A microraptorine (Dinosauria Dromaeosauridae) from the Late Cretaceous of North America

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1 A microraptorine (Dinosauria Dromaeosauridae) from the Late Cretaceous of North America Nicholas R. Longrich a,1 and Philip J. Currie b a Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4; and b Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E9 Edited by James P. Kennett, University of California, Santa Barbara, CA, and approved January 30, 2009 (received for review November 17, 2008) The fossil assemblages of the Late Cretaceous of North America are dominated by large-bodied dinosaur species. Associated skeletons of small dinosaurs are exceedingly rare, and small (<10 kg) carnivorous theropods have not previously been reported from these beds. Here, we describe a small dromaeosaurid from the 75-millionyear-old Dinosaur Park Formation of Alberta, Canada. Hesperonychus elizabethae gen. et sp. nov. is represented by a pelvic girdle from an animal weighing 1,900 g. Despite its size, the pubes and ilia are coossified, indicating that the animal was somatically mature. This is the smallest carnivorous, nonavian dinosaur known from North America. Phylogenetic analysis of Hesperonychus reveals that it is not closely related to previously described North American dromaeosaurids. Instead, Hesperonychus is a member of the dromaeosaurid clade Microraptorinae, a group containing the 4-winged Microraptor and the feathered Sinornithosaurus, both from the Lower Cretaceous Jehol Group of China. Hesperonychus is the youngest known member of this lineage, extending the temporal range of the clade by 45 million years, and it is the first microraptorine known from North America, providing further evidence for an affinity between the dinosaur faunas of North America and Asia. Study of fossil collections from the Dinosaur Park and Oldman formations of Alberta has revealed numerous isolated bones of small, basal dromaeosaurids, which are tentatively referred to Hesperonychus. These fossils suggest that small dromaeosaurids were a significant component of the carnivore community in this Late Cretaceous biota. Campanian Dinosaur Park Formation microraptorinae theropoda The vast majority of known nonavian dinosaurs are mediumand large-bodied forms, ranging in size from tens to thousands of kilograms (1). This pattern is especially evident in the Late Cretaceous of North America (2 8). Here, large-bodied ( 1,000 kg) dinosaurs dominate fossil assemblages in terms of number of skeletons (2, 3, 7) and number of species (4 6, 8). Small-bodied ( 10 kg) carnivorous dinosaurs have not previously been described from these assemblages. Until now, the smallest carnivorous dinosaur known from the Late Cretaceous of North America was the dromaeosaurid Saurornitholestes langstoni, which weighed 10 kg [see Dinosaur Mass Data and Equations in supporting information (SI) Appendix]. Other animals are unlikely to have filled the small-carnivore niche. The alvarezsaurid theropod Albertonykus (9, 10) appears to have been a specialized insectivore (10) rather than a carnivore in the strict sense, and the stagodont marsupials were highly specialized durophages (11, 12) that may have fed on aquatic invertebrates rather than terrestrial prey. The apparent absence of small, endothermic (or, in the case of dinosaurs, presumably endothermic) carnivores in these Late Cretaceous ecosystems is remarkable: In modern, mammaldominated terrestrial communities, small-bodied animals outnumber large-bodied animals, both in terms of the number of individuals in a given area (13, 14) and number of species (15, 16). That dinosaurs might have left the small-carnivore niche vacant in North America is still more perplexing when one considers that many small carnivorous dinosaurs are known from Europe, Asia, and Gondwana (17 28). This raises the question of whether small-bodied, endothermic carnivores were truly rare in these assemblages, or whether our picture of these ecosystems is incomplete. Recently, study of museum collections resulted in the identification of a specimen of a previously unknown dinosaur from the Upper Cretaceous (upper Campanian) Dinosaur Park Formation (29) of Alberta, Canada. The specimen was collected in 1982 but lay unstudied for 25 years. Preparation of the fossil revealed that it represents a new genus of the clade Dromaeosauridae, a group of birdlike, carnivorous theropods (30). This animal, Hesperonychus elizabethae, gen. et sp. nov., is almost an order of magnitude smaller than any other carnivorous dinosaur known from the Dinosaur Park Formation (see Dinosaur Mass Data and Equations in SI Appendix) or any North American Cretaceous assemblage. This find demonstrates that in the Late Cretaceous of North America, dromaeosaurids exploited the small-carnivore niche. Surprisingly, Hesperonychus does not appear to be related to previously known North American dromaeosaurids, such as Dromaeosaurus albertensis (31) and Saurornitholestes langstoni (32). Instead, it shares features with the Microraptorinae, a clade of small, basal dromaeosaurids that includes the feathered Sinornithosaurus and the bizarre 4-winged Microraptor, both from the Lower Cretaceous Jehol Group of China (20 22, 24). Systematic Paleontology Systematics are as follows: Theropoda Marsh, 1881; Coelurosauria von Huene, 1914; Maniraptora Gauthier, 1986; Dromaeosauridae Matthew and Brown, 1922; Microraptorinae Senter et al., 2004; Hesperonychus elizabethae gen. et sp. nov. Holotype UALVP (University of Alberta Laboratory for Vertebrate Palaeontology, Edmonton) 48778, a partial pelvic girdle comprising the pubes and ilia (Fig. 1). Referred Material A number of isolated pedal phalanges are tentatively referred to Hesperonychus (see Specimen Data in SI Appendix and Fig. 2). Phalanx II-1 is represented by TMP (Royal Tyrrell Museum of Palaeontology, Drumheller, AB, Canada) , TMP , and UALVP Phalanx II-2 is represented by TMP and TMP Phalanx II-3 is represented by TMP , TMP , TMP , TMP , TMP , and UALVP Author contributions: N.R.L. designed research; N.R.L. and P.J.C. performed research; N.R.L. analyzed data; and N.R.L. and P.J.C. