A new theropod assemblage from the Late Cretaceous of Western Gondwana, La Rioja Province, Argentina

LUCAS E. FIORELLI; MARTÍN EZCURRA; MARTÍN HECHENLEITNER; DIEGO POL; GERALD GRELLER-TINNER

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

International Palaeontological Association

Late Cretaceous vertebrate assemblages are well known in southern Argentina, but they areconsiderably less sampled in the north of the country. Indeed, Cretaceous dinosaur-bearing beds inLa Rioja Province (NW Argentina) were virtually unknown until the discovery in 2007 of dinosaureggshells and subsequently the Sanagasta sauropod-nesting site in beds of the Los Llanos Formation. Recent field work in the Tama locality of this unit resulted in the discovery of a new anddiverse terrestrial vertebrate assemblage in a semi-arid palaeoenvironment with an active eoliansedimentation. These vertebrate remains are associated with Late Cretaceous microfossils(ostracods and charophytes) and include turtles, notosuchian crocodyliforms, and titanosaur,ornithischian and theropod dinosaurs. Four theropod specimens (CRILAR-Pv 501, 503, 505, 524) have been collected and their phylogenetic affinities are reported here. CRILAR-Pv 501 isrepresented by a partial left frontal and fragments of both prootics. This specimen is assigned toAbelisauridae because it possesses a dorsoventrally thick frontal with a coarse dorsalornamentation, formed by low tubercles, and a median fossa between both supratemporal fossae.The prootics preserve partial natural casts of the innear ear and the osteological correlate of a smallflocculus, which may represent an additional abelisaurid apomorphy. CRILAR-Pv 524 consists of a partial pelvic girdle that was found in association with CRILAR-Pv 501 and may belong to thesame individual. In agreement, this pelvic girdle resembles that of abelisaurids in the presence ofco-ossified bones and a straight dorsal margin of the ilium. CRILAR-Pv 503 includes the proximalend of a right pubis and fragments of femoral, tibial, and fibular shafts. This specimen is assigned toManiraptora because it possesses a preacetabular tubercle and lacks a pubic apron on the proximal half of the bone. Lastly, CRILAR-Pv 505, a distal end of tibia with a weathered, articulated astragalus, is interpreted as an indeterminate averostran because of the presence of anteriorly projected condyles on the astragalar body. Within Averostra, this specimen resembles some abelisauroids, megaraptorans and coelurosaurs in the absence of a distinct step for the reception of the ascending process of the astragalus on the anterior surface of the tibia. The taxonomic composition of the theropod assemblage of the Los Llanos Formation contains theropod groups recorded in other Jurassic-Cretaceous beds of Gondwana and, as a result, is consistent with the Late Cretaceous dating of the unit recently suggested by micropalaeontological evidence and not with previous claims of a Cenozoic age.