ma. In addition, the Late Triassic timescale has suffered recent modifications. Muttoni et al. (2004), based on recent radiometric datings applied to biostratigraphically dated marine deposits, suggested that the Late Triassic, and in particular the Norian, are considerably longer than previously accepted, with durations of approximately 35 and 20 myrs respectively. As a result, Muttoni et al. (2004) placed the Carnian Norian boundary at 228 Ma. These re-calibrations of the Late Triassic timesacale, as well as corrected dating of the lower Ischigualasto bentonite ash, indicate that the deposition of the sedimentary unit began approximately 231 Ma during the late Carnian. Another age dating has been performed by Shipman (2004) on plagioclase crystals from a bentonite ash, approximately 610 m from the base of the formation (following Currie et al. 2009, fig. 3) (top of the Valle de la Luna Member, Section 1 of Currie et al. 2009). This dating yielded an 40 Ar/ 39 Ar date of ± 1.7 Ma (Shipman 2004). Nevertheless, following the corrections introduced by Schoene et al. (2006), the recalculated age obtained by Shipman (2004) is about ± 1.7 Ma. This age locates the top of the Valle de la Luna Member of the Ischigualasto Fm. in the mid-norian (sensu Muttoni et al. 2004). Accordingly, following the recalculated age datings of Rogers et al. (1993) and Shipman (2004), and the modified Late Triassic timescale of Muttoni et al. (2004), the Carnian Norian boundary should be located close to the middle of the formation probably within the Valle the la Luna Member. The type specimen of Chromogisaurus novasi (PVSJ 845) was excavated from the north of the Valle Pintado locality, in lower levels of the Cancha de Bochas Member (sensu Currie et al. 2009) of the Ischigualasto Fm. (Section 3 of Currie et al. 2009, Ischigualasto National Park, San Juan Province) (Fig. 1). The Valle Pintado locality exposes the lower third of the formation, very close to the Herr Toba bentonite level dated by Rogers et al. (1993). This considered, the holotype of Chromogisaurus is late Carnian in age. It was found in a very fossiliferous grey mudstone, in the same layer (approximately three metres thick) from which were collected a specimen of the herrerasaurid Herrerasaurus ischigualastensis (PVSJ 380) and a cynodont jaw (F. Novas unpublished field trip notes).

4 A new early dinosaur and reassessment of dinosaur origin and phylogeny 373 Figure 1. Map of SE Agua de la Peña Group (San Juan and La Rioja provinces, NW Argentina), showing the type locality of Chromogisaurus novasi (modified from Alcober 1996). The lower third of the Ischigualasto Fm. contains a rich fossil tetrapod fauna including temnospondyls (Pelorocephalus ischigualastensis and Promastodonsaurus bellmanni), rhynchosaurs (Hyperodapedon sanjuanensis and H. mariensis), dicynodonts (Ischigualastia jenseni), cynodonts (Exaeretodon frenguellii, Ischignathus sudamericanus, Ecteninion lunensis, Chiniquodon sanjuanensis and cf. Probainognathus sp.), crurotarsan archosaurs (Saurosuchus galilei, Aetosauroides scagliai, Sillosuchus longicervix and Trialestes romeri) and dinosaurs. Hitherto, only three dinosaur species have been recorded in the lower third of the unit (Herrerasaurus ischigualastensis, Eoraptor lunensis and Panphagia protos). Two probably new herrerasaurian species are also present in the lower levels of the unit (Ezcurra & Novas 2007a, 2008; Martinez & Alcober 2007). Pisanosaurus mertii (Casamiquela 1967; Bonaparte 1976) and a new theropod species (Martinez et al. 2008) are known from the middle of the Ischigualasto Fm. The assemblage in the lower third of the Ischigualasto Fm. is dominated by the rhynchosaur Hyperodapedon, followed by the cynodont Exaeretodon (Bonaparte 1982; Benton 1983; Rogers et al. 1993). The same pattern occurs in the Hyperodapedon Acme Zone of the Santa Maria Fm. of Brazil (Langer 2005b; Langer et al. 2007a), which has formed the basis for correlating the two assemblages biostratigaphically. Pv: Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Paleontologia de Vertebrados, Buenos Aires, Argentina; MB: Museum fur Naturkunde der Humboldt Universitat, Berlin, Germany; MCN: Museu de Ciencias Naturais da Fundacao Zoobotanica do Rio Grande do Sul, Porto Alegre, Brazil; MCP: Museo de Ciencias e Tecnología, Porto Alegre, Brazil; MCZ: Museum of Comparative Zoology, Cambridge, USA; MLP: Museo de La Plata, La Plata, Argentina; NHM: Natural History Museum, London, UK; PEFO: Petrified Forest National Park, Arizona, USA; PULR: Paleontología, Universidad Nacional de La Rioja, La Rioja, Argentina; PVL: Paleontología de Vertebrados, Instituto Miguel Lillo, San Miguel de Tucumán; PVSJ: División de Paleontologia de Vertebrados del Museo de Ciencias Naturales y Universidad Nacional de San Juan, San Juan, Argentina; SAM: South African Museum, South Africa; SMNS: Staatliches Museum fur Naturkunde, Stuttgart, Germany; UCMP: University of California Museum of Paleontology, Berkeley, CA, USA; UFRGS: Universidade Federal de Rio Grande do Sul, Porto Alegre, RS, Brazil; UMMP: University of Michigan Museum of Paleontology, Ann Arbor, MI, USA; QG: Zimbabwe Natural History Museum, Bulawayo, Zimbabwe; ZPAL: Institute of Paleobiology of the Polish Academy of Sciences, Warsaw, Poland. Institutional abbreviations GR: Ghost Ranch Ruth May Museum of Paleontology, New Mexico, USA; ISIR: Geological Studies Unit of the Indian Statistical Institute, Calcutta, India; MACN- Systematic palaeontology Dinosauria Owen, 1842 Saurischia Seeley, 1888 Sauropodomorpha von Huene, 1932 Guaibasauridae Bonaparte et al., 1999

5 374 M. D. Ezcurra Figure 2. Preserved bones of Chromogisaurus novasi (PVSJ 845). Scale bar = 50 cm. Definition. The family Guaibasauridae is defined here as all archosaurs more closely related to Guaibasaurus candelariensis Bonaparte et al., 1999 than to Carnotaurus sastrei Bonaparte, 1985 or Saltasaurus loricatus Bonaparte & Powell, Saturnaliinae new taxon Definition. The subfamily Saturnaliinae is defined here as Saturnalia tupiniquim Langer et al., 1999, Chromogisaurus novasi nov. gen. et nov. sp., and all the descendants from their most common ancestor. Chromogisaurus novasi gen. et sp. nov. (Figs 2 13, 14B, D, F, 16B, 17C) Etymology. The generic name is derived from the Greek words chroma (colour, paint), gi (ground, land) and saurus (reptile); in allusion to the Valle Pintado (Painted Valley) locality of the Ischigualasto Fm. in which the new taxon was found. The specific name is in honour of Dr. Fernando Novas, for his outstanding research on early dinosaur evolution. Holotype. PVSJ (Paleontologia de Vertebrados Museo de San Juan) 845, partial skeleton including a proximal caudal vertebra, a mid-caudal vertebra, proximal end of the right ulna, right ilium, a probable fragment of ischial shaft, both femora, left tibia, left fibula, left metatarsals II and V, pedal phalanges, including a complete left digit II and some unidentified bone fragments (Fig. 2; Tables 1, 2). Locality and horizon. Lower levels of the Cancha de Bochas Member (sensu Currie et al. 2009) of the Ischigualasto Fm. (Late Triassic: late Carnian, ± 0.3 Ma; Rogers et al. 1993; Furin et al. 2006), northern margin of Valle Pintado site (Section 3 of Currie et al. 2009), Agua de la Peña Group, Ischigualasto- Villa Unión Basin, Ischigualasto National Park, San Juan Province, Argentina (Fig. 1). Diagnosis. Chromogisaurus novasi is a small basal saurischian dinosaur diagnosed by the following combination of characteristics (autapomorphies ): proximal caudals without median notch separating the postzygapophyses; ilium with strongly posteriorly developed postacetabular process; incipiently perforated acetabulum; a femoral lateral surface with deep and large fossa immediately below the trochanteric shelf; and a metatarsal II with strongly dorsoventrally asymmetric distal condyles. Description The size of the appendicular elements (Table 2) of the holotype of Chromogisaurus closely matches that of Saturnalia (MCP 3844-PV, 3846-PV, Langer 2003) and Table 1. Selected measurements (in millimetres) of axial and forelimb elements of Chromogisaurus novasi gen. et sp. nov. Abbreviations: = incomplete. Measurements Element of PVSJ 845 Proximal caudal vertebra Maximum length (neural arch) 29.7 Maximum height (centrum + neural arch) 28 Length of centrum 24.7 Anterior height of centrum 14.9 Anterior width of centrum 12.3 Posterior height of centrum 13.6 Posterior width of centrum 12.5 Maximum width of transverse process 18.1 Mid-caudal vertebra Maximum length (neural arch) 31.6 Maximum height (centrum + neural arch) 38.7 Length of centrum 25.4 Anterior height of centrum 11.8 Anterior width of centrum 6.9 Posterior height of centrum 12.6 Posterior width of centrum 8.1 Right ulna Maximum length 29.6 Maximum width 11.8 Length of olecranon 20.7