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. 1 To whom correspondence should be addressed. longrich@ucalgary.ca. This article contains supporting information online at /DCSupplemental PNAS March 31, 2009 vol. 106 no cgi doi pnas

2 Fig. 1. Holotype pelvic girdle (UALVP 48778) of Hesperonychus elizabethae from the late Campanian Dinosaur Park Formation of Alberta, Canada. (A) Ventral view. (B) Anterior view. (C) Right lateral view. (D) Posterior view. (E) Dorsal view. ace, acetabulum; apr, pubic apron; fen, fenestra; il, ilium; ldp, lateral depression; lpr, lateral process; msb, medial shelf of brevis fossa; pat, pathology; pbt, pubic boot; pub, pubis. Etymology The name Hesperonychus derives from hesperus (Latin, west) and onychos (Greek, claw). The specific epithet elizabethae honors the late Dr. Elizabeth Nicholls, who discovered the holotype. Horizon and Locality The holotype was collected from exposures of the Dinosaur Park Formation located on the south side of the Red Deer River, 20 km east of Dinosaur Provincial Park, Alberta. The Dinosaur Park Formation was deposited during the middle of the late Campanian, between 76.5 and 74.8 Ma (29). Referred material was collected from Dinosaur Provincial Park and surrounding badlands and other locations in southern Alberta, including Devil s Coulee, Manyberries, Onefour, Irvine, and Sandy Point (Specimen Data in SI Appendix); all are within 200 km of the type locality. All specimens for which precise stratigraphic data are available come from a narrow chronostratigraphic interval that encompasses the Dinosaur Park Formation and coeval beds of the uppermost Oldman Formation (Specimen Data in SI Appendix). This interval represents no more than 1.7 million years of time (29). Diagnosis Small dromaeosaurid characterized by the following autapomorphies: pubic peduncle of ilium with medial surface deeply excavated; posterior wing of ilium with medial shelf split to form anterior and posterior processes; lateral tubercles of pubis wing-like and curving anteriorly; pubis with fossa on lateral surface ventral to acetabulum; pubic apron shifted onto posterior surface of pubis; pubic symphysis teardrop-shaped in lateral view; ischiadic process of pubis reduced to a narrow lamina. Description and Comparisons Hesperonychus is a remarkably small theropod. By using a regression of pubis length against body mass, the holotype is estimated to have weighed 1,900 g, approximately half the weight of a domestic cat. Despite the small size of the animal, the pubes and ilia are completely fused to each other, indicating that the animal was somatically mature. The pubic peduncle of the ilium is long anteroposteriorly and has a deep fossa medially, which extends onto the medial surface of the pubis. The lateral surface of the pubic peduncle is convex, with no trace of a cuppedicus fossa. The ilia are strongly inclined medially and may have contacted each other dorsally along the midline. Unlike the situation in Velociraptor, in which the ilia diverge posteriorly and the dorsal margin is laterally everted (30, 33, 34), the posterior alae of the ilia would have been approximately parallel, and the postacetabular blade projects vertically. As in basal paravians (23, 35 37), the posterior wing of the ilium is tapered and curved ventrally in lateral view. Along the dorsal edge of the posterior ala, there is a distinct tubercle, a common maniraptoran feature (36). The brevis shelf projects ventrolaterally away from the posterior blade of the ilium. Medially, the medial shelf of the brevis fossa is split into separate anterior and posterior processes, a condition unique to Hesperonychus. The acetabulum is similar to those of other dromaeosaurids in that it lacks a prominent supracetabular crest (30, 36). However, anteriorly, the contribution of the ilium to the acetabulum is broad, and the anterior rim projects strongly laterally, as it does in Unenlagia (36). The medial opening of the acetabulum is partially closed, as it is in other Dromaeosauridae (36). The acetabulum opens dorsolaterally rather than laterally, as is the case in Velociraptor (38), suggesting the ability to partially abduct the hindlimbs. This morphology is of interest in light of proposals that Microraptor gui abducted its feathered hindlimbs to function as airfoils (24). As in many other paravians (20, 23, 24, 30, 33 35, 37), the pubic shaft projects posteroventrally. The proximal end of the pubis has a distinct scar on its anterior surface, in the same position as the large tubercle found in Velociraptor (33). The proximal end also bears a deep depression on its lateral surface, just ventral and anterior to the acetabulum. The ischiadic