New insights into Unenlagiine (Theropoda: Paraves: Dromaeosauridae) diversity, paleoecology, and paleobiogeography derived from Late Cretaceous fossils from Patagonia and Antarctica

LAMANNA, M.; PORFIRI, J.; BAIANO, MATTIA A.; DOS SANTOS, D. ; GIANECHINI, FEDERICO A.; CASE, J.

Cincinnati

Congreso; 83rd Annual Meeting of the Society of Vertebrate Paleontology; 2023

Unenlagiine dromaeosaurids are among the most bird-like of Gondwanan non-avian theropoddinosaurs, yet the fossil record of these paravians is highly incomplete, with most taxa represented by fragmentary remains and/or separated by lengthy temporal gaps. Here we report a new unenlagiine taxon from the Late Cretaceous of northern Patagonia and reinterpret the enigmatic, similarly aged Antarctic paravian Imperobator antarcticus as a probable member of Unenlagiinae. The new Patagonian taxon comes from the Santonian Bajo de la Carpa Formation of the Neuquén Basin of Neuquén Province, Argentina, thereby filling a substantial stratigraphic gap in the unenlagiine fossil record. Although known only from a very incomplete postcranial skeleton, the bones of the new taxon exhibit morphological differences from corresponding elements in other unenlagiines. Moreover, in several aspects, the humerus of the new form appears intermediate between those of geologically older unenlagiines from the Neuquén Basin (e.g., Unenlagia spp. from the Turonian–Coniacian Portezuelo Formation) and that of the stratigraphically younger, larger-bodied Austroraptor cabazai from the Campanian–Maastrichtian Allen Formation, potentially indicating a transitional stage in unenlagiine evolution. Furthermore, this humerus offers paleoecological information in exhibiting subcircular punctures near its distal end that are interpreted as feeding traces most likely left by a conical-toothed crocodyliform or theropod.The holotype of Imperobator was collected from an uppermost Campanian–lowermost Maastrichtian (ca. 72 Ma) exposure of the Cape Lamb Member of the Snow Hill Island Formation on the Naze Peninsula of James Ross Island, Antarctic Peninsula. A recent phylogenetic analysis recovered this taxon as a paravian of uncertain position within that maniraptoran clade. Based in part on our study of previously undescribed material, including multiple craniomandibular fragments, we tentatively reinterpret Imperobator as an exceptionally large-bodied unenlagiine comparable in size to Austroraptor. If correct, this would extend the paleobiogeographic range of Unenlagiinae to include Antarctica—thereby enhancing the already considerable similarities between the latest Cretaceous dinosaur faunas of southern South America and the Antarctic Peninsula—and furthermore, demonstrate that unusually large unenlagiines inhabited the Gondwanan high latitudes.