6 A new early dinosaur and reassessment of dinosaur origin and phylogeny 375 Table 2. Selected measurements (in millimetres) of pelvic girdle and hindlimb available bones of Chromogisaurus novasi gen. et sp. nov. Abbreviations: = incomplete; () = estimated; =deformed. The estimated lengths of the complete elements are based on extrapolations with Saturnalia tupiniquim (following the measurements of Langer 2003). Measurements of PVSJ 845 Element Left Right Ilium Maximum length 96.4 Maximum length along iliac blade 75.7 Maximum length along pedicles 53.9 Maximum height 61.5 Maximum width of the supraacetabular 15.5 crest Length of the postacetabular process 49.9 Length between pubic and ischiadic 28.2 embayments Femur Maximum preserved length Maximum depth (anteroposteriorly) at level of the 4th trochanter Maximum length of the lateral fossa Circumference below 4th trochanter 56 Tibia Length 175 Maximum proximal depth 41.8 (anteroposteriorly) Maximum proximal width (lateromedially) 14.4 Maximum distal depth (anteroposteriorly) 19.1 Maximum distal width (lateromedially) 13.9 Fibula Maximum preserved length Maximum preserved proximal depth 21.0 Depth at mid-length 10.6 Width at mid-length 5.8 Metatarsal II Maximum preserved length 58.0 (70) Distal width 12.8 Maximum height of medial condyle 8.6 Maximum height of lateral condyle 10.6 Metatarsal V Preserved length 38.5 Proximal width 10.3 Proximal depth 6.5 Distal width 2.1 Phalanx II-1 Length 19.6 Maximum proximal height 7.0 Maximum proximal width 12.2 Maximum distal height 5.7 Maximum distal width 8.7 Phalanx II-2 Length 17.1 Maximum proximal height 6.1 Maximum proximal width 10.1 Maximum distal height 4.6 Maximum distal width 7.4 Phalanx II-3 (ungual) Length 17.8 Maximum proximal height 7.7 Maximum proximal width 6.9 Eoraptor (PVSJ 512), indicating that it was a small animal of around 2 metres long (Fig. 2), with an approximate mass of 10 kg (following the regression of Anderson et al. 1985). The preserved caudal vertebrae present fully closed neurocentral sutures, suggesting that the specimen is not a young juvenile (Irmis 2007). Caudal vertebrae. A proximal and a mid-caudal vertebra are preserved (Figs 3, 4; Table 1). The centrum of the mid-caudal vertebra is slightly longer than that of the proximal vertebra, but the proximal caudal centrum is taller. The length of the centrum of the proximal caudal vertebra is 1.5 times its height (Fig. 3A, D). The centrum is strongly compressed laterally at mid-length, and hourglassshaped in ventral view. The ventral surface of the centrum is convex, without a ventral groove. Both anterior and posterior articular surfaces are concave and oval, with their major axis dorsoventrally oriented (Fig. 3B, E). In the neural arch, the walls of the neural canal are low. The left transverse process is almost completely preserved, only lacking its distal tip, and it is dorsolaterally oriented, as also occurs in Panphagia (Martinez & Alcober 2009). The transverse process is elongated, representing 82% of the length of the centrum as preserved (Fig. 3C, F). The transverse process curves gently anteriorly, with concave and convex anterior and posterior margins, respectively. The latter condition is also observed in Saturnalia (MCP 3846-PV), but is absent in Guaibasaurus (UFRGS PV 0725T), Herrerasaurus (PVL 2556), Eoraptor (PVSJ 512) and neotheropods (e.g. Dilophosaurus; UCMP 37302). The prezygapophyses are damaged, but it is possible to see that they extended only slightly beyond the anterior end of the centrum. The articular surface of the prezygapophysis is laterodorsally oriented. The postzygapophyses are much better preserved, with lateroventrally oriented articular surfaces, extending slightly beyond the posterior end of the centrum. No hyposphene-hypantrum articulation complex is present, contrasting with the proximal and mid-caudal vertebrae of basal neotheropods (e.g. Dilophosaurus, Lophostropheus; UCMP 37302, Ezcurra & Cuny 2007). No median notch separates the postzygapophyses from each other. In contrast, in Saturnalia (MCP 3846-PV) and Guaibasaurus (UFRGS PV 0725T) the postzygapophyses are separated by a deep median notch. A well developed and sharp prezygo-postzygopophyseal lamina is present. The centrum of the mid-caudal vertebra is two times longer than the height of its posterior articular facet (Fig. 4A, B, E, F). The anterior articular surface of the centrum is not preserved (Fig. 4G, H). The posterior articular surface is strongly concave and oval, with its major axis dorsoventrally oriented (Fig. 4C, D). Indeed, the height of the posterior articular surface is around 1.3 times its maximum width. The centrum is only gently transversely constricted at mid-length, with slightly concave lateral surfaces

7 376 M. D. Ezcurra Figure 3. Proximal caudal vertebra of Chromogisaurus novasi in A, D, right lateral; B, E, posterior; and C, F, dorsal views. Abbreviations: ns, neural spine; paf, posterior articular facet; poz, postzygapophysis; ppl, prezygo-postzygopophyseal lamina; prz, prezygapophysis; tp, transverse process. Scale bar = 1cm. (Fig. 4I, J). An incipient longitudinal groove is present along the posterior third of the ventral surface of the centrum. Only the bases of the transverse processes are preserved, arising slightly more posteriorly than mid-length of the neural arch (Fig. 4K, L). The bases of the transverse processes project laterally. Only the base of the left prezygapophysis is preserved. It projects anteriorly in side view and anterolaterally in dorsal view. The postzygapophyses are available, but they are deformed in the transverse plane. They arise directly below the neural spine, and no median notch separates one from the other, contrasting with Eoraptor (PVSJ 512), Panphagia (PVSJ 874), Guaibasaurus (UFRGS PV 0725T) and Saturnalia (MCP 3846-PV). The articular surface of the postzygapophysis is lateroventrally oriented. It is extended well posteriorly, surpassing the posterior-most level of the centrum. A well developed and sharp prezygo-postzygopophyseal lamina is also present. The neural spine arises anteriorly between the base of both prezygapophyses, but remains as a very low and transversely thin lamina up to the level of the postzygapophyses, as in Silesaurus (Dzik 2003), Eoraptor (PVSJ 512) and other basal dinosauriforms. Between both postzygapophysis, the neural spine curves strongly dorsally, reaching a height that exceeds that of the centrum plus the base of the remaining neural arch. The neural spines of Eoraptor (PVSJ 512) are proportionally lower than those of Chromogisaurus. The neural spine is more posteriorly extended than the postzygapophyses, as occurs in other basal dinosauriforms. Ulna. A partial proximal end of the right ulna is available, preserving the complete olecranon processes (Fig. 5; Table 1). This fragmentary bone was preliminarily interpreted as the posterior end of the right jaw (Ezcurra 2008). The preserved portion of the ulna indicates that the forelimb of Chromogisaurus was very stout in relation to the hindlimb elements, resembling the condition present in Saturnalia (Langer et al. 2007b) and more derived sauropodomorphs. The olecranon process is extremely proximodistally enlarged, in relation to its anteroposterior depth and the inferred depth of the proximal end of the bone. This condition is almost identical to that of Saturnalia (Langer et al. 2007b), and clearly contrasts with the less developed olecranon of Herrerasaurus (Sereno 1993), Eoraptor (PVSJ 512), neotheropods (e.g. Dilophosaurus, Liliensternus, Coelophysis; UCMP 37302; HMN MB R. 2175; QG 1), Guaibasaurus (Bonaparte et al. 2007) and basal plateosaurian sauropodomorphs (e.g. Unaysaurus, Plateosaurus, Adeopapposaurus; Leal et al. 2004; Bonnan & Senter 2007; Martinez 2009). The posterolateral surface of the olecranon is formed by a sheet of bone, bearing strongly marked longitudinal striations (Fig. 5A). Exactly the same condition is present in Saturnalia, and Langer et al. (2007b) interpreted this sheet of bone as an independent ossification. The latter seems to be also true for Chromogisaurus. This probable ossification forms a distinct posteriorly-projected knob, which exceeds the posterior margin of the ulnar shaft, as is also present in Saturnalia (Langer et al. 2007b). Langer et al. (2007b) pointed

8 A new early dinosaur and reassessment of dinosaur origin and phylogeny 377 Figure 4. Mid-caudal vertebra of Chromogisaurus novasi in A, B, right lateral; C, D, posterior; E, F, left lateral; G, H, anterior; I, J, ventral; and K, L, dorsal views. Abbreviations: ns, neural spine; paf, posterior articular facet; poz, postzygapophysis; ppl, prezygopostzygopophyseal lamina; prz, prezygapophysis; tp, transverse process; vg, ventral groove. Scale bar = 1cm. out that the striated surface of this ossification represents the insertion of the M. triceps tendon. This ossification is anteroproximally limited by a vertical sharp edge which posteriorly bounds a concave and smooth surface, proximoanteriorly oriented. Below this concavity, an anteriorly projected knob is present, proximally covering the humeral articulation area. Langer et al. (2007b) interpreted the latter as a second independent ossification, which defines a proximally hollow olecranon, and the same seems to be the case for Chromogisaurus. These authors pointed out that the presence of these ossifications and extremely enlarged olecranon in two specimens of Saturnalia suggest that it is not a pathological trait, but typical for the taxon. Its presence in Chromogisaurus clearly bolsters the claim of these authors. The humeral articulation area, for the reception of the ulnar condyle, is strongly concave, and ventrolaterally bounded by a conspicuous, but low, ridge. This ridge extends distally, as a vertical structure, onto the lateral process (Godefroit et al. 1998) directly below the humeral