peduncle of the pubis is mediolaterally compressed and its contact with the ischium is reduced to a thin blade of bone. The distal shaft of the pubis sharply curves posteriorly, as it does in the Jehol dromaeosaurids Microraptor (24) and Sinornithosaurus (20); Unenlagia shows a similar curvature but it is more weakly developed (35). Proximally, the shaft of the pubis is mediolaterally compressed. Unlike the condition in Velociraptor, where the distal shaft of the pubis is anteroposteriorly flattened (33, 34), the distal shaft is subcircular in section. The right pubic shaft has an unusual swelling not seen on the left, apparently representing a well-healed fracture. On the lateral surface of each pubic shaft, there is a distinct process, as in other microraptorines (20 22, 24). However, in the new taxon, these processes are larger, winglike, and curve anteriorly. The laminae that form the ECOLOGY GEOLOGY Longrich and Currie PNAS March 31, 2009 vol. 106 no

3 Fig. 2. Pedal phalanges, cf. Hesperonychus.(A) Pedal phalanx II-3 (TMP ), from Dinosaur Provincial Park, Canada (reversed from left) in medial (A1) and lateral (A2) views. (B) Right pedal phalanx II-2 (TMP ) in medial (B1) and lateral (B2) views. (C) Right pedal phalanx II-1 ( ) in medial (C1) and lateral (C2) views. (D and E) cf. Hesperonychus specimens (D) compared with Saurornitholestes langstoni (E), a typical member of the Eudromaeosauria. Derived features characterizing the Eudromaeosauria include (1) paired grooves on the medial surface of the ungual that are shifted ventrally relative to the lateral groove; (2) phalanx II-2 with an elongate heel of the proximal articular surface, and (3) phalanx II-1 with strong dorsal projecton of the distal articular surface. Absence of these eudromaeosaur features indicates that these fossils are from a basal dromaeosaur, rather than juveniles of Saurornitholestes or Dromaeosaurus. pubic apron are greatly reduced, and their midline contact is all but lost, resulting in a deep, broad pelvic canal. Unusually, these laminae are located on the posterior surface of the shaft rather than on the medial surface, as is the case in other deinonychosaurs (21 23, 30, 33 35). At their ends, the pubes fuse to form a symphysis. No distinct anterior or posterior processes are present. Instead, the symphysis is spatulate in lateral view, as in other microraptorines (20, 24). In addition to the holotype, numerous small pedal phalanges have been recovered from the Dinosaur Park and Oldman formations (Fig. 2 A C), which are tentatively referred to Hesperonychus. Phalanx II-1, the proximal phalanx, is long and slender, as in basal deinonychosaurs (20 24, 39) (Fig. 2C). Its distal articular surface is broad and spool-shaped in distal view (Fig. 2D), as in other dromaeosaurids (30). However, in medial view, the distal articular surface is weakly expanded and subcircular (Fig. 2D). In contrast, the distal articular surface is dorsally extended in Eudromaeosauria (Fig. 2E). The proximal articular surface has a small medial cotyle. Phalanx II-2, the penultimate phalanx, resembles those of basal deinonychosaurs (20 22, 39) in being relatively long and gracile, with a short proximal articular heel and weak expansion of the proximal and distal articular surfaces (Fig. 2B). In ventral view, the heel is triangular, as in Sinovenator and saurornitholestines. Ventrally, the flexor tendon groove is only weakly developed, in contrast to the condition in saurornitholestines, in which this groove is prominent. Phalanx II-3, the ungual phalanx (Fig. 2A), resembles the sickle claw of basal dromaeosaurids such as Rahonavis (39) and microraptorines (20, 22). In cross-section, the claw is broad for its depth; the medial surface is almost flat, and the lateral surface is highly convex (Fig. 2D), resulting in a semilunate cross-section. A similar cross-section occurs in Rahonavis UA (University of Antananarivo, Antananarivo, Madagascar) 8656 and the troodontid Sinovenator IVPP (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, Peoples Republic of China) V In contrast, derived dromaeosaurids (Eudromaeosauria), including Saurornitholestes (Fig. 2E), Velociraptor (33, 34), Deinonychus (40), and Utahraptor (41), have a more blade-like claw. Phalanx II-3 is also distinguished by the more symmetrical arrangement of the vascular grooves. In Eudromeosauria, the lateral groove is shifted toward the dorsal edge of the claw, and the medial groove is shifted ventrally (40, 41) This asymmetry is only weakly developed in the ungual phalanges of the new taxon (Fig. 2), although it is developed to a greater degree than in troodontids. Eudromaeosauridae also exhibit an accessory vascular groove on the medial surface of pedal ungual II, which is absent from the fossils described here (Fig. 2). The linear dimensions of the referred specimens range from 71% to 105% of the dimensions of the corresponding elements in the microraptorine Sinornithosaurus millennii (IVPP V12811), whereas the pubis of the holotype is 80% of the length of the cgi doi pnas Longrich and Currie

