Oldest skeleton of a plesiadapiform provides additional evidence for an exclusively arboreal radiation of stem primates in the Palaeocene

Abstract

Palaechthonid plesiadapiforms from the Palaeocene of western North America have long been recognized as among the oldest and most primitive euarchontan mammals, a group that includes extant primates, colugos and treeshrews. Despite their relatively sparse fossil record, palaechthonids have played an important role in discussions surrounding adaptive scenarios for primate origins for nearly a half-century. Likewise, palaechthonids have been considered important for understanding relationships among plesiadapiforms, with members of the group proposed as plausible ancestors of Paromomyidae and Microsyopidae. Here, we describe a dentally associated partial skeleton of Torrejonia wilsoni from the early Palaeocene (approx. 62 Ma) of New Mexico, which is the oldest known plesiadapiform skeleton and the first postcranial elements recovered for a palaechthonid. Results from a cladistic analysis that includes new data from this skeleton suggest that palaechthonids are a paraphyletic group of stem primates, and that T. wilsoni is most closely related to paromomyids. New evidence from the appendicular skeleton of T. wilsoni fails to support an influential hypothesis based on inferences from craniodental morphology that palaechthonids were terrestrial. Instead, the postcranium of T. wilsoni indicates that it was similar to that of all other plesiadapiforms for which skeletons have been recovered in having distinct specializations consistent with arboreality.

Keywords: Palaeocene; evolution; paleontology; plesiadapiforms; primates.

Figure 1.

Skeleton composite of Torrejonia wilsoni (NMMNH P-54500). Most elements of the composite skeleton are in ventral view, but some elements are oriented differently to better illustrate articular surfaces. Descriptions and orientations of skeletal elements organized from left to right and then from top to bottom: (a) cranial fragment of R frontal in dorsal, left lateral, right lateral views; cranial fragment of parietals in dorsal, ventral views. (b) R maxilla M1–M3 in occlusal view; L maxilla P4, M2–M3 in occlusal view; R dentary p2–m2 in buccal, lingual, occlusal views (also see [13]). (c) R distal humerus in ventral, dorsal, distal views; R distal radial epiphysis in distal view; R proximal radius in proximal, ventral, lateral, dorsal, medial views; R proximal ulna in ventral, lateral, dorsal, medial views. (d) R scapula fragment in ventral, lateral, dorsal views; L proximal humerus in proximal, ventral, lateral, dorsal, medial views. (e) R partial astragalus in dorsal, lateral, plantar, medial, proximal, distal views; R calcaneus in dorsal, lateral, plantar, medial, proximal, distal views; R cuboid in dorsal, lateral, plantar, medial, proximal, distal views. (f) R partial innominate in ventral, lateral, dorsal, medial views; concretion with proximal femora and L tibia with L proximal femur in ventral view, R proximal femur in ventral view; R tibia in ventral, lateral, dorsal, medial, distal views; L distal femur in ventral, lateral, dorsal, medial, distal views. Scale bars, 1 cm.

Figure 2.

Comparison of distal humerus, innominate and distal femur of Torrejonia wilsoni (NMMNH P-54500) with those of other euarchontan mammals. Photographs of (a) left and (b) right distal humeri of T. wilsoni and micro X-ray CT scan reconstructions of those of paromomyid plesiadapiforms (c) cf. Phenacolemur simonsi (USNM 442260) and (d) Ignacius clarkforkensis (UM 108210), and extant treeshrews (e) arboreal Ptilocercus lowii (MCZ 51736) and (f) terrestrial Tupaia gracilis (FMNH 140928) in ventral view. Photograph of (g) partial right innominate of T. wilsoni and micro X-ray CT scan reconstructions of innominates of (h) I. clarkforkensis (UM 82606), (i) arboreal P. lowii (MCZ 51736) and (j) terrestrial T. gracilis (FMNH 140928) in lateral view. Photograph of (k) left distal femur of T. wilsoni and micro X-ray CT scan reconstructions of distal femora of (l) I. clarkforkensis (UM 82606), (m) arboreal P. lowii (MCZ 51736) and (n) terrestrial T. gracilis (FMNH 140928) in distal view. Some elements reversed for clarity. Scale bars, 1 mm. AC, acetabulum; HC, humeral capitulum; IS, anterior inferior iliac spine; ME, medial epicondyle; PG, patellar groove; RF, radial fossa; SC, supinator crest; ZC, zona conoidea. See the electronic supplementary material for institutional abbreviations.

Figure 3.

Comparison of astragalus and calcaneus of Torrejonia wilsoni (NMMNH P-54500) with those of other euarchontan mammals. Columns illustrate micro X-ray CT scan reconstructions of tarsals of (a) terrestrial condylarth cf. Protungulatum (AMNH 118260, 118060), (b) colugo Cynocephalus (UNSM 15502, AMNH 207001), (c) arboreal treeshrew Ptilocercus (USNM 488072), (d) purgatoriid plesiadapiform cf. Purgatorius (UCMP 197507, 197517), (e) micromomyid plesiadapiform Dryomomys (UM 41870), (f) paromomyid plesiadapiform Ignacius (USNM 442235, 442240), and photographs of (g) Torrejonia wilsoni. Right astragali (rows 1–3) and calcanei (rows 4–6) in dorsal (top), plantar (middle), distal (bottom) views. Some elements reversed for clarity. Scale bars, 1 mm. AEF, astragalar ectal facet; ASF, astragalar sustentacular facet; CEF, calcaneal ectal facet; CF, calcaneal cuboid facet; CSF, calcaneal sustentacular facet; LTF, astragalar lateral tibial facet; PT, calcaneal peroneal tubercle. See the electronic supplementary material for institutional abbreviations.

Figure 4.

Hypothesis of evolutionary relationships of Torrejonia wilsoni and other eutherian mammals. (Left) Resulting single most parsimonious cladogram based on modified morphological dataset of Bloch et al. [4], sampling a total of 240 morphological characters (68 postcranial, 45 cranial and 127 dental) with Primates sensu lato indicated in blue and Torrejonia wilsoni supported as a stem primate and indicated in orange. Numbers below branches represent Absolute Bremer Support values. See the electronic supplementary material for detailed methods, descriptions of morphological characters, specimens examined (also see [5]), and the taxon-character matrix in TNT format. (Bottom) Simplified subset of resulting tree topology focused on Primates. Boxes (a–f) illustrate tarsals of select primates with great mobility at the upper ankle joint (yellow: lateral tibial facet extends distally onto neck of astragalus in dorsal view), lower ankle joint (red: sustentacular facet extends distally onto body of calcaneus in dorsal view) and transverse tarsal joint (orange: round, concave cuboid facet of calcaneus in distal view) indicating arboreality. Boxes (a–f) also illustrate micro X-ray CT scan reconstructions of (a) purgatoriid Purgatorius unio p4-m3 (UCMP 107406) with tall molar cusps in buccal view, (b) micromomyid Dryomomys szalayi cranium (UM 41870) in right lateral view with large IOF, (c) Torrejonia wilsoni partial skeleton (NMMNH P-54500), (d) paromomyid Ignacius graybullianus cranium (USNM 421608) in right lateral view with relatively large olfactory bulbs (OB) of endocast (violet), (e) carpolestid Carpolestes simpsoni cranium (USNM 482354) in right lateral view and tarsals (UM 101963) and (f) notharctid Notharctus tenebrosus cranium (AMNH 127167) in right lateral view. Some elements reversed for clarity. See figure 3 legend for specimen numbers of tarsals not listed above. See the electronic supplementary material for institutional abbreviations.