9 378 M. D. Ezcurra Figure 5. Proximal end of the right ulna of Chromogisaurus novasi in A, B, posterolateral; and C, D, anterior views. Abbreviations: io1 2, independent ossification 1 and 2; its, insertion M. triceps scapularis; lp, lateral process; oas, olecranon articular surface; ol, olecranon process; oss, olecranon striated surface; r, ridge. Scale bar = 5 mm. articulation. Langer et al. (2007b) pointed out that this lip-like structure might represent attachment areas for ligaments of the elbow joint (Baumel & Raikow 1993; Meers 2003). This vertically oriented ridge seems to represent the base of the vertical crest which runs longitudinally across the proximal half of the ulnar shaft. Posteriorly to the ridge which bounds the posteroventral border of the humeral articulation area is a concave and slightly striated surface. This area has been interpreted as the insertion of the M. triceps scapularis in Saturnalia (Langer et al. 2007b). The lateral process of Chromogisaurus is less developed than in Saturnalia (MCP 3845-PV). The anterior process is not preserved in the available specimen of Chromogisaurus. The medial surface of the ulna, directly below of the olecranon process, is concave, for the reception of the radius. At the level where the ulnar shaft is broken off, it is elliptical in cross-section and with an anteroposterior main axis. Ilium. The available right ilium is well preserved, only lacking its preacetabular process (Fig. 6; Table 2). The ilium of Chromogisaurus closely approaches that of Guaibasaurus (UFRGS PV 0725T), Saturnalia (MCP 3844-PV, 3846-PV), Panphagia (PVSJ 874; Martinez & Alcober 2009) and Agnosphitys (Fraser et al. 2002), sharing an incipiently perforated acetabulum and an extremely long postacetabular process. The posterior end of the postacetabular process of Chromogisaurus has a thick and trapezoideal rugose area on its lateral surface (Fig. 6A, B), which tapers anterodorsally, a feature also present in Saturnalia (MCP 3844-PV; Langer 2003) (Fig. 14A). Langer (2003) suggested that these muscle scars are the origin areas for the M. flexor tibialis externus (ventrally) and M. iliotibialis (anterodorsally, close to the dorsal margin of the iliac blade). In a paratype specimen of Saturnalia (MCP 3845-PV), these muscle scars are less developed than in the holotype of the species (MCP 3844-PV) and Chromogisaurus, but still exhibit the same shape and position. In other basal dinosaurs (e.g. Herrerasaurus, Caseosaurus, Panphagia; PVL 2566, UMMP 8870, Martinez & Alcober 2009), the postacetabular process also present thick rugosities, but their shape and development are clearly distinct from that of Saturnalia and Chromogisaurus (Fig. 17). The posterior end of the process bears a pointed posteroventral prong and a rounded posterodorsal margin (Fig. 6A, B, E, F), as in Saturnalia and some other sauropodomorphs, but this is absent in Panphagia (PVSJ 874) and Guaibasaurus (UFRGS PV 0725T). The lateral surface of this posteroventral prong presents a striated surface, which Langer (2003) interpreted as the origin area of the M. flexor tibialis internus. A well developed brevis shelf is present, but does not merge anteriorly with the supraacetabular crest as occurs in Eoraptor (PVSJ 512) and neotheropods (e.g. Dilophosaurus, Carnotaurus; UCMP 37302; MACN-CH-PV 894). The brevis shelf, together with a prominent posteromedial lamina, delimits a well developed brevis fossa, which gently flares transversely at its posterior end (Fig. 6G, H). The brevis fossa corresponds to the origin area of the M. caudofemoralis brevis (Gatesy 1990). The posteromedial lamina of Chromogisaurus is much less developed than in Panphagia (Martinez & Alcober 2009). As is characteristic for dinosaurs (Ezcurra 2006), the iliac blade of Chromogisaurus is high and presents a straight dorsal margin in lateral view. In dorsal view, the iliac blade of Chromogisaurus strongly curves laterally (Fig. 6E, F), as occurs in other sauropodomorphs, such as Saturnalia (MCP 3846-PV), Guaibasaurus (UFRGS PV 0725T), Panphagia (PVSJ 874), Riojasaurus (PVL 3808), Efraasia (Galton 1984) and Lessemsaurus (PVL 4822) (Fig. 16). Directly

10 A new early dinosaur and reassessment of dinosaur origin and phylogeny 379 Figure 6. Right ilium of Chromogisaurus novasi in A, B, lateral; C, D, medial; E, F, dorsal; and G, H, ventral views. Abbreviations: aw, acetabular wall; bs, brevis shelf; bf, brevis fossa; cs?, probable insertion of the caudosacral rib; d, depression, origin M. iliofemoralis cranialis; ip, ischiadic peduncle; omi, origin M. iliotibialis; omfi, origin M. flexor tibialis internus; omfe, origin M. flexor tibialis externus; plp, posterolateral prong; pms, posteromedial shelf; pp, pubic pedunlce; sac, supraacetabular crest; sr1 2, sacral 1 and 2 rib attachment areas; t; tuberosity. Scale bar = 2cm. above the supraacetabular crest, a triangular depression exists on the iliac blade, probably for the origin of the M. iliofemoralis cranialis (following Langer 2003). The base of the preacetabular process is represented by a thick and well developed buttress, which contacts the supraacetabular crest ventrally, resembling other basal dinosauriforms (e.g. Herrerasaurus, Silesaurus, Saturnalia, Eoraptor; Novas 1993; Dzik 2003; Langer 2003; PVSJ 512). The supraacetabular crest is conspicuous above the acetabulum and continues as a well laterally developed crest along the extension of the pubic peduncle. Indeed, the supraacetabular crest reaches the distal end of the pubic peduncle, as occurs in Saturnalia (MCP, 3844-PV, 3846-PV), Guaibasaurus (UFRGS PV 0725T), Panphagia (Martinez & Alcober 2009, fig. 8) and Eoraptor (PVSJ 512), but contrasting with Silesaurus (Dzik 2003), ornithischians (UCMP ), Herrerasaurus (PVL 2556; MACN-PV 18060) and neotheropods (UCMP 37302, 77270, ; QG 1). This crest projects directly laterally, contrasting with basal neotheropods, in which this structure is lateroventrally deflected, partially overlapping the acetabulum in lateral view (e.g. Dilophosaurus, Lophostropheus, Coelophysis,

11 380 M. D. Ezcurra Liliensternus; UCMP 37302; Ezcurra & Cuny 2007; QG 1; MB R. 2175). The supraacetabular crest of Chromogisaurus is strongly ventrally curved in lateral view, contrasting with the almost straight crest present in Panphagia (Martinez & Alcober 2009, fig. 8). Contrasting with Guaibasaurus (Langer et al. 2007c), no excavation is observed on the lateral surface of the actetabular wall, just anterior to the ischiadic peduncle. The pubic peduncle is very long, resembling Saturnalia (MCP 3844-PV), Guaibasaurus (UFRGS PV 0725T), Panphagia (Martinez & Alcober 2009, fig. 8) and more derived sauropodomorphs (PVL 3808, 4822; Benton et al. 2000). The acetabulum is only incipiently open, as in Saturnalia (Langer 2003), Guaibasaurus (Bonaparte et al. 1999, 2007), Panphagia (Martinez & Alcober 2009) and Agnosphitys (Fraser et al. 2002). Contrasting with the holotype of Saturnalia (MCP 3844-PV), the ventral border of the iliac wall of Chromogisaurus is concave (Fig. 14A, B). However, different specimens of Guaibasaurus present a straight (Bonaparte et al. 1999; MCP 2355-PV) or a concave (UFRGS PV 0725T) ventral margin, showing that the ventral shape of the acetabular wall is intraspecifically variable. The ischiadic peduncle is anteroposteriorly broad, with a mostly ventrally and slightly posteriorly oriented articular surface for the ischium. The antitrochanter is positioned on the anteroventral corner of this peduncle. The medial surface of the iliac blade presents a complex topography, exhibiting scars for the attachment of the two primordial sacral ribs and a probable caudosacral vertebra (Fig. 6C, D). The anteriormost scar, for the first sacral rib, is positioned at the base of the preacetabular process. The attachment for the first primordial sacral rib presents a C contour in basal saurischians (Langer & Benton 2006, fig. 7), and it seems to be also the case in Chromogisaurus. The ventral component of the scar for the first sacral rib is very conspicuous, being longitudinally extended along the base of the ischiadic and pubic peduncles and reaching the base of the preacetabular process. The dorsal component is very shallow and is situated close to the dorsal margin of the iliac blade. The shape of the first primordial sacral rib scar closely approaches the morphology exhibited by Herrerasaurus (PVL 2556), but clearly contrasts with the more dorsoventrally reduced and posteriorly restricted ventral component of Saturnalia (Langer & Benton 2006). The scar for the second sacral rib originates at the level of the ischiadic peduncle and extends posteriorly up to threequarters of the length of the postacetabular process. This scar is subrectangular and oblique to the longitudinal axis of the bone, with its highest point situated at its posterior end. Contrasting with Herrerasaurus, but resembling Saturnalia (Langer & Benton 2006), the scar of the second primordial sacral of Chromogisaurus is formed by a single longitudinal depression, and no well-developed dorsal component is observed. The ventral border of this scar is delimited by a well developed posteromedial lamina, which forms the Figure 7. Probable ischial shaft of Chromogisaurus novasi in A, medial?; B, cross-section; and C, dorsal? views. Abbreviations: lr, longitudinal ridge. Scale bar = 1cm. medial border of the brevis fossa. This shelf-like structure continues anteriorly onto the medial surface of the ilium up to directly above the ischiadic peduncle, and it is much less medially developed than in Panphagia (Martinez & Alcober 2009). On the other hand, the dorsal border of the second sacral scar is formed by a medially bulged surface. This medially inflatened surface occupies a large area on the iliac blade and is triangular. At the posterior end of the medial surface of the postacetabular process, a concave and rounded depression is present. This depression would represent the scar for the third sacral rib. As occurs in Saturnalia (Langer 2003), this third sacral scar would indicate the adding of a caudal vertebra to the sacral series (i.e. a caudosacral). The medial surface of the iliac blade, above the subtriangular inflatened area and the scar for the second sacral, is almost flat. The medial surface of the acetabular wall is flat and slightly convex. Ischium? A fragment of a rod-like bone is here interpreted as a probable partial ischial shaft (Fig. 7). This purported portion of ischial shaft is straight and oval in cross-section. A well developed and sharp longitudinal ridge is present. This structure may represent the contact area for the other ischium. Femur. Remains of both femora are preserved (Figs 8, 9; Table 2). Most of the right femur is available, lacking its proximal end and distal condyles (Fig. 8). Otherwise,

