A new maniraptoran femur with alvarezsaurian affinities from the Plottier Formation (Coniacian-Santonian), northern Patagonia

A new specimen, MAU-Pv-PH-453, comprising an isolated femur from the Plottier Formation (Coniacian-Santonian), northern Patagonia, is described here. Its characteristics, i.e., an anteriorly curved diaphysis; fused lesser and greater trochanters forming a trochanteric crest; a prominent medially directed head; and the absence of a fourth trochanter, allow us to identify it as belonging to a maniraptoran theropod. Among Patagonian maniraptorans, MAU-Pv-PH-453 shares similarities with alvarezsaurians, such as the absence of a posterior trochanter, a trochanteric shelf, and the fourth trochanter. However, in Patagonian alvarezsaurs, the lesser and greater trochanters are generally separated by a cleft. Notably, MAU-Pv-PH-453 exhibits some features seen in femora of parvicursorine alvarezsaurids, including a proximally projected trochanteric crest and an L-shaped profile in proximal view. However, MAU-Pv-PH-453 presents differences with parvicursorines, such as a knob partially separating the lesser and greater trochanters. Phylogenetically, MAU-Pv-PH-453 may have parvicursorine affinities, although its fragmentary nature might generate a bias in its phylogenetic position. Due to the lack of more diagnostic characters, MAU-Pv-PH-453 is assigned to Alvarezsauria indet. This material represents the second theropod record from the Plottier Formation and it could fill a temporal gap (between Coniacian and Santonian) in the record of Upper Cretaceous Patagonian alvarezsaurians.