4 Fig. 3. Strict consensus of 5,292 most parsimonious trees resulting from phylogenetic analysis of 23 in-group taxa, 4 out-group taxa, and 114 characters. H. elizabethae was found to be part of the clade Microraptorinae. Tree length 223, consistency index , retention index , rescaled consistency index pubis in IVPP V12811 (see Specimen Data in SI Appendix). Thus, the referred elements and the holotype of Hesperonychus belong to individuals of comparable size. The pedal phalanges are virtually identical to the pedal phalanges of digit II in Sinornithosaurus and Microraptor. However, they lack features found in the Eudromaeosauria, indicating that they do not represent juveniles of Saurornitholestes langstoni or Dromaeosaurus albertensis, the eudromaeosaurs known from the Dinosaur Park assemblage (31, 32). As described above, the second pedal digit of Eudromaeosauria is characterized by numerous derived characters (blade-like ungual with asymmetrically arranged vascular grooves; elongate proximal articular heel of phalanx II-2; dorsally extended medial condyle of phalanx II-1) that are conspicuously absent in these specimens (Fig. 2 D and E). The presence of these synapomorphies in a juvenile eudromaeosaur, the holotype of Bambiraptor feinbergorum (42), demonstrates that the absence of such features cannot be explained as the result of ontogenetic changes in morphology. For these reasons, we interpret the referred specimens as coming from a small-bodied basal dromaeosaur. In the absence of an associated skeleton, referral to Hesperonychus must be considered tentative, but this identification is consistent with the available evidence. Systematics Phylogenetic analysis places Hesperonychus in the dromaeosaurid clade Microraptorinae (Fig. 3). Referral of Hesperonychus to the Microraptorinae is supported by the presence of a lateral process of the pubis, strong posterior curvature of the pubic shaft, and a spatulate pubic foot. This study agrees with previous studies in recovering the clades Microraptorinae, Velociraptorinae, Dromaeosaurinae, and Unenlagiinae (26 28, 43, 44), although the membership of some of these clades differs in our analysis. Shanag ashile, for instance, was found to belong to the Microraptorinae, and Adasaurus mongoliensis was found to be a member of the Velociraptorinae (Fig. 3). Another result of this study is the recovery of a clade containing 3 taxa from the Campanian of North America: Saurornitholestes langstoni, Atrociraptor marshalli, and Bambiraptor feinbergorum. This clade is here named Saurornitholestinae. Bambiraptor has been referred to the Microraptorinae in some studies (43), but our analysis shows that it is not a member of this clade. Although Bambiraptor resembles microraptorines in having a curved pubis, this resemblance is superficial: It is the pubic symphysis that is curved, not the pubic shaft (as is the case in Microraptorinae). Rather, the animal shares derived features with Saurornitholestes (prominent depression caudal to the accessory antorbital fenestra, ridge on the medial surface of the ischium). Saurornitholestinae was found to be the sister taxon of a clade consisting of Velociraptorinae, Dromaeosaurinae, and Deinonychus (Fig. 3). These taxa and the Saurornitholestinae form a monophyletic group (26 28, 30, 43, 44) of derived dromaeosaurids, here termed Eudromaeosauria. Discussion and Conclusions The recognition of Hesperonychus results in a remarkable extension of the temporal range of the Microraptorinae. Previously, the geologically youngest known microraptorines came from the Jiufotang Formation of northeastern China s Jehol Group (24) ( 120 Ma) (45). The discovery of Hesperonychus in the Dinosaur Park Formation ( 75 Ma) (29) therefore extends the range of the clade by 45 million years, more than half the length of the Cretaceous. It remains unknown whether Hesperonychus represents the persistence of a 4-winged morphology, as seen in Microraptor gui (24), or a flightless form, such as Sinornithosaurus (20). The latter seems more likely in light of the fact that Hesperonychus approached Sinornithosaurus in size. Surprisingly, there appears to have been little evolution in body size in the Microraptorinae during their long history. The discovery of Hesperonychus indicates that microraptorines continued to exploit the small predator niche for at least 50 million years, whereas the Eudromaeosauria continued to exploit the medium- and large-bodied predator niche over the same span of time (27). In this respect, the evolution of the Dromaeosauridae appears to have been surprisingly conservative. Additionally, Hesperonychus is the first definitive member of the Microraptorinae known from North America. This results in a geographic range extension for the clade, and provides further evidence of an affinity between the dinosaur faunas of North America and Asia (46, 47). The discovery of Hesperonychus also alters our understanding of the predator community in the Late Cretaceous of North America. It now appears that North American carnivorous dinosaurs ranged widely in body size, from 2 kg to many tonnes, 4 orders of magnitude (Fig. 4). The large difference in size between Hesperonychus and other, contemporary dromaeosaurids is consistent with the suggestion that dinosaurian predators were highly segregated in terms of body size so as to reduce competition for prey (48). Despite the absence of associated skeletons, Hesperonychus may have been a relatively common animal in the Dinosaur Park fauna. The most common element, phalanx II-3, is represented by at least 10 specimens in the TMP collections. For the same element, Saurornitholestes is known from 30 isolated specimens; only 2 Dromaeosaurus specimens have been recognized. Therefore, considering that the small bones of Hesperonychus have a low preservation potential and that their size makes them easy to overlook in the field, fossils of this microraptorine are relatively common. It remains to be seen whether microraptorine ECOLOGY GEOLOGY Longrich and Currie PNAS March 31, 2009 vol. 106 no

