A new basal ornithopod (Dinosauria: Ornithischia) from the Early Cretaceous of Texas.

Kate A Andrzejewski,Louis L Jacobs,Dale A Winkler,Catherine Forster

doi:10.1371/journal.pone.0207935

Abstract

Material from a minimum of twenty-nine individuals of a new ornithopod, represented by nearly every skeletal element, was recovered from the Proctor Lake locality in the Twin Mountains Formation (Aptian) of north-central Texas. This material includes various ontogenetic stages, providing insight into the growth patterns of this species. The new ornithopod, Convolosaurus marri gen. et sp. nov., is recovered outside of Iguanodontia, but forms a clade with Iguanodontia exclusive of Hypsilophodon foxii. The presence and morphology of four premaxillary teeth along with a combination of both basal and derived characters distinguish this taxon from all other ornithopods. Basal characters present in C. marri including the presence of premaxillary teeth, the shape of the dentary teeth, and position of the pterygoid wing on the quadrate, whereas the presence of opisthocoelous cervical vertebrae, large proximal caudal neural spines, and curved maxillary tooth roots suggest C. marri is more derived than 80% of the basal neornithischians included in this analysis.

Highlights

Abundant remains of a small ornithopod dinosaur were first discovered near Proctor Lake, Texas in May 1985 in sediments of the Lower Cretaceous Trinity Group (Fig 1)
Autapomorphies recovered for C. marri include the presence of four premaxillary teeth (112:2), opisthocoelous cervical vertebrae (134: 1); convergently present in Agilisaurus louderbacki [23] and Iguanodontia, an expanded ischial ‘foot’ (182:1); convergently present in Heterodontosaurus tucki [47] and members of Iguanodontia, and the presence of a medial sulcus on the buccal surface of the premaxillary teeth (233:1)
Iguanodontia is supported unambiguously by possessing diamond shaped maxillary and dentary crowns (115: 2), the absence of cingulum on the maxillary and dentary teeth (123:1); convergently present in Heterodontosaurus tucki [47], and a laterally inflated medial condyle which partial covers the opening of the flexor groove of the femur (204: 1)

Summary

Introduction

Abundant remains of a small ornithopod dinosaur were first discovered near Proctor Lake, Texas in May 1985 in sediments of the Lower Cretaceous Trinity Group (Fig 1). More dinosaur specimens have been recovered from this locality than from any other Early Cretaceous site in Texas. At least 29 individuals of a new ornithopod, including several larger articulated individuals and mass accumulations of partially articulated smaller individuals were recovered (Fig 2). Initial studies suggested that some mass accumulations of smaller individuals occur in depressions possibly reflecting a nesting site, no egg shell has been recovered [1], and that these groupings may have represented precocious flocks. The Proctor Lake locality sits stratigraphically low in the Twin Mountains Formation, the lowest of the three formations in the Trinity Group in this area (Twin Mountains, Glen Rose, Paluxy).

Methods

Results

Discussion

Conclusion