12 A new early dinosaur and reassessment of dinosaur origin and phylogeny 381 Figure 8. Right femur of Chromogisaurus novasi in A, B, lateral; C, D, medial; and E, F, posterior views. Abbreviations: cd, concave depression; ft, fourth trochanter; imc, insertion of the M. caudofemoralis longus; lf, lateral fossa; pf, plopiteal fossa; pll, posterolateral intermuscular line. Scale bar = 2cm. the left femur is represented by the proximal half of the bone, lacking the proximal end (Fig. 9), and a fragmentary distal end. The femoral shaft of Chromogisaurus is bowed posteriorly in side view. A well developed and thick trochanteric shelf is present (Fig. 9A, E) (not preserved in the right femur), as in several dinosauriforms (e.g. Chindesaurus, Herrerasaurus, Dilophosaurus, Saturnalia, Pseudolagosuchus, Silesaurus; PEFO 10395; PVL 2556; UCMP 37302; MCP 3844-PV; PULR 053; Dzik 2003). By contrast, in all the available specimens of Guaibasaurus the trochanteric shelf is absent (Langer & Benton 2006; MCP 2355-PV; UFRGS PV 0725T; contra Bonaparte et al. 2007). The trochanteric shelf of Chromogisaurus originates on the anterior surface of the femur and continues along most of the lateral surface of the bone. The shelf slopes distally, merging with the femoral shaft, on the posterolateral corner of the bone. On the other hand, contrasting with Chromogisaurus, in Saturnalia the trochanteric shelf continues on the posterior surface of the femur (MCP PV, 3845-PV) (Fig. 14C). The trochanteric shelf presents a rugose surface, which seems to indicate the insertion of the M. iliofemoralis externus (Langer 2003). A very large and deep elliptical fossa is present on the lateral surface of both femora (Figs 8A, B, 9A, E), immediately below the trochanteric shelf. This fossa opens widely anteroposteriorly, whereas distally it ends sharply and abruptly. The fossa exhibits the same position, size, and contour in both available femora, and there is no crushing that would suggest deformation of the medullary cavity. The surface of this fossa is smooth, thus it seems to not represent a muscle origin area. Thus, the function of this fossa is unknown. This feature is unknown in other basal dinosauriforms, thus, it seems to represent an autapomorphy of Chromogisaurus. Furthermore, the lateral fossa confers a B- shaped cross-section at the level of the fourth trochanter Figure 9. Left femur of Chromogisaurus novasi in A, E, lateral; B, H, medial; C, G, posterior; and D, H, cross-section views. Abbreviations: cd, concave depression; ft, fourth trochanter; lf, lateral fossa; pll, posterolateral intermuscular line; ts, trochanteric shelf. Scale bar = 1cm.

13 382 M. D. Ezcurra (Fig. 9D, H). The fourth trochanter lies in the proximal half of the femur and is asymmetric (Fig. 8A D), as in non-neotheropod basal saurischians. Distally to the level of the trochanteric shelf, the posterior surface of the femur is concave (Figs 8E, F, 9C, G). This concavity is deeper proximally, and bounded by the posterolateral intermuscular line and the fourth trochanter. On the medial surface of the femur, at the anteroventral base of the fourth trochanter, a fossa bearing strongly marked longitudinal striations is present. This area corresponds to the insertion of the M. caudofemoralis longus (Langer 2003). The distal end of the available femora of Chromogisaurus is incomplete. However, it can be seen that it is somewhat transversely expanded and bears a long and deep longitudinal popliteal fossa. Tibia. The right tibia is completely preserved, but somewhat transversely compressed artificially (Fig. 10; Table 2). The proximal end is well anteroposteriorly expanded. The cnemial crest is moderately developed and slightly curved laterally, as occurs in other basal dinosaurs (Fig. 10C, D). The posterior condyles of the proximal end of the bone are asymmetric, with the medial condyle much more posteriorly extended than the lateral one. This condition is also present in sauropodomorphs (Yates 2007a, b; including Panphagia; PVSJ 874, contra Martinez & Alcober 2009), Eoraptor (PVSJ 512), ornithischians (UCMP ), Marasuchus (Sereno & Arcucci 1994) and Pseudolagosuchus (PULR 053). The proximal end of the tibia reaches its highest point at the cnemial crest (Fig. 10A, B). In lateral view, the cnemial crest has a rounded profile. The lateral surface of the cnemial crest is rugose, which could be the origin area of the M. tibialis cranialis proximally and M. extensor digitorum longus more distally (sensu Langer 2003; McGowan 1979; Dilkes 2000). The posterior condyles are strongly projected backwards. They are distinct from one another, yet are not separated by a median notch, as occurs in other dinosaurs (e.g. Saturnalia, Panphagia, Chindesaurus; Langer 2003; PVSJ 874; PEFO 10395). The proximal articular surface of the bone is concave, due to a proximally inflated medial margin. As occurs in Saturnalia (Langer 2003; MCP 3844-PV), a wide, but very low, longitudinal tuberosity bounds posteriorly the lateral concavity formed by the curved cnemial crest (Fig. 10A, B). Curiously, this tuberosity is located in the same position to that of the fibular crest of basal neotheropods (e.g. Dilophosaurus, Liliensternus, Segisaurus; UCMP 37302; MB R. 2175; UCMP 32101) and silesaurids (e.g. Silesaurus, Sacisaurus; Dzik 2003; Ferigolo and Langer 2007; MCN PV10020). This tuberosity participates in attachment Figure 10. Right tibia of Chromogisaurus novasi in A, B, lateral; C, D, proximal; E, F, distal; G, H, posterior; and I, J, anterior views. Abbreviations: ad, anterior depression; cn, cnemial crest; fap, facet for the reception of the ascending process of the astragalus; lc, lateral condyle; ln, lateral notch; lt, lateral tuberosity; mc, medial condyle; plc, posterolateral concavity; plp, posterolateral process; plt, posterolateral tuberosity; se, sharp edge. Scale bar = 2cm.

14 A new early dinosaur and reassessment of dinosaur origin and phylogeny 383 of the Lig. tibiofibularis (Langer 2003). The latter structure is bounded, anteriorly and posteriorly, by concave surfaces for the reception of the fibula. On the posterolateral border of the bone, a low but well laterally inflatened tuberosity is present (Fig. 10A, B, G, H), directly below the distal level of the cnemial crest. On the medial side, a large and deep concave area exists directly below the proximal end of the tibia. The tibial shaft is straight along all its extension. The lateral surface of the shaft is strongly convex, with a conspicuous and sharp median edge. On the other hand, the medial surface of the tibial shaft is planar. Both anterior and posterior borders of the tibial shaft are convex, but the former is more acute. The distal articular surface of the tibia of Chromogisaurus closely resembles that of Saturnalia (MCP 3844-PV), being more anteroposteriorly deep than transversely wide (Fig. 10E, F). Nevertheless, the transverse compression of the distal tibia of Chromogisaurus could be exaggerated by post-mortem deformation. Otherwise, a paratype specimen of Saturnalia exhibits a distal end of the tibia transversely wider than it is anteroposteriorly deep (MCP 3846-PV), contrasting with the holotype specimen. Accordingly, the proportions of the distal outline of the tibia seem to depend on intraspecific variability, at least in Saturnalia tupiniquim. The borders of the distal end of the tibia of Chromogisaurus are straight, thus lacking a rounded distal articular surface (Langer 2003, MCP 3844-PV). The latter condition resembles that of other eusaurischians (e.g. Panphagia, Saturnalia, Guaibasaurus, Eoraptor, neotheropods, Riojasaurus; Martinez & Alcober 2009; MCP 3844-PV, 2356-PV; PVSJ 512; PULR 076; PVL 3808), but contrasts with that of more basal dinosauromorphs (e.g. Dromomeron, Lagerpeton, Pseudolagosuchus, Silesaurus, Herrerasaurus; GR 220; PULR 53; Dzik 2003; Novas 1993). A conspicuous lateral notch starts with the distal end of the bone. This longitudinal notch opens distally into the anterior articular facet for the reception of the ascending process of the astragalus. On the anterior surface, a proximodistally short but deep sulcus is present. This sulcus does not reach the distal articular end of the bone. The posterolateral surface of the distal end of the tibia is concave, resembling the condition present in some eusaurischians (e.g. Panphagia, Saturnalia, Guaibasaurus, neotheropods; Martinez & Alcober 2009; MCP 3844-PV, 2356-PV; Ezcurra & Novas 2007b). The posterolateral process of the distal tibia is moderately developed, as is the case in Herrerasaurus (Novas 1989), Saturnalia (MCP 3844-PV), Panphagia (Martinez & Alcober 2009) and other basal sauropodomorphs (e.g. Riojasaurus; PVL 3808). However, in Chromogisaurus this process is not as developed as in Eoraptor (PVSJ 512), Guaibasaurus (MCP 2356-PV), Chindesaurus (Nesbitt et al. 2007) and neotheropods (Dilophosaurus, Liliensternus, Segisaurus, Zupaysaurus; UCMP 37302; MB R. 2175; UCMP 32101; Ezcurra and Novas 2007b). In anterior view, the articular facet for the reception of the ascending process of the astragalus is diagonal to the longitudinal axis of the bone (Fig. 10I, J). Thus, the articular facet faces laterodistally and its distal-most region is located at the anteromedial corner of the distal end. This condition contrasts with that of basal dinosauriforms, such as Herrerasaurus (Novas 1989; MACN-PV 18060; PVL 2556) and Silesaurus (Dzik 2003), but resembles that of Guaibasaurus (MCP 2356-PV), Saturnalia (MCP 3844-PV), Eoraptor (PVSJ 512), neotheropods (e.g. Dilophosaurus, Zupaysaurus; UCMP 77270; Ezcurra and Novas 2007b) and plateosaurians (e.g. Riojasaurus; PVL 3845). The distal articular surface of the tibia of Chromogisaurus presents a moderately deep posteromedial notch, bounded by convex surfaces. Fibula. The right fibula is preserved, with a damaged proximal end and lacking its distal end (Fig. 11; Table 2). The shaft is straight in side view and its anteroposterior depth decreases gradually towards its distal end (Fig. 11A, B). The fibular shaft is thinner than that of the tibia at mid-length, as in other basal dinosaurs (e.g. Staurikosaurus Herrerasaurus, Saturnalia, Guaibasaurus and Eoraptor), but contrasting with the more pronounced difference present in neotheropods and ornithischians (Langer & Benton 2006). The proximal end is well anteroposteriorly expanded but strongly compressed transversely. In crosssection the proximal fibula is oval, with sharp anterior and posterior edges. The lateral surface of the proximal end is slightly convex, but along the shaft the lateral surface of the bone becomes strongly convex. On the other hand, the medial surface of the proximal end is planar, but along the shaft, it bears a deep longitudinal median fossa (Fig. 11C, D), mostly for contact of the lateral surface of the tibia. Thus, in cross-section the fibular shaft acquires a semilunate shape. The anterior and posterior borders of the shaft are rounded. Below the proximal expansion of the fibula, a low tuberosity is present on the anterolateral edge of the shaft (Fig. 11A, B, E, F), probably for the insertion of the M. iliofibularis (Carrano & Hutchinson 2002). This tuberosity is clearly less developed than in neotheropods (e.g. Segisaurus, Dilophosaurus; UCMP 32101, 37302). Metatarsal II. The overall morphology of the metatarsals resembles that of Silesaurus and other basal dinosaurs, contrasting with the extremely gracile metatarsus of more basal ornithodirans (Langer & Benton 2006). The left metatarsal II is preserved, only lacking its proximal end (Fig. 12A I; Table 2). The shaft of the metatarsal II is slightly bowed medially in dorsal view (Fig. 12A, B). In cross-section the shaft is circular at mid-length. A distinct articular facet is present at the most proximal preserved portion of the bone. This facet is for the reception of the metatarsal I, and it lies at the posteromedial corner of the bone (Fig. 12E, F). Thus, the proximal end of the metatarsal