5 Fig. 4. Size of H. elizabethae and other carnivorous dinosaurs from the Dinosaur Park assemblage of Alberta. (A) Comparison of body mass of H. elizabethae and other carnivorous dinosaurs (a mass estimate is not given for Ricardoestesia, which is known only from teeth and dentaries). (B) Relative size of Hesperonychus and other predatory dinosaurs from the assemblage. elements will be recognized in other North American fossil assemblages. Whereas Hesperonychus is diminutive when compared with contemporary theropods, by the standards of Mesozoic mammals it was a relatively large animal. The metatherian Eodelphis cutleri is perhaps the largest mammal in the Dinosaur Park assemblage, but it weighed just 600 g (49). Ironically, therefore, our discovery of a small theropod in the assemblage actually emphasizes the lack of substantial overlap in body size between dinosaurs and mammals in this fauna. The patterns seen in the Dinosaur Park assemblage are therefore consistent with the conventional wisdom that dinosaurian predation and competition limited the evolution of large body size in Mesozoic mammals, the existence of uncommon giant taxa such as Gobiconodontidae (50) notwithstanding. Relatively few Mesozoic mammals approached or exceeded 500 5,000 g (51), the size of the smallest nonavian dinosaurs, and few if any nonavian dinosaurs were 100 grams (24), the size of the average Mesozoic mammal (51). However, one could also argue that nonavian dinosaurs were precluded from evolving small body size by mammalian competition. Such interpretations are not mutually exclusive. That the existence of Hesperonychus remained undetected until now, 100 years after the discovery of the Dinosaur Park Formation assemblage, underscores the degree to which preservational and collecting biases alter our picture of dinosaurdominated faunas. The Late Cretaceous of the Western Interior boasts one of the world s richest and most intensively studied Mesozoic terrestrial fossil assemblages, but skeletons of small vertebrates are vanishingly rare (2, 3, 7). In contrast, isolated microfossils document the presence of a rich fauna of small, terrestrial vertebrates. This fauna included amphibians, squamates, birds, mammals (52 58), and, as shown here, small dromaeosaurids. Only a small percentage of these taxa are known from associated remains, demonstrating the existence of a strong taphonomic bias against the preservation of small vertebrate skeletons (2, 3, 59). Historically, this problem has been compounded by a collecting bias that has favored the recovery of large, relatively complete dinosaur skeletons. The existence of similar biases in other fossil assemblages has doubtless served to obscure the diversity, abundance, and ecological importance of the small-bodied members of the Dinosauria. Materials and Methods Mass was estimated for Hesperonychus by regressing mass estimates for Archaeopteryx and dromaeosaurids against pubis length by using ordinary least-squares (OLS) regression (see Dinosaur Mass Data and Equations in SI Appendix) to generate an allometric equation. Systematics of Hesperonychus were established by coding the holotype into a morphological data matrix (see Data Matrix in SI Appendix) of 114 characters (Taxa and Characters in SI Appendix) with 23 in-group taxa. The matrix includes previously published (26 28, 36, 43, 44, 60, 61) and heretofore unpublished characters. Four outgroup taxa were used to polarize the characters. Out-group taxa used were the troodontids Troodon, Byronosaurus, and Sinovenator and the basal avian Archaeopteryx. Phylogenetic analysis was performed in PAUP* version 4b10 in branch and bound search mode. Five characters were ordered, and all characters were equally weighted. Tree statistics were calculated with uninformative characters excluded. ACKNOWLEDGMENTS. We thank Clive Coy for his skillful preparation of the holotype. For discussions, we thank David Evans, Mark Norell, Anthony Russell, Jessica Theodor, Alan Turner, Xu Xing, and the University of Calgary Vertebrate Morphology Research Group. We also thank the anonymous reviewers whose suggestions have greatly improved this manuscript. We appreciate the assistance of James Gardner (Royal Tyrrell Museum of Palaeontology, Drumheller, AB, Canada), Farish Jenkins (Museum of Comparative Zoology, Cambridge, MA), Walter Joyce (Yale Peabody Museum, New Haven, CT), Scott Hartman (Wyoming Dinosaur Center, Thermopolis, WY), Mark Norell (American Museum of Natural History, New York), Ken Stadtman (Brigham Young University, Provo, UT), Brandon Strilisky (TMP), and Xing Xu (Institute of Vertebrae Paleontology and Paleoanthropology, Beijing, Peoples Republic of China) for specimen access and T. 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7 Supporting Information For: Longrich and Currie, A microraptorine (Dinosauria- Dromaeosauridae) from the Late Cretaceous of North America Supporting Information 1. Dinosaur mass data and equations Supporting Information 2. Specimen data Supporting Information 3. Taxa and characters Supporting Information 4. Data matrix

