Overoraptor (, meaning "piebald thief") is an extinct genus of paravian theropod of uncertain affinities from the Early Cretaceous Huincul Formation of Argentinian Patagonia. The genus contains a single species, O. chimentoi, known from several bones of the hands, feet, and hips alongside some vertebrae.
Discovery and naming
The fossil specimens of Overoraptor were unearthed by Dr. Roberto Nicolás Chimento in the Violante Farm locality, which is southeast of the Ezequiel Ramos-Mexía lake, in the northwestern portion of Rio Negro Province, Argentina. This outcrop is part of the Huincul Formation, which is dated to the Aptian stage in the Cretaceous period. This formation has yielded some of the largest South American dinosaurs including the massive sauropod Argentinosaurus and the theropod Mapusaurus. The precise date of the discovery of the type series of Overoraptor is not publicly known, but the specimens were reposited in the collections of the Carlos Ameghino Provincial Museum in Cipolletti, which is near the outcrop of the Huincul Formation where the specimens were discovered. The fossils were described as a new genus and species of paravian theropod in May 2020 by Matías Motta, Federico Agnolín, Federico Brissón Egli, and Fernando Novas in the journal The Science of Nature (or Naturwissenschaften). The holotype specimen, MACN-Pv 805, was disarticulated when it was found and it is relatively fragmentary. It consists of two phalanges and a metacarpal of the right hand, two hemal arches, the right scapula, the right ulna, a partial ilium, a partial pubis, two metatarsals from each foot, and several phalanges and unguals from the left foot. The paratype specimen, MPCA-Pv 818, was discovered at the same locality in association with the holotype. It is about 20% smaller than the holotype, which led the researchers describing it to conclude that they were likely not from a fully-grown animal. The paratype consists of two phalanges from the right hand, a fragment of the ilium, the right pubis, one of the right metatarsals, and one pedal phalanx from each foot. The generic name, Overoraptor, comes from the Spanish word "overo", meaning "piebald"—referencing the coloration of the fossils—and the Latin word "raptor", meaning "thief". The specific epithet, "chimentoi", was given in honor of Roberto Nicolás Chimento, who initially discovered its remains.
Description In their 2020 description of the genus, Motta and colleagues estimated that Overoraptor would have been about 1.3 metres (4.3 ft) in length, although they do not provide an estimate of the animal's mass. Gregory S. Paul estimated the size of Overoraptor to be about 3 metres (9.8 ft) long and weighing about 30 kilograms (66 lb). Motta and colleagues diagnosed Overoraptor as being distinct from all other paravians from the following unique autapomorphies: a medial deflection at the distal end of the scapula, a reduced and ridge-like acromial process, an extensive crest on metacarpal I, a crest ending in a distal posterior tubicle on metatarsal II, and a metatarsal III that is taller than it is wide at the midpoint along its length. Motta and colleagues further diagnosed the genus by the co-occurrence of the following autapomorphies: lateral ridges on the caudal centra, a robust ulna, a saddle-shaped radial cotyle on the ulna, a non-hinged joint at the distal end of metatarsal III, and dorsally displaced pits on the second toe. In the description of its osteology, Motta and colleagues considered both the holotype and paratype together, rather than describing each specimen individually. The scapular anatomy of Overoraptor resembles that of stem-avialans in the presence of a cup-shaped glenoid fossa, but it differs from most other paravians in that the acromial process projects dorsally rather than medially. The overall morphology of the scapula resembles the enigmatic paravian taxon Rahonavis because the scapular blade projects medially outward at its distal-most end. The ratio of the lengths of the ulna to the metacarpals is similar to the ratio found in dromaeosaurs and less than that of more derived avialans. The ulna itself is also curved, which is the plesiomorphic condition in paravians, unlike the straight ulnae of Anchiornis and related taxa. However, the ulna of Overoraptor resembles the derived avialan condition in the presence of a saddle-shaped cotyle on the ulnar-radial articular surface. The metacarpal bones of Overoraptor were noted by Motta and colleagues to be unusual in shape, namely that it is proportionally much wider and shorter than those known from other non-avialan paravians and there is a distinctly asymmetrical ginglymoidal surface which articulates with the lateral condyle. The manual phalanges are slender and elongate, which is common in the paravian taxa that retain their fingers. Similarly, the manual unguals are mediolaterally compressed, sharp, and curved, similar to the condition seen in dromaeosaurs. The foot bones of Overoraptor are sub-arctometatarsalian, meaning the middle metatarsal narrows at the medial end but it is not completely pinched together by the other two bones of the ankle. The foot also bears the archetypal raptorial second digit (the characteristic toe claw of dromaeosaurs), which is short in length and proportionally robust relative to other taxa with this claw.
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