Achillesaurus manazzonei, a new alvarezsaurid theropod (Dinosauria) from the Late Cretaceous Bajo de la Carpa Formation, Río Negro Province, Argentina

Abstract

A new genus and species, Achillesaurus manazzonei gen. et sp. nov., of the enigmatic clade Alvarezsauridae (Theropoda, Coelurosauria), recovered from the Santonian Bajo de la Carpa Formation (Río Negro Province, Argentina), is here described. A. manazzonei is a relatively large alvarezsaurid different from Alvarezsaurus calvoi (from the same Age and Formation) in having a lateral fossa in the proximal caudal centra, a less developed supraacetabular crest, the brevis shelf not reaching the base of the ischial pedicel, and the lateral malleolus of the tibia at the same level of the medial one. Achillesaurus differs from Patagonykus puertai, from the Portezuelo Formation (Neuquén Province, Argentina), by the presence of an almost undeveloped supracetabular crest of the ilium and the unfused condition of the astragalus and the calcaneum. The new species is excluded from the Asian Mononykinae due to the unreduced fibula distally and a non-arctometatarsalian pes. The autapomorphies of Achillesaurus are the presence of a biconcave caudal vertebra (possibly the fourth) with the cranial surface 30% larger in diameter than the caudal one. The inclusion of Achillesaurus in a phylogenetic framework resulted in an unresolved polytomy among the new taxon, Alvarezsaurus, and Patagonykus plus Mononykinae, the latter clade being weakly supported. The result here presented shows a basal stem radiation of South American alvarezsaurids. New material of the Patagonian alvarezsaurids is necessary to evaluate relevant traits to test further the phylogenetic relationships of the basal alvarezsaurids.