Pumiliornis tessellatus MAYR, 1999 revisited - new data on the osteology and possible phylogenetic affinities of an enigmatic Middle Eocene bird

Palfontologische Zeitschrift 2008, Vol. 82/3, p. 247-253, 30-09-2008 Pumiliornis tessellatus MAYR, 1999 revisited - new data on the osteology and possible phylogenetic affinities of an enigmatic Middle Eocene bird GERALD MAYR, Frankfurt/Main with 4 figures and 1 table MAYR, G. 2008. Pumiliornis tessellatus MAYR, 1999 revisited - new data on the osteology and possible phylogenetic affinities of an enigmatic Middle Eocene bird. - Pal/iontologische Zeitschrift 82 (3): 247-253, 4 figs., 1 tab., Stuttgart, 30.9. 2008. Abstract: Although the avian species Pumiliornis tessellatus MAYR, 1999 is known from two skeletons from the Middle Eocene of Messel in Germany, its phylogenetic affinities remained enigmatic. The new osteological data presented in this study document that P. tessellatus had an at least semizygodactyl foot, with a very wide basal phalanx of the fourth toe, and lacked an ossified pons supratendineus on the distal tibiotarsus. Compared to the known zygodactyl and semizygodactyl birds, this tiny Middle Eocene species resembles the late Eocene/early Oligocene taxon Eocuculus CHANDLER, 1999. A new, tentatively referred wing of Eocuculus from the early Oligocene of France is described and compared with Pumiliornis. Keywords: fossil birds Eocene Oligocene Messel phylogeny Kurzfassung: Obwohl die Vogelart Pumiliornis tessellatus MAYR, 1999 von zwei Skeletten aus dem mittleren Eoz~in von Messel in Deutschland bekannt ist, blieben ihre Verwandtschaftsbeziehungen r~itselhaft. In dieser Studie vorgestellte neue osteologische Einzelheiten belegen, dab P. tessellatus einen zumindest semizygodactylen Fug mit sehr breiter Grundphalanx der vierten Zehe hatte und der Pons supratendineus des distalen Tibiotarsus nicht verkn6chert war. Verglichen mit den bekannten zygodactylen und semizygodactylen Vogelgruppen ~ihnelt die winzige mitteleoz~ine Art am st~irksten dem obereoz~inen/unteroligoz~inen Taxon Eocuculus CHANDLER, 1999. Ein neuer, unter Vorbehalt zugeordneter Fltigel von Eocuculus aus dem unteren Oligoz~in Frankreichs wird beschrieben und mit Pumiliornis verglichen. Schliisselwbrter: fossile V6gel Eoz~in Oligoz~in Messel Phylogenie Introduction Classification of early Paleogene birds can be a challenging task and quite a number of taxa have so far defied a phylogenetic assignment. Among these is Pumiliornis tessellatus MAYR, 1999, a tiny species from the Middle Eocene of Messel in Germany, which is known from two fairly well preserved skeletons (MAYR 1999a). The species had an overall body size similar to that of a small wren (Passeriformes, Troglodytidae), and combines a long and most likely schizorhinal beak with short and very robust feet (Figs. 1-3). In an attempt to shed some light on the phylogenetic affinities of P. tessellatus, the specimens have been restudied, and after removal of matrix additional osteological features of some bones became visible. Re-examination showed that Pumiliornis resembles Eocuculus CHANDLER, 1999, which is known from a postcranial skeleton from the late Eocene of Colorado (USA) (CHANDLER 1999) and another partial skeleton from the early Oligocene of Southern France (MAYR 2006a). Eocuculus had zygodactyl feet, i.e., retroverted fourth toes, and was assigned to the Cuculidae by CHAND- LER (1999). This assignment was considered uncertain by MAYR (2006a) who concluded that it is morphologically quite different from extant Cuculidae and at best constitutes a stem lineage representative of this taxon. Address of the author: Gerald Mayr, Forschungsinstitut Senckenberg, Sektion Omithologie, Senckenberganlage 25, D-60325 Frankfurt/Main, Germany; e-mail <gerald.mayr@ senckenberg.de>. 0031-0220/08/0082-247 $ 3.15 2008 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart

248 GERALD MAYR The French Eocuculus specimen consists of the sternum, pelvis, and hindlimbs (MAYR 2006a). After publication of its description, I recognized that another fossil from the same locality, represented by wings and pectoral girdle elements, may also belong to Eocuculus. This specimen and the other remains of Eocuculus are compared with Pumiliornis in the present study, and previously unknown osteological features of the latter are described. Material and methods Osteological terminology follows BAUMEL & WITMER (1993). Institutional abbreviations: SMF, Forschungsinstitut Senckenberg, Frankfurt am Main, Germany; DM, Denver Museum of Natural History, Denver, Colorado (USA). Systematic paleontology Aves LINNAEUS, 1758 Pumiliornis tesseuatus MAYR, 1999 Figs. 1, 2, 3A, C, E, F G, 4E, K; Tab. 1 Referred specimens: SMF-ME 2092A+B (holotype), SMF- ME 2475A+B. Locality and horizon: Messel near Darmstadt, Germany; Middle Eocene. Description of new osteological features and comparisons with Eocuculus: In the holotype (SMF-ME 2092A) and on the x-ray pictures of SMF-ME 2475A+B it can be discerned that the corpi of the thoracic vertebrae bear deep lateral excavations (Figs. l, 2), a presumably plesiomorphic feature which is also found in, e.g., Ichthyornis, a stem lineage representative of Neornithes (CLARKE 2004), the palaeognathous Lithornithidae (LEONARD et al. 2005), stem lineage representatives of the Galliformes (landfowl, DYKE 8 GULAS 2002), and extant Procellariiformes (tubenoses and allies) and Charadriiformes (shorebirds and allies). Deep depressions on the lateral surfaces of the corpus of at least one thoracic vertebra are also visible in the specimen of Eocuculus cf. cherpinae described by MAYR (2006a). The proximal end of the tibiotarsus bears a marked crest along its medial side (Fig. 2B). This feature was overlooked in the original description (MAYR 1999a) and, in a comparable development, is otherwise known only from Psittaciformes (parrots), Coliiformes (mousebirds), Pici (woodpeckers and allies), and the upupiform Phoeniculidae (woodhoopoes). Orientation of the tibiotarsi of the French specimen of Eocuculus (SMF Av 425) does not allow unambiguous assessment of this feature; the tibiotarsi of the holotype of E. cherpinae were not described by CHANDLER (1999). After removal of matrix from the distal end of the tibiotarsus of specimen SMF-ME 2475B, it is now visible that Pumiliornis lacks an ossified pons supratendineus and a well-delimited sulcus extensorius (Fig. 3A). Among extant birds, a ports supratendineus is absent in the adults of the palaeognathous ratites (except Apterygidae [kiwis] and Dinornithidae [moas]), Strigiformes (owls), some crown group representatives of Psittaciformes, as well as in Nyctibiidae (potoos), Steatornithidae (oilbird), Bucerotidae (hornbills), and Opisthocomidae (hoatzin). An ossified pons supratendineus is further absent in Mesozoic stem group representatives of Neornithes (e.g., CLARKE 2004), and the early Paleogene Ameghinornithidae (MAYR 2005a), Horusornithidae (MOURER-CHAUVIRI~ 1991), and Messelasturidae (MAYR 2005b). All of these taxa otherwise show little similarity to Pumiliornis. The distal tibiotarsus of Eocuculus bears an ossified pons supratendineus but resembles that of Pumiliornis in the size and shape of the condyli (Figs. 3A, B). After some further preparation of the proximal end of the tarsometatarsus of specimen SMF-ME 2475B, it can be seen that the tuberositas musculi tibialis cranialis is very prominent and situated at the medial margin of the bone (Fig. 3C). In most birds, this tubercle is much lower and positioned more or less centrally, which by outgroup comparison with palaeognathous birds and Galloanseres represents the plesiomorphic condition. Foramina vascularia proximalia are not visible, as the corresponding area of the tarsometatarsus is only preserved as a resin mould. The dorsal surface of the distal end of the bone is convex. The trochlea metatarsi IV is turned plantad (MAYR 1999a: 80) but lacks a well-developed trochlea accessoria. The tarsometatarsus of Pumiliornis is very similar to that of Eocuculus (Fig. 3D). Shared characteristics include the shape and proportions of the bone, the medial position and prominence of the tuberositas musculi tibialis cranialis, the absence of a crista rnedianoplantaris, the convex dorsal surface of the distal end, the presence of a depression at the base of the wide trochlea metatarsi III, and the short and plantarly deflected trochleae metatarsorum II