Hadrosaurs. David A. Eberth
whereas it was originally described as the basal-most member of Hadrosauridae, subsequent analyses have recovered it as a non-hadrosaurid hadrosauroid (Norman, 2002; Horner et al., 2004; Prieto-Márquez, 2010a).
Given the proximity of fossiliferous strata of the Woodbine Formation to a major and expanding metropolitan area, it is surprising that more significant fossil material has not been recovered from the unit. Such material would likely provide valuable insight into the turnover of taxa and the origin of major clades (e.g., hadrosaurids) that occurred during the middle Cretaceous, for which there is little fossil evidence from North America (Jacobs and Winkler, 1998).
This contribution describes the stratigraphy, depositional setting, taphonomy, and postcranial anatomy of a basal hadrosauroid from the Woodbine Formation, and thus, further elaborates the nature of the formation’s hadrosauroid fauna. The new ornithopod material consists of specimens UTA-AASO-2003 and UTA-AASO-125 (University of Texas, Arlington–Arlington Archosaur Site field numbers), now stored in the collections of the Perot Museum of Nature and Science, Dallas, Texas. (Field numbers provided by C. R. Noto, August 2013. At the time of publication all material described here has been received by the Perot Museum of Nature and Science, Dallas, Texas, where it will be accessioned and assigned specimen numbers [A. R. Fiorillo, pers. comm., August 2013]). All specimens described here were recovered from the AAS, intermittently, from 2003 to 2008. The adult material includes a femur; scapula; coracoid; ilium; ischium; pubis; ribs; an axis; and cervical, dorsal, and caudal vertebrae. Although an assemblage of hadrosauroid postcrania such as this is potentially important for assessing phylogenetic relationships (Brett-Surman, 1979; Davies, 1983; Maryànska and Osmólska, 1984; Casanovas et al., 1999; Prieto-Márquez et al., 2006; Brett-Surman and Wagner, 2007; Poole, 2008; McDonald et al., 2010; Campione, this volume) we are not yet able to determine whether the AAS ornithopod material represents a known taxon, such as Protohadros byrdi, or a new taxon. Such conclusions must await further discoveries and research.
5.1. Location of the Arlington Archosaur Site (AAS) on a Texas map, with outlines of counties in which Woodbine Formation exposures occur. The AAS study area is in the city of Arlington, in northeastern Tarrant County.
Institutional Abbreviations SMU, Shuler Museum of Paleontology, Southern Methodist University, Dallas, Texas; UTA-AASO, University of Texas, Arlington–Arlington Archosaur Site, Arlington, Texas.
Anatomical Abbreviations ac, acetabulum; ap, pseudoacromion process; C, coracoid; ccv, cranial convexity; cf, coracoid foramen; cp, capitulum; cr, coracoid ridge; crp, cranial pubic process; cs, coracoid suture; ctr, base of cranial trochanter; dia, diapophysis; dr, deltoid ridge; eb, expanded boot; fn, femoral neck; ftr, fourth trochanter; gl, glenoid; gtr, greater trochanter; haf, hemal arch facet; IL, ilium; IS, ischium; ilp, iliac peduncle; isp, ischial peduncle; obp, base of obturator process; od, odontoid process; P, pubis; pap, preacetabular process; par, parapophysis; pop, postacetabular process; poz, postzygapophysis; prz, prezygapophysis; pup, pubic peduncle; R, rib; S, scapula; sac, suprailiac crest; scl, scapula labrum; scs, scapula suture; svf, facets for sacral vertebrae; tb, tuberculum; V, vertebra; vl, ventral lip.
GEOLOGIC SETTING
Woodbine Formation and Members
R. T. Hill was the first geologist to map and describe the Cretaceous of Texas, which extends from North Texas to the Big Bend region (Hill, 1901; Alexander, 1976), and named the Woodbine Formation of North Texas for the village of Woodbine in Cooke County. Woodbine exposures extend as an irregular and narrow, north–south-oriented band from Cooke to Johnson counties (Johnson, 1974; Fig. 5.1). Woodbine sediments were sourced from the Ouachita Mountains in southern Oklahoma and were deposited in fluvial, deltaic, and shelf environments in coastal plain and shoreline settings of the subsiding East Texas Basin (Dodge, 1952; Oliver, 1971; Trudel, 1994). The Woodbine Formation unconformably overlies the Grayson Marl of the Washita Group and, in turn, is overlain conformably by the Eagle Ford Group (Dodge, 1952, Oliver, 1971; Fig. 5.2). A minimum age for the Woodbine Formation has been determined using the ammonite biostratigraphy of the overlying Eagle Ford Group, which lies within the Conlinoceras tarrantense zone at the base of the middle Cenomanian. This approach indicates that the Woodbine is no younger than 95 Ma (Kennedy and Cobban, 1990; Head, 1998).
5.2. North Texas Cretaceous stratigraphy, with the Woodbine Formation highlighted in gray. The Arlington Archosaur Site lies in the lower to middle Lewisville Member of the Woodbine Formation. The site consists of ~1.7 m of sediments representing a delta-plain environment. The adult ornithopod material discussed here was discovered in a paleosol overlying a carbonaceous peat bed containing crocodyliform, turtle, and juvenile ornithopod remains (not discussed herein). North Texas stratigraphic column modified after Jacobs and Winkler (1998).
The Woodbine Formation comprises four members. In ascending order these are Rush Creek, Dexter, Lewisville, and Arlington (Dodge, 1968, 1969; Fig. 5.2). The Rush Creek Member consists of a southern province distinguished by offshore bars and lagoonal deposits, and a northern province distinguished by lower shoreface mudstones and sandstones, and estuarine sandstones (Bergquist, 1949; Johnson, 1974). The Dexter Member consists of coastal-plain distributary and meander belt facies (Oliver, 1971). The Lewisville Member consists of deltaic sandstones and mudstones, and marine mudstones and shales deposited adjacent to active deltas (Oliver, 1971; Trudel, 1994; Main, 2005). The Lewisville Member is highly fossiliferous, containing mollusks, ammonites, and foraminifers (Trudel, 1994), and the material described here. A basal unit of lenticular, cross-bedded sandstones with localized lignite beds was designated as the Red Branch Member by Bergquist (1949), but subsequently was included as part of the Lewisville Member (Beall, 1964; Dodge, 1969; Trudel, 1994). The Arlington Member comprises nonmarine delta plain deposits dominated by fluvial sandstones (Murlin, 1975). The type locality of Protohadros occurs within this member (Head, 1998).
Stratigraphy and Paleoenvironments of the AAS
The AAS occurs within the Lewisville Member of the Woodbine Formation based on lithostratigraphic and biostratigraphic criteria. The presence of the invertebrates Anchura and Gyrodes place the AAS in the lower to middle strata of the Lewisville Member (Stephenson, 1952). The palynomorphs Dichastopollenites and Stellatopollis, recovered from the AAS, affirm a mid-Cenomanian age (May, 1975).
The sediments of the AAS suggest sedimentation in a low-lying coastal plain setting. All beds dip slightly to the east. The lowermost part of the section consists of rippled shallow marine sandstones that are heavily marked by traces of Diplocraterion, Thalassinoides, and Arenicolites (Skolithos ichnofacies [Seilacher, 2007]). Overlying these nearshore sandstones is a thin (50–60 cm thick) peat that has locally high claystone, sulphur, and pyrite content (Main, 2009). The peat is laterally continuous, cropping out along the base of adjacent hillsides. It is also highly fossiliferous, containing the mixed remains of numerous vertebrates, and carbonized wood, branches, twigs, and fallen logs (Main, 2009: Main et al., in press). Overlying the peat