New caenagnathid (Theropoda: Oviraptorosauria) dinosaur specimens from middle and upper Campanian strata of West Texas

Abstract

New caenagnathid dinosaur specimens from the upper Aguja Formation of West Texas comprise the most complete examples yet described from southern North America. Two individuals are represented. Osteohistology indicates that both were mature at their times of death. Although they share no overlapping elements, the two individuals are separated by overall body size, paleohabitat, and stratigraphic position and so they almost certainly represent different species. One individual consists of a partial hindlimb, but exhibits too few (3) characters to be phylogenetically informative and is, therefore, referred to an indeterminate caenagnathid – possibly one of two species previously recognized in coastal facies of the Aguja Formation. The second individual is more complete. It was recovered higher in section from more inland fluvial paleoenvironments and preserves fragmentary elements from throughout the postcranial skeleton. A histology-based growth model – the first for a caenagnathid – indicates that the second individual required at least five years to approach fully adult size. However, only 11 character states could be derived from its preserved remains. Phylogenetic analyses found this specimen deeply nested within Caenagnathidae, but were otherwise inconclusive and so it, too, is referred to an indeterminate caenagnathid. Nevertheless, several combined morphologies (e.g., absence of cervical epipophyses, dorsoventral depth of the hypapophysis on the second cervicodorsal vertebra, and greater trochanter of the femur only weakly separated from the femoral head) implies that the second individual represents an unknown species. One critical aspect of both specimens is that they provide significant new comparative material representing the sparse ‘southern’ caenagnathid record.