et IV. In contrast to Pumiliornis, however, Eocuculus exhibits a marked sulcus for the tendon of musculus extensor brevis digiti IV on the dorsal surface of the tarsometatarsus, between the trochleae metatarsorum III et IV, and the trochlea metatarsi lli bears a shallower furrow. Preserved distal to the left tarsometatarsus of SMF- ME 2475B there is a damaged flat bone, which represents the proximal phalanx of the fourth toe (Fig. 3C; with a length of 2.1 mm it is too short for the corresponding phalanx of the third toe; see the measurements in MAYR 1999a). A very wide proximal phalanx of the fourth toe is also visible in the holotype SMF-ME 2092B (Figs. 3E, F). The fact that the mediolateral width of this phalanx is much greater than that of the trochlea metatarsi IV precluded arrangement of the fourth toe in an

Pumiliornis tessellatus MAYR revisited - new data on an enigmatic Middle Eocene bird 249 Fig. 1. A: Pumiliornis tessellatus MAYR, 1999, holotype (SMF-ME 2092A), coated with ammonium chloride. B: Pumiliornis tessellatus, combined x-ray picture of specimens SMF-ME 2092A and SMF-ME 2092B. The framed areas in A indicate the position of the details in Figs. 2A, B. - Abbreviations: exc, excavation on corpus of thoracic vertebrae. - Scale bars = 5 mm. B anterior position and is indicative of an at least semizygodactyl foot. In Eocuculus the proximal phalanx of the fourth toe is also very flat and wide, but the hallux is proportionally shorter than that of Pumiliornis (Figs. 3D, H). In SMF-ME 2092B a cream-colored, heart-shaped substance covers the pygostyle (Fig. 2C). Such a patch also occurs in other Messel birds and probably constitutes the remains of the uropygial gland waxes (MAYR 2006b).?Eocuculus sp. CHANDLER, 1999 Figs. 4A, C, D, F; Tab. 1 Tentatively referred specimen: SMF Av 424, both wings, furcula, right coracoid and scapula (Fig. 4A). Locality and horizon: Region of Vach~res, Alpes de Haute Provence, Southern France; early Oligocene. Remarks on identification: The wing bones of SMF Av 424 are slightly larger than those of the holotype of Eocuculus cherpinae CHANDLER, 1999, but very similar in proportions and all osteological details that can be compared (Figs. 4A, B; Tab. 1). In particular and as in E. cherpinae, the humerus and ulna are very robust, the ulna exceeds the humerus only slightly in length, and the phalanx proximalis digiti majoris bears a well-developed processus internus indicis. The specimen comes from the same private collection as SMF Av 425, which was identified as Eocuculus cf. cherpinae by MAYR (2006a). Although it lacks exact locality data, it may be from the same quarry as the latter, an assumption that is in concordance with the structure of the matrix. Other avian taxa known from the early Oligocene "Calcaires lithographiques" of the C6reste/Vach~res region are clearly distinguished from SMF Av 424 in wing bone morphology (BESSONAT & MICHAUT 1973; MAYR 1999b, 2000, 2005c, in press; ROUX 2002; MAYR & MANEGOLD 2006; MAYR & KNOPF 2007, 2006; LOU- CHART et al. 2008). Measurements: see Tab. 1. Description and comparison with Pumiliornis: The slab contains the cranialmost seven cervical vertebrae. The first four of these are poorly preserved, but the fifth to seventh vertebrae resemble those of, e.g., extant parrots in overall morphology. The furcula (SMF-ME 2092A; Fig. 4C) is U- shaped and has robust scapi clavicularum. Although the bony substance of the extremitas sternalis is missing, its

250 GERALD MAYR Fig. 2. Purniliornis tessellatus MAYR, 1999, holotype. A: Thoracic vertebrae in ventral view (SMF-ME 2092A), coated with ammonium chloride. B: Proximal end of left tibiotarsus in medial view (SMF-ME 2092A), coated with ammonium chloride. C: Tail vertebrae; arrows indicate remains of the uropygial gland waxes (SMF-ME 2092B). - Abbreviations: cre, crest on medial side of proximal tibiotarsus; exc, excavation on corpus of thoracic vertebrae. - Scale bars = 5 mm. impression in the slab allows the recognition of a short apophysis furculae. According to CHANDLER (1999: fig. 3), an apophysis furculae is also present in the holotype of E. cherpinae, although the extremitas sternalis is not preserved in the mould ("plastotype") of