8 Supporting Information 1. Dinosaur mass data and equations Table 1. Body masses and pubis lengths used for mass estimate Taxon Specimen Pubis, mm Reference Mass, g Reference Archaeopteryx recurva JM this study 69 (2) Archaeopteryx lithographica HMN this study 252 (3) GS Paul, Microraptor gui IVPP V this study 600 AMNH Bambiraptor feinbergorum this study 2100 Sinornithosaurus millennii IVPP this study 3000 Velociraptor mongoliensis IGM 100/ (1) Deinonychus antirrhopus MCZ This study (2) unpublished data GS Paul, unpublished data GS Paul, unpublished data GS Paul, unpublished data Notes: Institutional abbreviations AMNH; American Museum of Natural History, New York, New York; HMN, Humboldt Museum fur Naturkunde, Berlin, Germany; IGM, Geological Institute, Ulan Bator, Mongolia; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China; JM, Jura Museum, Willibaldsburg, Germany; MCZ, Harvard Museum of Comparative Zoology, Cambridge, Massachusetts ; NMC/CMN, Canadian Museum of Nature, Ottawa, Canada; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Canada; UALVP, University of Alberta Laboratory for Vertebrate Palaeontology, Edmonton, Canada. Methods Length of the pubis was measured from casts (JM 2257; HM 1880), from photographs of specimens using ImageJ image analysis software, or taken from the literature. When the data are log 10 transformed and mass is regressed against pubis length, the resulting equation is M = L ; R 2 = The resulting allometric equation is: M = L Where M is mass in grams, and L is pubis length in mm. A pubis length of 95 mm therefore predicts a mass of 1918 g (4.2 lbs) for the holotype of Hesperonychus elizabethae.

9 Table 2. Body mass of predatory theropods from the Dinosaur Park Formation Taxon Specimen Mass, kg Reference Daspletosaurus torosus NMC/CMN (4) Gorgosaurus libratus AMNH (4) Troodon inequalis NMC/CMN (5) Dromaeosaurus albertensis AMNH (4) Saurornitholestes langstoni TMP Estimated using equation 9 from reference(5) and femur circumference = 67.6mm Ricardoestesia gilmorei NMC/CMN 343? too incomplete for mass estimate Hesperonychus elizabethae UALVP this study

10

11 Supporting Information 2. Specimen data Table 3: Specimens of Hesperonychus Element Specimen Stratigraphy Locality Dimensions (mm) Pelvic girdle Belly River Group: Dinosaur E of DPP, Southern UALVP Park Fm. Alberta 95 Pedal phalanx II-1 Cripple Creek (near TMP Belly River Group: Dinosaur Park Fm. Onefour) Southern Alberta 14.2 Pedal phalanx II-1 Manyberries, Southern Pedal phalanx II-1 TMP Belly River Group Alberta Irvine Mammal Locality, Southern Alberta incomplete Belly River Group: Upper UALVP Dinosaur Park Fm. Pedal phalanx II-2 TMP Belly River Group DPP, Southern Alberta 11.1 Pedal phalanx II-2 TMP Belly River Group DPP, Southern Alberta 17.1 Pedal phalanx II-2 TMP Belly River Group DPP, Southern Alberta 4.8 Pedal phalanx II-3 TMP Belly River Group DPP, Southern Alberta 4.8 Pedal phalanx II-3 Pedal phalanx II-3 Pedal phalanx II-3 TMP TMP incomplete Belly River Group: Oldman Fm. (DPF equivalent) Devil's Coulee, Southern Alberta 5.6 Belly River Group: Dinosaur Park Fm. DPP, Southern Alberta incomplete Belly River Group: Dinosaur Steveville RR Grade, Park Fm. NW of DPP 5.42 TMP Pedal phalanx II-3 TMP Belly River Group DPP, Southern Alberta 5.6 Pedal phalanx II-3 TMP Belly River Group DPP, Southern Alberta 6.5 Pedal phalanx II-3 Pedal phalanx II-3 Pedal phalanx II-3 Pedal phalanx II-3 TMP TMP TMP UALVP Belly River Group Onefour, Southern Alberta 6.6 Sandy Point, Southern Belly River Group Alberta incomplete Belly River Group: Dinosaur Park Fm. DPP, Southern Alberta 6.4 Belly River Group: Upper Dinosaur Park Fm. Irvine Mammal Locality, Southern Alberta 4.8 Notes: Institutional abbreviations TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Canada; UALVP, University of Alberta Laboratory for Vertebrate Palaeontology, Edmonton, Canada. Locality Most of the specimens described here (including the holotype) come from Dinosaur Provincial Park, Alberta, and nearby exposures along the Red Deer River. Seven referred

12 specimens come from other areas of Alberta; none of these are more than 200km from Dinosaur Park (SI Figure 1). Stratigraphy All specimens for which precise stratigraphic data are available come from the chronostratigraphic interval bounded by the Dinosaur Park Formation and coeval beds of the uppermost Oldman Formation. These beds encompass less than 1.7 million years of time (6). Some of the referred specimens lack precise stratigraphic data. In particular, many specimens are recorded as coming from the Judith River Formation ; these beds are now divided into the beds of the lower Oldman Formation and the Dinosaur Park Formation, which overlies the Oldman in the Dinosaur Provincial Park area. Both formations are part of the Belly River Group (6). Most (if not all) of these Belly River Group fossils probably come from the Dinosaur Park Formation, which is more extensively exposed and more fossiliferous than the underlying Oldman. Dimensions: dimensions measured for Hesperonychus are lengths of the pubis, pedal phalanx II- 1, and pedal phalanx II-2, and height of the articular surface for pedal phalanx II-3. Corresponding dimensions for the holotype of Sinornithosaurus millennii are 120 mm, 14.9 mm, 16.8 mm, and 6.8 mm, respecticely. SI:2 Fig. 1. Map showing localities of Hesperonychus holotype and referred specimens: 1, Dinosaur Provincial Park (DPP); 2 Sandy Point; 3, Irvine; 4, Manyberries/Onefour, 5, Devilʼs Coulee.

