Monday, June 8, 2020

[Paleontology • 2020] Overoraptor chimentoi • New Theropod Dinosaur from the Upper Cretaceous of Patagonia sheds light on the Paravian Radiation in Gondwana

Overoraptor chimentoi
Motta, Agnolín, Egli & Novas, 2020

llustration: Gabriel Lio

The fossil record of basal paravians in Gondwana is still poorly known, being limited to the Cretaceous unenlagiids from South America and the problematic Rahonavis from Madagascar. Here we report on a new paravian from the Cenomanian-Turonian (Late Cretaceous) of Río Negro province, NW Patagonia, Argentina. The new taxon exhibits a derived bird-like morphology of the forelimbs (e.g., robust ulna with prominent, anteriorly oriented, and proximally saddle-shaped radial cotyle and wide medial flange on metacarpal I) and a plesiomorphic foot with a raptorial pedal digit II. Phylogenetic analysis recovers the new taxon in a monophyletic clade with Rahonavis, being the sister group of the remaining Avialae and more derived than other non-avian dinosaurs. Both exhibit derived forelimb traits in opposition with their plesiomorphic hind limbs. The position of the new taxon and Rahonavis as stem avialans indicates that Gondwanan basal paravians are represented by two different clades, at least. The new taxon probably constitutes a previously unknown grade in the avian-line theropods in which some flight-related adaptations of the forelimbs are present in cursorial taxa. The present discovery sheds light on the acquisition of flight-related traits in non-avian dinosaurs and on the still poorly known paravian radiation in Gondwana.

Keywords: Theropoda, Paraves, South America, Upper Cretaceous

Fig. 1: Silhouette of Overoraptor chimentoi gen. et sp. nov. (MPCA-Pv 805) showing selected skeletal elements.
(a) Pelvic girdle elements including fragments of ilium in lateral (top) view and left pubis in medial (left) view, (b) right scapula in dorsal (top) and lateral (bottom) views, (c) mid-caudal vertebra in left lateral view, (d) right ulna in lateral (bottom) and proximal (top) views, (e) right metacarpal I in dorsal (top), medial, ventral, and distal (bottom) views, (f) right manual phalanx I-1 and manual ungual of digit I in medial view, (g) left pedal phalanx II-2 and left pedal ungual of digit II in medial view, (h) left metatarsal II in lateral (left), dorsal (right), and distal (bottom) views, (i) left pedal phalanx I-1 and pedal ungual of digit I in medial view, (j) left metatarsal III in dorsal (right), lateral (left) and distal (bottom) views.
 Abbreviations: a anterior, acr acromion, cg collateral groove, cp collateral pit, ft flexor tubercle, gf glenoid fossa, ip ischium pedicle of ilium, l lateral, lc lateral crest, lr lateral ridge, lvc lateroventral crest, ldc laterodorsal crest, mvc medioventral crest, rp radial process of ulna, sra surface for radial articulation, tc bump representing the m. tibialis cranialis insertion, tsb tapered scapular blade, pa pubic apron, pr protuberance, pt proximal tubercle, vh ventral heel. Scale bar equals 50 cm in the silhouette; 4 cm in a, b, d, h, and j; and 2 cm in c, e, f, g, and i

Systematic paleontology
SAURISCHIA Seeley, 1888

THEROPODA Marsh, 1881
MANIRAPTORA Gauthier, 1986

PARAVES Sereno, 1997

Overoraptor chimentoi gen. et sp. nov.

Etymology: Overo from “overo” the Spanish word meaning piebald, in reference to the coloration of the O. chimentoi bones, which consists of a pattern of light and dark spots, and raptor from the Latin for thief. The species name chimentoi honors its discoverer, the paleontologist Dr. Roberto Nicolás Chimento.

Diagnosis: The new taxon differs from other paravians in the following combination of characters (autapomorphies are marked by an asterisk): posterior caudal centra with a complex system of lateral longitudinal ridges and concavities (also present in Buitreraptor and Rahonavis); scapula with a medially deflected distal end*; acromial process reduced and ridge-like*; robust ulna; radial cotyle of ulna saddle-shaped and prominent; metacarpal I with extensive medioventral crest*; metatarsal II with longitudinal lateroventral crest on distal half, ending distally in a posterior tubercle* (present on both specimens); metatarsal III distal end non-ginglymoidal (also present in Pamparaptor); metatarsal III distal end dorsoventrally deeper than transversely wide*; and strongly dorsally displaced collateral pits on pedal phalanx II-2.


Matías J. Motta, Federico L. Agnolín, Federico Brissón Egli and Fernando E. Novas. 2020. New Theropod Dinosaur from the Upper Cretaceous of Patagonia sheds light on the Paravian Radiation in Gondwana. The Science of Nature. 107, 24.  DOI: 10.1007/s00114-020-01682-1