the specimen figured by CHANDLER (1999: fig. 2). The small extremi- tas omalis of SMF Av 424 is slightly widened, with a straight end. The furcula of Purniliornis tessellatus is not very well preserved, but has similar overall proportions and a likewise small and simple extremitas omalis (SMF-ME 2475A). The coracoid (Fig. 4D) bears a long and narrow processus procoracoideus; a foramen nervi supracoracoidei is absent. The extremitas sternalis forms a long processus lateralis and exhibits a small projection on its medial margin. The coracoid of the holotype of E. cherpinae has not been described and, judging from the published figures, seems to be poorly preserved. According to CHANDLER (1999: fig. 3) the processus lateralis was, however, long as in SMF Av 424. The coracoid of Pumiliornis is somewhat more elongated and the processus procoracoideus wider (Fig. 4E); details of the extremitas sternalis are not visible in the specimens. Except for the more strongly bent distal end, the scapula resembles that of P. tessellatus, especially with regard to the shape of the long and pointed acromion (Figs. 4F, G). The humerus is very compact and with a large extremitas proximalis. In its proportions it is similar to the humerus of the holotype of E. cherpinae, but stouter than the humerus of any representative of crown-group Cuculidae. There is a marked sulcus transversus. The crista deltopectoralis is short and rounded, its margin cranially deflected. The distal end resembles the distal humerus of P. tessellatus in the proportions and orientation of the condyli and the size of the tuberculum supracondylare ventrale (Figs. 4H, I). The ulna only slightly exceeds the humerus in length and is thus proportionally somewhat longer than the ulna of the holotype of E. cherpinae, which has about the same length as the humerus (Tab. 1). By contrast, the ulna of P. tessellatus is distinctly longer than the humerus. Tab. 1. Dimensions of major limb bones of Pumiliornis and Eocuculus (left/right; maximum length in mm). humerus ulna carpometacarpus femur tibiotarsus tarsometatarsus Pumiliornis tessellatus SMF-ME 2475 t 13.3/13.4 SMF-ME 2092 t 13.4/- M 5.7/M5.5 7.5/7.5 -/10.8 18.1/18.7 9.9/- 16.3/- 6.5/- ~12.0/- 17.7/- 8.9/8.9 Eocuculus cherpinae DM 10682/106832 27.0/- 27.0/27.7 15.2/- -/- 33.7/32.5 17.0/17.0 Eocuculus cf. cherpinae SMF Av 4253 -/- -/- -/- -/25.1 35.0/35.7 ~17.7/17.6?Eocuculus sp. SMF Av 424 29.5/28.1 ~31.7/32.1 ~ 17.6/17.4 -/- -/- -/- 1 after MAYR (1999a) 2 after CHANDLER (1999) 3 after MAYR (2006a)

Pumiliornis tessellatus MAYR revisited - new data on an enigmatic Middle Eocene bird 251 Fig. 3. Hindlimb elements of Pumiliomis tessellatus MAYR, 1999 and Eocuculus cf. cherpinae CHANDLER, 1999 in comparison. A: Distal end of right tibiotarsus of P. tessellatus (SMF-ME 2475B, cranial view). B: Distal end of left tibiotarsus of E. cf. cherpinae (SMF Av 425, cranial view). C: Left tarsometatarsus of P. tessellatus in dorsal view (SMF-ME 2475B). D: Right tarsometatarsus of E. cf. cherpinae in dorsal view (SMF Av 425). E: Left tarsometatarsus of P. tessellatus in lateral view (SMF-ME 2092B). F: Right tarsometatarsus of P. tessellatus in plantar view (SMF-ME 2092B). G: Left tarsometatarsus of P. tessellatus in plantar view (SMF-ME 2475A). H: Left tarsometatarsus of E. cf. cherpinae in medioplantar view (SMF Av 425). - Abbreviations: dep, depression at base of trochlea metatarsi IV; mtl, os metatarsale I; phlv, proximal phalanx of fourth toe; pst, pons supratendineus; tmll, trochlea metatarsi II; truly, trochlea metatarsi IV; ttc, tuberositas musculi tibialis cranialis. - All specimens coated with ammonium chloride. - Scale bars = 5 mm. The crus longum of the os carpi ulnare is short but not as greatly abbreviated as in crown group representatives of the Cuculidae. The carpometacarpus (Fig. 4J) is of similar proportions to that of the holotype of E. cherpinae. As in P. tessellatus it is craniocaudally narrow, with a straight os metacarpale minus. However, the spatium intermetacarpale widens distally whereas it has a constant width in P. tessellatus. The phalanx proximalis digiti majoris bears a distinct processus internus indicis (STEGMANN 1963), as does that of