13 Appendix 3: Taxa and characters used in phylogenetic analysis I. List of Taxa Archaeopteryx Sinovenator changii Byronosaurus jaffei Troodon Mahakala omnogovae Rahonavis ostromi Unenlagia comahuensis Unenlagia? paynemili Buitreraptor gonzalezorum Neuquenraptor argentinus Austroraptor cabazai Microraptor Graciliraptor lujiatunensis Shanag ashile Sinornithosaurus millenii Hesperonychus elizabethae Saurornitholestes langstoni Atrociraptor marshalli Bambiraptor feinbergi Velociraptor mongoliensis Itemirus medullaris Tsaagan mangas Adasaurus mongoliensis Deinonychus antirrhopus Dromaeosaurus albertensis Utahraptor ostrommaysi Achillobator giganticus II. List of characters Skull 1. Premaxilla, maxilla, and dentary bearing distinct interdental plates [0] or interdental plates absent [1] (7) 2. Premaxilla elongate [0], or body of premaxilla short, no more than 15% length of maxilla [1] 3. Premaxilla with short maxillary process [0] or elongate maxillary process separating nasal and maxilla [1] (8) 4. Premaxilla: body of premaxilla longer than tall [0] or at least as tall as long [1] 5. Nasal process of premaxilla projecting posterodorsally [0] or posteriorly [1] 6. Premaxilla with limited exposure of narial fossa on lateral surface [0] or with prominent anteroventral extension of narial fossa onto lateral surface of premaxilla [1]

14 7. Maxilla does not contribute to narial fossa [0] or narial fossa expanded onto anterior ramus of maxilla [1] 8. Maxilla, postantral wall concealed in lateral view [0] or caudally projecting into antorbital fenestra and exposed in lateral view [1] (9) 9. Maxilla, palatal shelf concealed in lateral view [0], or palatal shelf projecting dorsally into the antorbital fenestra and visible in lateral view [1] 10. Maxilla, promaxillary fenestra subcircular and broadly exposed in lateral view [0] or slitlike, largely concealed in the anteroventral margin of antorbital fossa [1] (10) 11. Maxilla, anterior ramus elongate, 25% or more of the length of maxilla [0] or short, less than 25% length of maxilla [1] 12. Maxilla, accessory antorbital fenestra large and subcircular [0] or small and subcircular [1] or anteroposteriorly elongate [2] UNORDERED 13. Maxilla, accessory antorbital fenestra positioned low in antorbital fossa [0] or placed dorsally in antorbital fossa [1] (12) 14. Maxilla, accessory antorbital fenestra developed as a simple perforation [0] or located in distinct fossa [1] or located in a fossa containing a deep and pitlike excavation posterodorsal to the accessory antorbital fenestra [2] UNORDERED [Modified from (12)] 15. Maxilla, labial margin smooth [0] or maxilla with a distinct lip bounding the ventral margin of the antorbital fossa [1] 16. Maxilla, anterior ramus longer than tall [0] or short, at least as tall as long [1] 17. Maxilla, interfenestral bar separating maxillary fenestra and antorbital fenestra: narrow [0] or broad [1] 18. Nasals concave in lateral view [0] or straight to convex in lateral view [1] [2] 19. Frontal, orbital margin straight or smoothly concave in dorsal view [0] or postorbital process sharply offset, and orbital margin L-shaped in dorsal view [0] (13) 20. Frontal notched to recieve lacrimal: absent [0] or present [1] (13) 21. Frontal, supratemporal fossa smooth [0] or supratemporal fossa with a distinct pit [1] (13) 22. Frontal, supratemporal fossa restricted to the lateral half of the frontal [0] or supratemporal fossa extends medially [1] 23. Jugal, suborbital ramus slender [0] or dorsoventrally deep and robust [1] 24. Jugal, postorbital process slender; jugal triradiate [0] or postorbital process broad, and jugal triangular [1] 25. Quadratojugal with straight ascending ramus [0] or ascending ramus bowed anteriorly [1] 26. Quadrate shaft pierced by large foramen: present [0] or absent [1] 27. Quadrate shaft straight or weakly curved in lateral view [0] or strongly bowed anteriorly in lateral view [1] 28. Exoccipital, caudal surface with a bowl-like depression containing the exits of cranial nerves X and XII: absent [0], or present [1] (8) 29. Exoccipital, paroccipital processes of exoccipital project ventrolaterally [0] or laterally [1] [(8), polarity reversed] 30. Basioccipital tubera, caudal surfaces flat or smoothly concave [0] or basioccipital tubera with distinct, ovoid depressions on the caudal surface [1] 31. Basioccipital tubera separated by weak notch [0] or separated by a deep, broad, U-shaped ventral notch [1] 32. Basisphenoid recess with paired openings: absent [0] or present [1] 33. Prominent lateral depression of braincase bounded by otosphenoidal crest: absent [0] or present 34. Foramen magnum subcircular [0] or distinctly taller than wide [1] 35. Dentary subtriangular, with dorsal and ventral margins diverging posteriorly [0] or dentary dorsal and ventral margins subparallel [1] (13) 36. Dentary with prominent groove along the length of the lateral surface [0] or lateral groove reduced anteriorly or absent [1] (8) 37. Dentary straight or weakly curved in lateral view [0] or strongly bowed, with curved dorsal and ventral margins [1]

15 38. Dentary with distinct midlength constriction and terminal expansion: absent [0] or present [1] 39. Articular with tall, columnar process on retroarticular process: absent [0] or present [1] 40. Skull short [0] or elongate, at least 125% length of femur [1] (14) Dentition 41. Teeth constricted between crown and root [0] or constriction absent [1] (15) 42. Premaxillary and anterior dentary teeth unserrated [0] or bearing denticles on posterior carina [1] (8) 43. Premaxillary teeth 2-4 subequal in size [0] or second premaxillary tooth larger than third and fourth [1] (13) 44. Maxillary teeth number fewer than 20 [0] or numbering at least 20 [1] [modified from (8)] 45. Maxillary teeth strongly raked posteriorly: absent [0] or present [1] (16) 46. Maxillary teeth subequal in length along the jaw [0] or posterior maxillary teeth elongate and fanglike, approximately 200% the length of anteriormost maxillary teeth [1] 47. Maxillary and posterior dentary teeth unserrated [0] or bearing denticles on posterior carina only [1] or bearing denticles on anterior and posterior carina [2] UNORDERED [modified from (17)] 48. Maxillary and posterior dentary teeth, denticles: anterior denticles smaller than posterior denticles [0] or anterior and posterior denticles subequal in size [1] (7) 49. Anterior dentary teeth closely packed: absent [0] or present [1] Axial skeleton 50. Axis vertebra bearing short epipophyses [0] or elongate axial epipophyses that project laterally beyond postzygapophyses [1] (18) 51. Cervical vertebrae, carotid processes present [0] or carotid processes absent [1] (17) 52. Cervical vertebrae with low neural spines [0] or cervical neural spines at least as tall as long anteroposteriorly [1] 53. Dorsal vertebrae, parapophyses short [0] or borne on elongate pedicels [1] [1] 54. Dorsal vertebrae, pneumatopores absent [0] or present on dorsal centra [1] (19) 55. Dorsal vertebrae, neural spines low, height does not exceed anteroposterior length [0] or taller than long anteroposteriorly [1] 56. Dorsal vertebrae, neural arch bearing prominent anterior fossae on either side of neural canal: absent [0] or present [1] 57. Dorsal vertebrae, centra of posterior dorsals elongate [0] or short and massive, length of centrum less than diameter [1] 58. Dorsal vertebrae, distal end of neural spines transversely expanded by at least 200% to form a distinct spine table: absent [0] or present [1] 59. Sacral vertebrae, 5 vertebrae incorporated into sacrum [0] or sacrum incorporating at least 6 vertebrae [1] (10) 60. Sacral vertebrae lack pneumatopores [0] or pneumatopores present in one or more sacral vertebrae [1] (19) 61. Caudal vertebrae, distal caudal centra bearing prominent lateral depressions [0] or lateral surfaces of centra flat or convex [1] 62. Caudal vertebrae, distal caudals greatly elongated, up to 200% the length of proximal caudals [0] or moderately elongate, no more than 200% the length of the proximal caudals [1] 63. Caudal vertebrae, distal caudal vertebrae with a convex or flat dorsal surface [0] or bearing a prominent dorsal groove [1] (20) 64. Caudal vertebrae, prezygapophyses short [0] or elongate [1] or extended by ossified tendons of caudal epaxial muscles [2] (7)