UNENLAGIID AFFINITIES FOR IMPEROBATOR ANTARCTICUS (PARAVES: THEROPODA): PALEOBIOGEOGRAPHICAL IMPLICATIONS

Abstract

Imperobator antarcticus is a paravian dinosaur based on fossil remains of the left and right hind limbs from the Cape Lamb Member (lower Maastrichtian) of the Snow Hill Island Formation, Antarctica. The only known specimen includes the tibia, astragalus, calcaneum, fragments of metatarsals, and non-ungual and ungual phalanges. It was originally described as a paravian with uncertain affinities, as it exhibits a mosaic of characters, such as the fusion of the calcaneum and fibula, a non-ginglymoid metatarsal II, and the absence of hyper-specialization of pedal digit II, forming the raptorial digit typical of deinonychosaurian dinosaurs. Previous authors indicate that it may be related to dromaeosaurids and unenlagiids. Imperobator was included in the TWiG data matrix to recognize its phylogenetic affinities. This comprehensive dataset was further improved by re-scoring and reanalysis of most unenlagiids. Additionally, 11 new taxa (including Imperobator and South American paravians like Pamparaptor and Overoraptor) were added, resulting in a data matrix of 175 taxa and 850 characters. The STAR 1 methodology proposed by previous authors was followed to construct phylogenetic trees. As a result, Imperobator antarcticus is nested within the Unenlagiidae clade. Despite its incomplete nature, Imperobator exhibits a subarctometatarsalian pes, a condition regarded as a Unenlagiidae synapomorphy by the present analysis. In dromaeosaurids, such as eudromaeosaurians and halszkaraptorines, metatarsal III is not lateromedially compressed, shows subparallel medial and lateral margins, and is not constricted between metatarsals II and IV. Imperobator antarcticus constitutes an important addition to the list of Late Cretaceous dinosaurs shared by South America and Antarctica.