E. cherpinae (MAYR 2006a) and P. tessellatus (Figs. 4J, K). The fossa ventralis of this phalanx is marked. The phalanx digiti minoris exhibits a well-developed flexor process. Discussion As detailed above, the plantarly deflected trochlea metatarsi IV and very wide proximal phalanx of the fourth toe suggest that Pumiliornis had at least semizygodactyl feet. Compared to the known zygodactyl and semizygodactyl birds, the Middle Eocene taxon most closely resembles the late Eocene/early Oligocene Eocuculus. Shared characteristics include: (1) lateral excavations of the corpus of the thoracic vertebrae (Figs. 1, 2); (2) a processus internus indicis on the phalanx proximalis digiti majoris (Figs. 4J, K); (3) a prominent and medially situated tuberositas musculi tibialis cranialis on the tarsometatarsus (Figs. 3C, D); (4) a depression at the base of the trochlea metatarsi III dorsal surface of tar-

252 GERALD MAYR Fig. 4. Wing and pectoral girdle elements of Pumiliornis tessellatus MAYR, 1999 and?eocuculus sp. CHANDLER, 1999 (SMF Av 424) in comparison. A:?Eocuculus sp., wings from the early Oligocene of the region of Vacheres, Alpes de Haute Provence, Southern France (SMF Av 424). B: Left wing of Eocuculus cherpinae CHANDLER, 1999; resin mould of holotype (DM 10682; from CHANDLER 1999). C:?Eocuculus sp. (SMF Av 424), furcula. D:?Eocuculus sp. (SMF Av 424), right coracoid in ventral and left humerus in cranial view. E: P. tessellatus (SMF-ME 2475B), right coracoid in ventral view. F:?Eocuculus sp. (SMF Av 424), right scapula in lateral view. G: P. tessellatus (SMF- ME 2475A), right scapula in lateral and right coracoid in dorsal view. H:?Eocuculus sp. (SMF Av 424), right humerus in cranial view. I: P. tessellatus (SMF-ME 2475B), right humerus in cranial view. J:?Eocuculussp. (SMF Av 424) left manus in ventral view. K: P. tessellatus (SMF-ME 2475B), right manus in ventral view. - Abbreviations: apf, apophysis furculae; cst, carina sterni; cv, cervical vertebrae; exo, extremitas omalis; fur, furcula; Ico, left coracoid; pla, processus lateralis; ppc, processus procoracoideus; ppi, processus internus indicis; pro, projection on medial margin of extremitas sternalis; rco, right coracoid; rsc, right scapula; stv, sulcus transversus. - All figures except B coated with ammonium chloride. - Scale bars = 5 ram.

new Pumiliornis tessellatus MAYR revisited - data on an enigmatic Middle Eocene bird 253 sometatarsus (Fig. 3C, D); and (5) a very wide and dorsoventrally flat proximal phalanx of fourth toe (Fig. 3). With the exception of the first (see above), and by outgroup comparison with, e.g., palaeognathous birds and Galloanseres, these characters constitute the derived state within neornithine birds. Pumiliornis tessellatus is only half the size of Eocuculus cherpinae but exhibits similar length proportions of most major limb elements, which are also about half as long as those of E. cherpinae (Tab. 1). The ulna of P. tessellatus is, however, significantly longer than the humerus, whereas both bones have about the same length in the holotype of E. cherpinae. If SMF Av 424 is correctly assigned to Eocuculus, the humerus of this taxon was somewhat stouter than that of Pumiliornis. P. tessellatus further differs from E. cherpinae (referred specimen SMF Av 425) in the absence of an ossified pons supratendineus on the distal tibiotarsus. Loss of an ossified pons supratendineus may be an autapomorphic feature of Pumiliornis, and there is no evidence beyond that feature supporting a closer relationship to any other avian taxon without this bony bridge on the distal tibiotarsus. Certainly a close relationship between Pumiliornis and Eocuculus is not as robustly supported as in the case of other pairs of Eocene and early Oligocene avian taxa, such as the zygodactylids Primozygodactylus and Zygodactylus (MAYR in press). However, given the derived beak morphology of Pumiliornis, this hypothesis can be tested by future specimens of Eocuculus in which the skull is preserved. Whether any of these taxa is on the stem lineage of the Cuculidae, which have no unambiguous Paleogene fossil record (MAYR 2005d), is still an open question. 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