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| Oksoko avarsan Funston, Chinzorig, Tsogtbaatar, Kobayashi, Sullivan & Currie, 2020 Illustration: Michael W. Skrepnick |
Abstract
Late Cretaceous trends in Asian dinosaur diversity are poorly understood, but recent discoveries have documented a radiation of oviraptorosaur theropods in China and Mongolia. However, little work has addressed the factors that facilitated this diversification. A new oviraptorid from the Late Cretaceous of Mongolia sheds light on the evolution of the forelimb, which appears to have played a role in the radiation of oviraptorosaurs. Surprisingly, the reduced arm has only two functional digits, highlighting a previously unrecognized occurrence of digit loss in theropods. Phylogenetic analysis shows that the onset of this reduction coincides with the radiation of heyuannine oviraptorids, following dispersal from southern China into the Gobi region. This suggests expansion into a new niche in the Gobi region, which relied less on the elongate, grasping forelimbs inherited by oviraptorosaurs. Variation in forelimb length and manus morphology provides another example of niche partitioning in oviraptorosaurs, which may have made possible their incredible diversity in the latest Cretaceous of Asia.
Keywords: forelimb evolution, Theropoda, Late Cretaceous, Oviraptoridae, digit reduction
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| Figure 1. Holotype block of Oksoko avarsan MPC-D 102/110. (a,b) Holotype block with skeletons in ventral view. Colours distinguish different individuals; the holotype individual is in blue. |
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| Figure 2. Skeletal anatomy of Oksoko avarsan. (a) Skeletal reconstruction. (b,c) Skull of MPC-D 102/110.a in left lateral view. (d) Anterior cervical vertebra of MPC-D 102/12 in left lateral view. (e) Articulated sacrum and ilium of MPC-D 102/11 in ventral view, anterior is to the left. (f) Mid-caudal vertebra of MPC-D 102/12 in left lateral view. (g) Pygostyle vertebrae of MPC-D 102/12 in left lateral view. (h) Right scapulocoracoid of MPC-D 100/33 in lateral view. (i) Furcula of MPC-D 100/33 in anterior view. (j) Right and left sternal plates of MPC-D 100/33 in anterior view. (k) Articulated pelvis of MPC-D 102/11 in right lateral view. (l) Right femur of MPC-D 102/12 in posterior view. (m) Proximal metatarsus and distal tarsals of MPC-D 102/12 in proximal view. (n) Tibia, fibula and pes of MPC-D 102/11 in ventral view. acr, acromion process; astr, astragalus; bt, biceps tubercle; calc, calcaneum; cor, coracoid; corf, coracoid foramen; cr, cervical rib; crtb, ceratobranchial; cs, caudosacral vertebrae; dent, dentary; dist, distal end; dor, dorsal vertebrae; ds, dorsosacral; dt III, distal tarsal III; dt IV, distal tarsal IV; epi, epipophysis; epic, epicleidum; fib, fibula; for, foramen; fr, frontal; glen, glenoid; gt, greater trochanter; h, head; hypc, hypocleidum; idpf, infradiapophyseal fossa; il, ilium; isc, ischium; I–IV, pedal digits I–IV; lat, lateral trabecula; mt II–IV, metatarsals II–IV; mx, maxilla; nas, nasal; ns, neural spine; p1–p3, pygostyle vertebrae 1–3; par, parietal; ?path, possible pathology; pbt, pubic boot; pdp, proximodorsal process; pl, pleurocoel; pmx, premaxilla; pop, popliteal fossa; post, postorbital; prox, proximal end; pub, pubis; q, quadrate; qj, quadratojugal; sac, sacral vertebrae; scap, scapula; spdf, supradiapophyseal fossa; stcp, sternocoracoidal process; tib, tibia; tp, transverse process. |
Theropoda Marsh 1881
Oviraptorosauria Barsbold 1976
Oviraptoridae Barsbold 1976
Heyuanninae (=Ingeniinae) Barsbold 1981
Oksoko avarsan gen. et sp. nov.
Diagnosis. Oksoko avarsan is a small oviraptorid oviraptorosaur distinguished from other oviraptorosaurs by the following suite of autapomorphies (*) and other characters: apically thickened, dome-shaped cranial crest composed equally of nasals and frontals (figure 2)*; nasal recesses housed in a depression; postorbital with dorsally directed frontal process; cervical vertebrae with large epipophyses; functionally didactyl manus (figure 3)*; accessory ridge of brevis fossa of ilium*; anteriorly curving pubis; and large proximodorsal process of distal tarsal IV.
Etymology. Oksoko (pronounced ‘Oak-soak-oh') from the three-headed eagle of Altaic mythology, in reference to the fact that the holotype assemblage preserves three skulls; the specific name avarsan is from the Mongolian word ‘аварсан' (avarsan: rescued), reflecting their confiscation from poachers and/or smugglers.
Holotype. Institute of Paleontology, Mongolia (MPC-D) 102/110a, a nearly complete juvenile skeleton missing only the distal half of the tail (figures 1–3), preserved in an assemblage of four individuals.
Referred specimens. MPC-D 100/33, partial subadult postcranial skeleton; MPC-D 102/11, partial juvenile skeleton with skull; MPC-D 102/12, adult postcranial skeleton; MPC-D 102/110b, nearly complete juvenile skeleton; MPC-D 102/110c, partial juvenile postcranial skeleton (figures 1–3).
Localities and Horizon. Bugiin Tsav and Guriliin Tsav, Nemegt Basin. Nemegt Formation (lower Maastrichtian).
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| Figure 3. Forelimb elements of Oksoko avarsan. (a,b) Left humerus of MPC-D 100/33 in anterior (a) and lateral (b) views. (c) Right radius, ulna and manus of MPC-D 102/110.a in lateral view. (d) Left carpals and manus of MPC-D 102/110.a in dorsal (extensor) view. (e) Left metacarpal III of MPC-D 102/110.a in medial view. (f) Left manual phalanx III-1 of MPC-D 102/110.a in lateral (top), proximal (right), dorsal (bottom) and distal (left) views. (g) Semilunate carpal and sesamoid ossicles of MPC-D 102/110.a in proximal view. (h) Range of motion of digits I (white) and II (grey) of Oksoko avarsan based on manual manipulation, in full extension (top) and full flexion (bottom). dc, distal condyle; dist, distal end; dp, dorsal process; dpc, deltopectoral crest; ecte, ectepicondylar tuber; ente, entepicondylar tuber; h, head; I-1, manual phalanx I-1; I-2, manual ungual I-2; II-1, manual phalanx II-1; II-2, manual phalanx II-2; II-3, manual ungual II-3; III-1, manual phalanx III-1; mc I–III, metacarpals I–III; ole, olecranon; prox, proximal end; rad, radius; rade, radiale; sed, sediment; ses, sesamoid ossicles; slc, semilunate carpal; uln, ulna. Scale bars as indicated. |
Gregory F. Funston, Tsogtbaatar Chinzorig, Khishigjav Tsogtbaatar, Yoshitsugu Kobayashi, Corwin Sullivan and Philip J. Currie. 2020. A New Two-fingered Dinosaur Sheds Light on the Radiation of Oviraptorosauria. Royal Society Open Science. DOI: 10.1098/rsos.201184
Newly Discovered Toothless, Two-Fingered Dinosaur’s Lost Digits Point to Spread of Parrot-Like Species
