Silhouette of Overoraptor chimentoi gen. et sp. nov. (MPCA-Pv805) showing selected skeletal elements. From Motta et al., 2020.
The fossil record of basal paravians in Gondwana is restricted to a relatively small number of taxa. South American paravians are included within the clade Unenlagiidae. Overoraptor chimentoi, a new specimen paravian from Cenomanian-Turonian beds of Patagonia differs morphologically from unenlagiids and other non-avialan paravians. The new taxon comes from the Huincul Formation. This geological unit has yielded remains of different theropod clades including several abelisaurid theropods like Skorpiovenator, Tralkasaurus and Ilokelesia.
Overoraptor was a gracile theropod that reached about 1.3 m in total length. The name derived from the Spanish word “overo”, meaning piebald, in reference to the coloration of the fossil bones (a pattern of light and dark spots), and the word “raptor” from the Latin for thief. The species name honors Dr. Roberto Nicolás Chimento, who discovered the specimen. The holotype (MPCA-Pv 805) and paratype (MPCA-Pv 818) specimens of O. chimentoi were found in a quarry in association with disarticulated crocodilian and turtle bones.
Comparative image of the right scapulacoracoid of Bambiraptor, Buitreraptor, Overoraptor, and Archaeopteryx in lateral view. Image not to scale. From Motta et al., 2020.
Overoraptor exhibits the following features: posterior caudal centra with a complex system of lateral longitudinal ridges concavities (also present in Buitreraptor and Rahonavis); the scapula is proximally stout; the glenoid fossa is cup-shaped (also observed in Archaeopteryx and Jeholornis, but is absent in unenlagiids); the ulna is robust, and the posterior margin of the ulna is longitudinally convex so that the ulna is bowed as in most basal paravian; the radial process of Overoraptor has a saddle-shaped radial cotyle proximally (a condition also present in modern birds and some basal paravians as Bambiraptor); metacarpal I with extensive medioventral crest; metatarsal II with longitudinal lateroventral crest on distal half, ending distally in a posterior tubercle; absence of a distal ginglymoid articular surface on metatarsal III; and the foot exhibits the characteristic raptorial digit II. The unusual combination of a plesiomorphic hindlimb, with features that are correlated with cursorial habits (the characteristic raptorial digit II, unfused metatarsals, and poorly curved ungual phalanx I); and the more derived forelimb, with features that show some adaptations related to active flight, placed Overoraptor, together with Rahonavis in a clade that is sister to Avialae.
References:
Motta, M.J., Agnolín, F., Brissón Egli, F. et al. New theropod dinosaur from the Upper Cretaceous of Patagonia sheds light on the paravian radiation in Gondwana. Sci Nat 107, 24 (2020). https://doi.org/10.1007/s00114-020-01682-1
Letters from Gondwana. 