15 384 M. D. Ezcurra Figure 11. Right fibula of Chromogisaurus novasi in A, B, lateral; C, D, medial; and E, F, anterior views. Abbreviations: md, medial depression; til, iliofibularis muscle insertion. Scale bar = 2cm. II seems to have overlapped the metatarsal I in dorsal view. The facet for the reception of the metatarsal I is triangular, deep and with conspicuous borders. The distal end of the metatarsal II expands transversely. At this region, the bone presents well-developed collateral pits, with the medial one as the largest (Fig. 12C F). On the dorsal surface of the distal trochlea, a transversely wide depression is present, proximally bounded by a conspicuous lip. The lateral distal condyle is more ventrally developed and transversely thinner than the medial one (Fig. 12G, I), contrasting with the more symmetric condition present in other basal dinosauriforms (e.g. Eoraptor, Saturnalia, Herrerasaurus, Silesaurus, Dilophosaurus; PVSJ 512; Langer 2003; MCP 3844-PV; MACN-PV 18060; ZPAL Ab III 361/19; UCMP 37302) (Figs 12G, I, 14F). A distinct ventral notch separates the lateral and medial distal condyles. The distal end of the metatarsal II of Chromogisaurus also exhibits a strong proximodistal asymmetry, with the medial condyle more distally projected than the lateral one. Metatarsal V. The left metatarsal V is preserved (Fig. 13; Table 2). This metatarsal is more gracile than that of Plateosaurus (MACN-PV 10052), Adeopapposaurus (Martinez 2009) and Lessemsaurus (Pol & Powell 2007), but its overall proportions resemble that of Silesaurus (Dzik 2003), Herrerasaurus (Novas 1993), Saturnalia (Langer 2003), Guaibasaurus (Bonaparte et al. 2007), Pantydraco (Yates 2003a) and neotheropods (e.g. Dilophosaurus, Coelophysis; Welles 1984; Colbert 1989). The proximal end of the bone is gently expanded dorsoventrally, but exhibits a strong medial expansion. A high and proximodorsally oriented flange is present in the proximal end, resulting in a gently concave and oval proximal surface. This flange also produces a concave surface on the dorsal side of the proximal end of the bone. Otherwise, the ventral surface of the proximal end is gently convex. The metatarsal shaft is plate-like, strongly dorsoventrally compressed. Both dorsal and ventral surfaces are gently convex. The lateral and medial margins of the metatarsal are parallel along the shaft. The distal end of the bone is slightly transversely expanded and lacks a distal articular facet. The latter feature suggests the complete absence of the pedal digit V, as also occurs in Saturnalia (Langer 2003). Pedal phalanges. The complete left pedal digit II, in natural articulation (Fig. 12J M; Table 2), and a partial isolated phalanx are preserved. The proximal ends of the non-ungual phalanges are much more transversely and dorsoventrally expanded than the distal end (Fig. 12J M). At mid-length, the phalanges present a conspicuous compression. The first phalanx of the digit II exhibits a well developed proximodorsal lip, which is directly projected proximally (Fig. 12H, I). The proximoventral lip is well expanded transversely. The dorsal surface of the phalanx is convex, whereas the ventral surface presents a longitudinal concavity. The distal end exhibits a distinct trochlea with well developed collateral fossae. The dorsal surface of the trochlea bears a well developed, but shallow, ligament pit.

16 A new early dinosaur and reassessment of dinosaur origin and phylogeny 385 Figure 12. Left metatarsal II and complete digit II of Chromogisaurus novasi in A, B, J, K, dorsal; C, D, H, I, lateral; E, F, medial; G, I, distal; J, K, proximal; and L, M, ventral views. Abbreviations: 1-II, first phalanx of digit II; 2-II, second phalanx of digit II; af, articular facet; afm, articular facet for metatarsal I; clf, collateral fossa; clg, collateral groove; df, dorsal fossa; dl, dorsal lip; lc, lateral condyle; mc, medial condyle. Scale bars = 1cm. The second phalanx of digit II is shorter than its preceding phalanx. The proximodorsal lip of 2-II contrasts with that of 1-II in its stouter and proximodorsally oriented body. The dorsal surface of the phalanx 2-II is concave, contrasting with the convexity seen in 1-II. The collateral fossae are larger than that of 1-II. The distal trochlea and the dorsal ligament pit are similarly developed to those in 1-II. The pedal ungual of the second digit is slightly ventrally curved, resembling other basal dinosaurs (e.g. Herrerasaurus, Guaibasaurus, Saturnalia, Dilophosaurus; PVSJ 373; MCP 2356-PV, 3845-PV; UCMP 37302), but contrasting with the almost straight pedal ungual of Eoraptor (PVSJ 512). The ungual of the second digit of Chromogisaurus is longer than its preceding phalanx, contrasting with the condition present in some basal dinosauriforms (e.g. Eoraptor, Lesothosaurus,

17 386 M. D. Ezcurra autapomorphic features of Chromogisaurus are discussed below: Figure 13. Left metatarsal V of Chromogisaurus novasi in A, dorsal; B, lateral; and C, proximal views. Abbreviations: mp, medial projection. Scale bar = 1cm. Herrerasaurus, Dilophosaurus, C. rhodesiensis, Pantydraco; PVSJ 512; Thulborn 1972; Novas 1993; Welles 1984; QG 1; Yates 2003a). Nevertheless, pedal unguals of the second digit which exceed the length of the phalanx 2-II are also observed in a wide variety of archosauriforms, such as Euparkeria (Ewer 1965), Lagerpeton (Sereno & Arcucci 1993; PULR 06), Heterodontosaurus (Santa Luca 1980), Guaibasaurus (MCP 2356-PV), Anchisaurus (Galton 1976) and C. bauri (Colbert 1989). The proximodorsal lip is thick and poorly developed. The ventral flexor tubercle is incipient. The ventral surface of the claw is wide and concave, contrasting with the almost planar condition of Eoraptor (PVSJ 512). The collateral grooves are well developed; they do not bifurcate proximally but strongly curve ventrally, distally to the flexor tubercle. The distal-most tip of the ungual is not preserved. Discussion Chromogisaurus novasi as a new species of early dinosaur As mentioned in the diagnosis, the available remains of Chromogisaurus novasi exhibit several features allowing its distinction from all other known dinosauromorphs. The 1. Femoral lateral surface with well developed fossa immediately below the trochanteric shelf (Figs 8A, B, 9A, E, 14D). In Chromogisaurus a deep, large and triangular fossa is present on the lateral surface of the femur, directly below the trochanteric shelf. However, contrasting with Chromogisaurus, in other basal dinosauriforms the femoral lateral surface presents gently convex surfaces (e.g. Silesaurus, Eoraptor, Herrerasaurus, Staurikosaurus, Chindesaurus, Saturnalia, Guaibasaurus, P. engelhardti, Riojasaurus, Lessemsaurus; Dilophosaurus, Liliensternus, Coelophysis sp.; Dzik 2003; PVSJ 512; Novas 1993; Colbert 1970; PEFO 10395; Langer 2003; UFRGS PV 0725T; MCP 2355-PV; Moser 2003; PVL 3808; PVL 4822; UCMP 37302, 77270; MB R. 2175; UCMP ). In particular, Saturnalia (MCP 3844-PV, 3846-PV) and other basal dinosaurs (e.g. Herrerasaurus; PVL 2566) exhibit a posterolateral fossa which lies between the fourth trochanter and the femoral posterolateral intermuscular line of Langer (2003). However, the latter feature is also present in Chromogisaurus, and it is clearly independent from the autapomorphic lateral fossa. Indeed, both traits are separated by the posterolateral intermuscular line in Chromogisaurus (Fig. 14C, D). 2. Metatarsal II with strongly dorsoventrally asymmetric distal condyles (Figs 12, 14). In basal dinosaurs (e.g. Saturnalia, Herrerasaurus, Dilophosaurus; MCP 3844-PV; MACN-PV 18060; UCMP 37302), the distal condyles of the metatarsal II are separated from one another by a deep and wide ventral groove, and the condyles are asymmetric in both proximodistal and dorsoventral axes. On the other hand, in non-dinosaurian archosaurs (e.g. Effigia, Silesaurus, Eucoelophysis, Marasuchus; Nesbitt 2007; Dzik 2003; Sullivan & Lucas 1999; PVL 3036) the distal condyles of the metatarsal II are almost symmetric in dorsal view. The dinosaur condition is present in Chromogisaurus, but the medial condyle is further ventrally developed and transversely thinner than in other basal dinosaurs (e.g. Saturnalia, Herrerasaurus, P. gracilis, Glacialisaurus, Liliensternus, Dilophosaurus; MCP 3844-PV; MACN-PV 18060; MACN-PV 10082; Smith & Pol 2007; MB R. 2175; UCMP 37302). The above set of autapomorphies allows the diagnosis of Chromogisaurus novasi among basal Dinosauriformes. Furthermore, although the morphology of Chromogisaurus and Saturnalia are very similar, the Argentinean taxon can be further distinguished from Saturnalia by the absence of a deep median notch separating the postzygapophyses in the proximal and mid-caudal vertebrae, the presence

