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Postcranial Anatomy of a Basal Hadrosauroid (Dinosauria: Ornithopoda) from the Cretaceous (Cenomanian) Woodbine Formation of North Texas
Derek J. Main, Christopher R. Noto, and David B. Weishampel
ABSTRACT
New fossil material recovered from the Arlington Archosaur Site (AAS; North Arlington, Tarrant County, Texas) includes the most complete hadrosauroid (Dinosauria: Ornithopoda) recovered from the Woodbine Formation to date, and allows for a detailed study of the postcranial anatomy of a basal hadrosauroid. The depositional setting of the AAS was a delta plain, and the host facies comprise fine-grained mudstones and claystones overlying a dense peat bed. The ornithopod fossil-bearing horizon occurs within a paleosol: a clay-rich, heavily rooted histic gleysol with numerous calcareous concretions. Hadrosauroid fossils recovered include cervical, dorsal, and caudal vertebrae; ribs, a scapula and coracoid; an ilium, ischium, and pubis; and a partial juvenile femur. Most of the remains represent a single adult individual, which experienced a complex biostratinomic and fossil-diagenetic history following death. The predominance of relatively dense bones from the axial column and limbs, and disarticulation are indicative of hydraulic sorting and limited transport of the remains. The new hadrosauroid material from the Woodbine Formation demonstrates a unique mix of plesiomorphic hadrosaurid characters and derived iguanodontian features. Whereas its exact taxonomic affinity cannot be determined presently, it is expected that this specimen will prove informative in understanding hadrosauroid evolution.
INTRODUCTION
The Arlington Archosaur Site (AAS) is located in the Dallas–Fort Worth Metroplex of North Texas, within the suburban city of Arlington, in Tarrant County (Fig. 5.1). It was discovered independently in 2003 by University of Texas, Arlington, students Phil Kirchhoff and Bill Walker, and local fossil collector Art Sahlstein. Among the first fossils found and reported were crocodyliform and ornithopod dinosaur remains, leading one of us (DJM) to propose the site’s name. Unfortunately, the landowner at the time of discovery refused to grant land access. From 2003 to 2007, the 2200 acres of fossil-rich land in northern Arlington was unavailable for paleontological exploration and scientific study. It was not until a new landowner, the Huffines family, purchased the land in 2007 that formal exploration and excavation began.
Amateur fossil hunters have collected scattered dinosaur remains in the Woodbine Formation for many years, but little of this material is described in the scientific literature. Isolated skeletal remains of theropods, nodosaurid ankylosaurs, and iguanodontian ornithopods are reported by Lee (1997a) from multiple localities in the greater Dallas–Fort Worth metroplex. A proximal left femur, complete tibia, and fibula are described from a locality in the Lewisville Member of the Woodbine Formation, and a humerus and teeth have been described from the overlying Arlington Member. All of this material is assigned to indeterminate hadrosaurids (Lee, 1997a). Ornithopod tracks from the Woodbine are described by Lee (1997b), and assigned to the ichnotaxon Caririchnium protohadrosaurichnos. The best known ornithopod from the Woodbine Formation, Protohadros byrdi, was discovered at Flower Mound in 1994, and was later named in honor of its discoverer, Dallas Paleontological Society member Gary Byrd (Head, 1998). Protohadros byrdi is based on a nearly complete skull and scant postcrania from the Arlington Member of the Woodbine Formation (Head,