18 A new early dinosaur and reassessment of dinosaur origin and phylogeny 387 Figure 14. Comparison between selected features of B, D, F, Chromogisaurus novasi; and A, C, E, Saturnalia tupiniquim. A, B, right ilia in lateral views; C, D, left femora in lateral views; and E, F, metatarsals II in distal views. (B, D, F, PVSJ 845; A, C, E, MCP 3844-PV) (E, reversed right metatarsal). Abbreviations: at, anterior trochanter; aw, acetabular wall; fh, femoral head; ft, fourth trochanter; lc, lateral distal condyle; lf, lateral fossa; pli, posterolateral intermuscular line; plp, posterolateral prong; pr, posterior rugosity for muscle origin; sac, supraacetabular crest; ts, trochanteric shelf. Not to scale. of an ilium with a lesser laterally projected supraacetabular crest, an iliac acetabular wall with concave ventral margin, well dorsoventrally extended articulation for the first primordial sacral rib, and a femoral trochanteric shelf which does not reach the posterolateral corner of the shaft (Fig. 14C, D). In addition, Chromogisaurus differs from Panphagia, both taxa coming from the same locality of the Ischigualasto Fm., in the presence of mid-caudal vertebrae with postzygapophyses not separated by a median notch, an ilium with an almost straight supraacetabular crest, postacetabular process with a pointed posteroventral corner and a rounded posterodorsal margin, a strong and anterodorsally tapering trapezoidal rugosity for the origin of the Mm. flexor tibialis and iliotibialis, and the absence of a strongly medially developed posteromedial lamina on the postacetabular process, and tibia with a median notch separating the medial and lateral proximal condyles. In this regard, all this evidence supports the assignment of PVSJ 845 to a new species of early dinosaur. Phylogenetic analysis Methods. A cladistic analysis was performed in order to assess the phylogenetic relationships of Chromogisaurus novasi (PVSJ 845). The data matrix was based on a modified version of that first published by Yates (2007a, b), and modified by Smith & Pol (2007). Three operational taxonomic units (Chromogisaurus novasi, Panphagia protos and MACN-PV 18649a, the latter specimen belongs to a dinosaur specimen from the Ischigualasto Fm. which will be formally described elsewhere; Ezcurra & Novas 2007a, 2008, in prep.) and 15 characters were added. The scorings for Chindesaurus, Crurotarsi, Eoraptor, Guaibasaurus, Herrerasaurus, Neotheropoda, Orithischia and Saturnalia were reviewed based on first-hand analysis of specimens, and several modifications have been made (see Appendix 2 for a full description of them). Only the holotype specimen of Agnosphitys (ilium) was considered in the analysis, because the assignment of the referred specimens to that taxon is not conclusive and it may represent a quimera (Langer 2004; Bonaparte et al. 2007). The resulting data matrix is composed of 378 characters and 50 taxa. The non-archosaurian archosauriform Euparkeria was used to root the recovered most parsimonious trees (MPTs). The data matrix was analysed under equally-weighted parsimony using TNT 1.1 (Goloboff et al. 2008). A heuristic search of 50 replications of Wagner trees (with random addition sequence) followed by TBR branch swapping

19 388 M. D. Ezcurra Figure 15. Phylogenetic relationships of Chromogisaurus novasi and other basal dinosaurs. A, strict consensus tree depicting the phylogenetic position of Chromogisaurus novasi and MACN-PV 18649a; and B, reduced strict consensus (after the exclusion of Agnosphitys) showing bootstrap (right, greater than 50%) and decay indexes (left). Abbreviations: Dino, Dinosauria; Guaiba, Guaibasauridae; Herr, Herrerasauridae; Plat, Plateosaurus; Satur, Saturnaliinae; Saur, Saurischia; Saurop, Sauropodomorpha; Ther, Theropoda. algorithm (holding 10 trees per replicate) was performed. The best trees obtained at the end of the replicates were subjected to a final round of TBR branch swapping. Zero length branches among any of the recovered MPTs were collapsed (rule 1 of Coddington & Scharff 1994). Multistate characters were treated as unordered. Results. The tree search resulted in 100 MPTs of 1186 steps, with CI = and RI = 0.697, and the best score hit 50 times out of the 50 replications. The obtained MPTs consistently place Chromogisaurus within Sauropodomorpha (Fig. 15), due to the presence of proximal caudal vertebrae with the base of the neural spine longer than half the length of the neural arch, iliac blade strongly curved laterally, length of the iliac pubic peduncle greater than twice the anteroposterior depth of its distal end, and proximal lateral condyle of the tibia more anteriorly placed than the medial one (Figs 16, 17). Within Sauropodomorpha, Chromogisaurus was found as a member of an early branch of basal forms, which includes Chromogisaurus, Saturnalia, Panphagia, Guaibasaurus and Agnosphitys. This group is referred here as the family Guaibasauridae, representing the sister-group of all remaining sauropodomorphs. The family Guaibasauridae was originally coined by Bonaparte et al. (1999) as a monospecific entity in order to include Guaibasaurus candelariensis. Although this family was diagnosed by a set of characters, no formal definition is available. Accordingly, Guaibasauridae is defined here as all archosaurs more closely related to Guaibasaurus candelariensis than to Carnotaurus sastrei or Saltasaurus loricatus (see Systematic Palaeontology). Within Guaibasauridae, a polytomy was obtained among Agnosphitys, Panphagia, Guaibasaurus and the Saturnalia + Chromogisaurus clade. Thus, Chromogisaurus was unequivocally depicted as more closely related to Saturnalia than to any other basal saurischian, and both are enclosed in the new subfamily Saturnaliinae. Due to its biostratigraphical importance (as a probable specifier of the Hyperodapedon-Acme Zone) and its well supported monophyly (see below) (Fig. 15B), the clade

20 A new early dinosaur and reassessment of dinosaur origin and phylogeny 389 Figure 16. Ilia of several saurischians in dorsal view. A, Herrerasaurus ischigualastensis (PVL 2566); B, Liliensternus liliensterni (MB R. 2175); C, Chromogisaurus novasi (PVSJ 845); D, Saturnalia tupiniquim (MCP 3844-PV); E, Guaibasaurus candelariensis (UFRGS PV 0725T); and F, Riojasaurus incertus (PVL 3808). Abbreviations: 372, curvature of iliac blade in dorsal view; fsr, first sacral rib; sac, supraacetabular crest; pms, posteromedial shelf; plp, posterolateral prong; pr, posterior rugosities. Not to scale. Saturnaliinae has been coined here. This clade includes Saturnalia tupiniquim, Chromogisaurus novasi, and all the descendants from their most common ancestor (see Systematic Palaeontology). The monophyly of Guaibasauridae is supported by the following unambiguous synapomorphies common to all the recovered MPTs: (1) ilium with an incipiently open acetabular wall; (2) iliac postacetabular process longer than the distance between the pubic and ischial peduncles (Fig. 17); (3) femur with the proximal tip of the anterior trochanter at level with the femoral head; (4) distal end of the tibia with a concave posterolateral corner; and probably (5) scapula with posterior margin of the acromion process which rises from the blade at an angle greater than 65 from the long axis of the bone at its steepest point (unknown in Guaibasaurus, Agnosphitys and Chromogisaurus); (6) presence of a caudosacral vertebrae (unknown in Agnosphitys and Panphagia); and (7) pubic shaft almost perpendicular to the longitudinal axis of the ilium (unknown in Agnosphitys, Chromogisaurus and Panphagia). Furthermore, Chromogisaurus and Saturnalia are nested within Saturnaliinae by an ulna with an extremely enlarged olecranon process with a strongly striated posterolateral surface, iliac postacetabular process with a pointed posteroventral corner and a rounded posterodorsal margin, and a strong and anterodorsally tapering trapezoidal rugosity for the origin of the Mm. flexor tibialis and iliotibialis (Langer 2003) (Fig. 14A, B), and probably proximal caudal transverse processes anteriorly bowed. Beyond the phylogenetic position of Chromogisaurus, the phylogenetic analysis performed here provides novel hypotheses regarding basal dinosaur phylogeny. Previous authors found Agnosphitys and Guaibasaurus to be basal theropods (Langer 2004; Langer & Benton 2006; Yates 2007a, b), but they are interpreted here as guaibasaurid sauropodomorphs. Yates (2007a, b) found Agnosphitys to be a basal theropod based on cranial apomorphies, but this taxon is restricted here to its holotype (i.e. an isolated ilium; Fraser et al. 2002). Accordingly, the present analysis found Agnosphitys as a guaibasaurid sauropodomorph due to the presence of an ilium with an incipiently open acetabular wall and strongly elongated postacetabular process. A constraint of the monophyly of the clades obtained in one of the MPTs was enforced but leaving Agnosphitys as a pivotal taxon. The search of suboptimal trees under this enforced constraint showed that three additional steps are required to obtain Agnosphitys as a non-dinosaurian dinosauriform, four for a non-eusaurischian position, and five to recover it as a basal theropod. Guaibasaurus candelariensis is currently known from three partial skeletons from the Norian of Brazil (Bonaparte et al. 1999, 2007), but cranial and cervical elements remain unknown. Guaibasaurus has been included in previous cladistic analyses, depicting it as a basal theropod (Langer 2004; Langer & Benton 2006; Yates 2007a, b) or a non-eusaurischian saurischian (Langer et al. 2007c). However, in the present analysis Guaibasaurus has been found as a member of Sauropodomorpha for the first time. Guaibasaurus shares with sauropodomorphs, but not with theropods, the presence of proximal caudal vertebrae with the base of the neural spine longer than half the length of the neural arch, ilium with strongly laterally curved blade

21 390 M. D. Ezcurra Figure 17. Iliac character-states among basal saurischians. A, Caseosaurus crosbyensis ( = Chindesaurus?) (UMMP 8870); B, Guaibasaurus candelariensis (UFRGS PV 0725T); C, Chromogisaurus novasi (PVSJ 845); and D, Saturnalia tupiniquim (MCP PV). Abbreviations: 251, medial wall of acetabulum; 252, length of the pubic peduncle; 255, length of the postacetabular process; 258, shape of the posterior margin of the postacetabular process; 362, muscle origin areas (Mm. flexor tibialis and iliotibialis) on the posterior portion of the postacetabular process. Not to scale. (Fig. 16), an elongated pubic peduncle (Fig. 17), and an ischial shaft with triangular transverse section. Furthermore, Guaibasaurus is included within Guaibasauridae due to the presence of the synapomorphies listed above. Bonaparte et al. (2007) suggested that Saturnalia, Guaibasaurus and Agnosphitys conform a natural group (Guaibasauridae), based on a very similar iliac morphology. The latter hypothesis is supported here for the first time in a quantitative analysis. It must be pointed out that Guaibasaurus exhibits some intriguing neotheropod-like features, such as the presence of a distal tibia with strongly externally projected posterolateral process (MCP 2355-PV). Nevertheless, the latter condition is also found in other basal saurischians, including Chindesaurus (Nesbitt et al. 2007) and Eoraptor (PVSJ 512). Thus, the distribution of this character is not currently clear within Saurischia. Suboptimal trees, under an enforced constraint topology, required two additional steps to recover Guaibasaurus as a basal theropod. Panphagia protos is an early dinosaur recently described by Martinez & Alcober (2009) from the lower third of the Ischigualasto Fm. (Valle Pintado locality, late Carnian). The phylogenetic analysis performed by Martinez & Alcober (2009), based on the data matrix of Langer & Benton (2006), recovered Panphagia as the most basal known sauropodomorph, being the sister-taxon of Saturnalia and more derived sauropodomorphs. As mentioned above, Panphagia was also found here to be a member of Sauropodomorpha; but within this clade, Panphagia was recovered as a non-saturnaliin guaibasaurid, and not as the most basal sauropodomorph. The inclusion of Panphagia in Sauropodomorpha is supported here by the following characters: (1) supraoccipital wider than high; (2) length of the cervical centra 3 5 is times the height of their anterior faces; (3) cervical neural arches 4 8 without postzygodiapophyseal lamina; (4) cervical neural arches 4 8 with weakly developed laminae; (5) ilium with strongly laterally curved blade; (6) length of the iliac pubic peduncle greater than twice the anteroposterior depth of its distal end; and (7) proximal lateral condyle of the tibia more anteriorly placed than the medial one. Furthermore, the following characters support the membership of Panphagia in Guaibasauridae: (1) scapula with posterior margin of the acromion process which rises from the blade at an angle greater than 65 from the long axis of the bone at its steepest point (unknown in Guaibasaurus, Agnosphitys and Chromogisaurus); (2) iliac acetabular wall incipiently open; (3) iliac postacetabular process longer than the distance between the pubic and ischial peduncles; and (4) tibial posterolateral corner concave (unknown in Agnosphitys). Panphagia is

22 A new early dinosaur and reassessment of dinosaur origin and phylogeny 391 more basal than Chromogisaurus and Saturnalia within Guaibasauridae due to the absence of the synapomorphies listed above for Saturnaliinae. The recovery of suboptimal trees under an enforced constraint topology showed that four additional steps were required to obtain Panphagia as the most basal sauropodomorph (cf. Martinez & Alcober 2009), and three additional steps to place it as a sauropodomorph more derived than guaibasaurids. Two additional steps were necessary to recover Panphagia as the sister-taxon of Chromogisaurus. Martinez & Alcober (2009) justified the position of Panphagia as less derived than Saturnalia and other sauropodomorphs based on three characters: (1) distally recurved crowns; (2) roughly semicircular distal outline of ischium; and (3) proximal lateral condyle of tibia posteriorly located. However, the following comments are warranted: 1. Distally recurved crowns are also found in the most anterior teeth of the dentary of Saturnalia (MACP 3845-PV) and Pantydraco (Yates 2003a, fig. 8B; evident in the second crown). Thus this trait is not only restricted to Panphagia within very basal sauropodomorphs. 2. Ischia with semicircular distal outline are also present in Staurikosaurus and Eoraptor (Langer & Benton 2006). The latter two taxa were found within successive sister-taxa of Sauropodomorpha in the present phylogenetic analysis, thus the distribution of this pelvic character is ambiguous within Eusaurischia. 3. A review of the type material of Panphagia shows that this taxon presents a lateral condyle more anteriorly positioned than the medial one (PVSJ 874), and not equally posteriorly projected as it was originally described by Martinez & Alcober (2009). Accordingly, Panphagia exhibits the apomorphic state of this character, as it occurs in Saturnalia, Chromogisaurus and more derived sauropodomorphs. The position of Herrerasauria as the most basal saurischians (Yates 2003a, 2007a, b; Langer 2004; Langer & Benton 2006; Ezcurra 2006; Irmis et al. 2007) is favoured here. MACN-PV 18649a (Ezcurra & Novas 2007a, 2008) was found to be a member of Herrerasauria. It is represented by a partial small forelimb collected from the Ischigualasto Fm, sharing with Herrerasaurus a manual phalanx 1-I longer than metacarpal I, strongly curved manual unguals, and metacarpals IV V ventral to the others. The new herrerasaurian specimen exhibits the following autapomorphies: (1) phalanx 1-II with conspicuous longitudinal ridge on its proximolateral border; and (2) manual unguals with a posteriorly bifurcated lateral groove. Furthermore, this specimen differs from Herrerasaurus in the presence of metacarpal I with strongly proximodistally asymmetric condyles, ulnar articular surface subequal to ulnar distal articular end, radial tapering medially, well developed proximodorsal lip of phalanx 1-I, and metacarpal V proportionally longer. These features show that this specimen belongs to a distinct species of early dinosaur from that previously described for the Ischigualasto Fm, which will be described elsewhere (Ezcurra & Novas in prep.). Chindesaurus, a fragmentary dinosauriform from the Norian of USA (Long & Murry 1995), was originally interpreted as a close relative of Herrerasaurus (Long & Murry 1995), an assignment subsequently followed by Novas (1997). Nevertheless, more recent numerical analyses alternatively recovered Chindesaurus as a basal theropod (Yates 2007a, b), a herrerasaurian (Irmis et al. 2007) or a herrerasaurid (Nesbitt et al. 2009). Otherwise, Nesbitt et al. (2007) claimed that Chindesaurus cannot be assigned beyond Saurischia indet. In the present analysis, several scorings for Chindesaurus have been modified from the original data matrix of Yates (2007b), some of them previously supporting its theropodan affinities (see Appendix 2). Character states which supported the theropod affinities of Chindesaurus, and were modified here, have been changed as follows: (1) femur without rounded fourth trochanter in profile (PEFO 10395; GR 226); (2) unknown presence of pubic tubercle on the lateral surface of the proximal end of the bone; and (3) medial peg of calcaneum fitting into astragalus (Nesbitt et al. 2007). In the phylogenetic analysis performed here, Chindesaurus is found to be either a basal theropod or a noneusaurischian saurischian, thus the strict consensus tree depicts the taxon within a trichotomy composed of Chindesaurus, Sauropodomorpha and Theropoda (Fig. 15). Chindesaurus shares with other saurischians the following apomorphies: (1) presence of hyposphene in dorsal vertebrae; (2) proximal tip of femoral anterior trochanter distal to the femoral head; (3) a sharp medial margin around the depression posterior to the ascending process of the astragalus. Within Saurischia, Chindesaurus would be more derived than herrerasaurids by the presence of an ilium with square-ended posterior margin of the postacetabular process, but this assignment is weak (Fig. 15B). Some of the recovered MPTs found Chindesaurus outside Eusaurischia, as its sister-taxon, due to the absence of a brevis fossa on the postacetabular process of the ilium. Furthermore, Chindesaurus is excluded from Theropoda in some of the MPTs due to the absence of a dorsosacral vertebra, an anterior end of brevis shelf not connected to the supraacetabular crest, and supraacetabular crest not flaring lateroventrally. Accordingly, in the present analysis neither the herrerasaurian or theropodan affinities of Chindesaurus are consistently favoured. In this regard, the recovery of suboptimal trees under an enforced constraint topology showed that only one additional step is required to obtain Chindesaurus as a herrerasaurian or a basal sauropodomorph.

23 392 M. D. Ezcurra Eoraptor lunensis was originally interpreted as the most basal known theropod (Sereno et al. 1993), but recent phylogenetic analyses have found this early dinosaur to be a non-eusaurischian saurischian (e.g. Langer 2004; Langer & Benton 2006; Yates 2007a, b). Numerous character-states for Eoraptor have been modified here from the original data set of Yates (2007b), and several of them previously supported the non-eusaurischian affinities of the taxon. Among these modified characters are the absence of a broad suture between the premaxilla and the nasal, the length of the radius less than the 80% of the humerus, the presence of a distal expansion in the ischium, and the presence of a distal tibial articular surface with the lateral side narrower than the medial side. After these modifications and the new characters added here, the present phylogenetic analysis consistently placed Eoraptor as the sister-taxon of Neotheropoda, supporting the hypothesis of Ezcurra (2006). Several apomorphies are shared between Eoraptor (PVSJ 512) and basal neotheropods (e.g. Dilophosaurus, Zupaysaurus, Liliensternus, Coelophysis; UCMP 37302, 37303, 77270, ; PULR 076; MB R. 2175; QG 1) (Fig. 18), such as the presence of a subnarial gap, maxilla with a dorsally upturned anterior end of the alveolar margin, an alveolar ridge, a subquadrangular anterior border of the maxillary antorbital fossa, lacrimal ventral ramus with lateral lamina interrupting the antorbital fossa only near its proximal end and ventrally restricted to its posterior margin, a dorsal added to the sacral series, and iliac brevis shelf connected with the supraacetabular crest. Indeed, several cranial characters previously claimed as coelophysoid synapomorphies (Rowe & Gauthier 1990; Rauhut 2003; Carrano et al. 2005; Tykoski 2005; Ezcurra and Cuny 2007; Ezcurra and Novas 2007b) are present in Eoraptor, and therefore re-interpreted as putative synapomorphies of Theropoda. Suboptimal trees, under enforced topology, show that four additional steps are required to obtain Eoraptor as a noneusaurischian saurischian or a basal sauropodomorph. Bootstrapping and Bremer support. In order to test the robustness of the obtained MPTs, both bootstrap and Bremer support were performed (Fig. 15B). This analysis was carried out after the exclusion of Agnosphitys from the data matrix. Due to the fragmentary nature of Agnosphitys, several potential guaibasaurid apomorphies will be treated as ambiguous characters, resulting in a lower support for the clade. The bootstrap analysis was carried out with replications. Bootstrap support is weak (lower than 50%) throughout much of the tree, including that of Guaibasauridae, but some high values are also present. The clade Dinosauriformes is supported by a bootstrap value of 99%, and the Silesaurus + Dinosauria clade is supported by 94%. Dinosauria presents a bootstrap frequency of 66%, and Saurischia a value of 51%. Within Dinosauria, the highest recorded bootstrap value is that of Saturnaliinae (85%). The monophyly of sauropodomorphs more derived than guaibasaurids and that of sauropodomorphs more derived than Thecodontosaurus are supported by bootstrap values of 68% and 75%, respectively. Bremer support values are shown in Fig. 15B. Decay indexes are very low (i.e. 1) in only a couple of nodes, namely Herrerasauridae and Chindesaurus + Eusaurischia. The decay index of the latter clade is low due to the pivotal position of Chindesaurus. Within basal Dinosauriformes, several nodes are well supported, with a decay index of 4. Among these clades are Dinosauria, Saurischia, Sauropodomorpha and sauropodomorphs more derived than gauibasaurids. Guaibasauridae and Saturnaliinae present an intermediate decay index (2). Accordingly, the monophyly of Dinosauria and Saurischia are both quite well supported by bootstrap and Bremer support in the present numerical analysis. Implications in the Late Triassic biostratigraphy The close affinities between Chromogisaurus and Saturnalia are biostratigraphically relevant. The common presence of saturnalliins in the lower levels of the Ischigualasto Fm. and the Brazilian Santa Maria Fm. allow reinforcing previous biostratigraphical correlations between the Hyperodapedon-Acme Zones (late Carnian) of these sedimentary units (Langer 2005b; Langer et al. 2007a). Implications in the rise of dinosaurs The new dinosaur remains reported here help to modify our understanding of the earliest dinosaur faunas. Chromogisaurus, the new herrerasaurian specimen (MACN- PV 18649a) and Panphagia protos (Martinez & Alcober 2009) drastically increase the dinosaur alpha-diversity of the lower levels of the Ischigualasto Fm., as well as the Carnian global record of the group. Accordingly, they demonstrate that the diversity of early dinosaurs was not as restricted as previously thought. In fact, the number of dinosaur species recognized in the lower levels of the Ischigualasto Fm. approaches that of the most diverse tetrapod groups documented in this assemblage, namely crurotarsans and cynodonts (Table 3). Otherwise, although predatory dinosaurs were abundant, dinosaurs as a whole represent around 6% of the total tetrapod sample of the Ischigualasto Fm. (Rogers et al. 1993). This stratigraphical unit shows that although dinosaurs were a numerically minor component of the Carnian terrestrial ecosystems (Benton 1988; Rogers et al. 1993), they were already quite diverse in this early stage of their evolution. Previous studies have suggested that Carnian dinosaur diversity was very low (Bonaparte 1982; Charig 1984; Benton 1988, 2004, 2006; Rogers et al. 1993) in comparison with that of Norian dinosaur-bearing assemblages, and a steady diversity increase has been described from the Carnian to the Early Jurassic (Brusatte et al. 2008a).

24 A new early dinosaur and reassessment of dinosaur origin and phylogeny 393 Figure 18. Cranial character-states among basal saurischians. A, Herrerasaurus (PVSJ 407); B, Eoraptor (PVSJ 512); and C, Zupaysaurus (PULR 076). Abbreviations: 38, shape of the lacrimal; 46, shape of the orbit; 364, subnarial gap; 365, alveolar margin of anterior-most maxilla; 366, anterior margin of maxillary antorbital fossa; 367, dorsoventrally compressed ridge on lateral surface of maxilla (alveolar ridge); 368, exposition of the lacrimal antorbital fossa in lateral view. Not to scale.

25 394 M. D. Ezcurra Table 3. Diversity of Late Triassic tetrapod species. Tetrapod sample of the lower Ischigualasto Fm. compared to those of Norian assemblages (lower Caturrita Fm., Petrified Forest Member of the Chinle Fm., upper Los Colorados Fm., and Lower Elliot Fm.). Abbreviations: Dino, Dinosauria (non-dinosaurian dinosauromorphs between brackets); Prot, Proterochampsidae; Ch, Chelonia; Cru, Crurotarsi; Sph, Sphenodontia; Proc, Procolophonidae; Dic, Dicynodontia; Cyn, Cynodontia. Dino Tem Prot Ch Cru Sph Rhy Proc Dic Cyn Ischigualasto Fm. 5 (0) Caturrita Fm. 2 (1) Chinle Fm. 3 (2) Los Colorados Fm. 4 (0) Lower Elliot Fm. 6 (0) However, the worldwide dinosaur Carnian record has been traditionally compared with the whole Norian record. This results in underestimation of Carnian relative to Norian dinosaur diversity, because Carnian dinosaurbearing assemblages are mostly restricted to three areas, corresponding to beds of the Ischigualasto (Argentina), Santa Maria (Brazil), and Lower Maleri (India) formations (Langer 2004, 2005a). In sharp contrast, rich Norian dinosaur-bearing assemblages are numerous and widely distributed globally, including the Los Colorados (Argentina; Bonaparte 1972), Laguna Colorada (Argentina; Casamiquela 1980), Caturrita (Brazil; Langer 2005b), Lower Elliot (South Africa, Kitching & Raath 1984; Yates & Kitching 2003; Butler et al. 2007) and Lower Dharmaram formations (India; Novas et al. 2006; Kutty et al. 2007), and several Norian depocenters of western Europe (Galton & Upchurch 2004; Rauhut & Hungerbühler 2000) and USA (Long & Murry 1995; Lucas et al. 1998; Irmis 2005; Irmis et al. 2007; Nesbitt & Chatterjee 2008). Nevertheless, if the early dinosaur alpha diversity of the lower Ischigualasto Fm. is compared with each of these Norian assemblages, Carnian dinosaur diversity closely resembles that of Norian times (Table 3). On the other hand, contrasting with Carnian beds, dinosaurs were more numerically abundant in Norian assemblages (Bonaparte 1982). Indeed, dinosaurs represent between 25% and 60% of the total number of terrestrial tetrapods in Norian assemblages (Benton 1983, 1994). Accordingly, although dinosaurs increased numerically during the Norian, they did not experience a major alpha diversification after the Carnian Norian boundary (Fig. 19). Therefore, increase in dinosaur diversity and their numerical dominance over other terrestrial tetrapods were diachronous processes. Thus, Norian dinosaur dominance (e.g. Casamiquela 1980; Benton 1983, 1994; Kitching & Raath 1984; Sander 1992) was preceded by a period of low abundance but high diversity during the late Carnian. In this context, the first recorded major step in the early radiation of dinosaurs (i.e. diversification) occurred during the Carnian. The Carnian increase of dinosaur diversity did not occur in an empty ecospace. Indeed, Carnian tetrapod terrestrial communities were dominated by herbivorous rhynchosaurs and cynodonts, whereas carnivorous crurotarsans and cynodonts were also common and diverse faunal components (Azevedo et al. 1990; Rogers et al. 1993). A high Carnian diversity but relatively poor abundance of dinosaurs may reflect competitive pressure by nondinosaurian tetrapods (Novas 1997). Alternatively, Brusatte et al. (2008a, b) demonstrated that Carnian and Norian Figure 19. Diversity of major terrestrial tetrapods during the Carnian Norian time span in South America. Samples correspond to the Ischigualasto (lower-third), Caturrita (lower levels) and Los Colorados (La Esquina Fauna) formations. Abbreviations: Ch, Chelonia; Di, Dicynodontia; Pr, Proterochampsidae; Rhy, Rhynchosauria; Tem, Temnospondyli. For detailed explanations see supplementary material 4.