INTRODUCTION

Sauropod fossils from the Jones Ranch locality, north-central Texas are intriguing because these remains are found within 25 km of numerous sauropod tracks and trackways (e.g., Shuler 1937, Langston 1974, Farlow 1987). Sauropod ichnofossils have been well documented in North America, but associations between skeletal remains and trace fossils have not been reported. Moreover, the Jones Ranch locality represents one of the richest known accumulations of Early Cretaceous sauropod remains in North America.

North American sauropods were most diverse during the Late Jurassic period (Hunt et al. 1994, Maxwell and Cifelli 2000). Sauropod discoveries in North America from Cretaceous strata, however, are sparse, with the exception of ichnofossils (Weishampel et al. 2004). There is no record of Cretaceous sauropods in North America prior to the Barremian, but they are known from the Barremian or Aptian through the Albian and possibly into the Cenomanian (Kirkland et al. 1998, Maxwell and Cifelli 2000). Sauropods then disappear from the North American record until the Maastrichtian (Lucas and Hunt 1989). Furthermore, much of what is known of Early Cretaceous sauropods is based on fragmentary and unassociated elements, making detailed comparisons difficult. Consequently, the taxonomy and systematics of Early Cretaceous sauropods from North America are problematic, which has serious implications for biogeographic studies. Correlating among different Early Cretaceous faunas is made difficult by the lack of precise ages for sauropod-bearing strata of that interval in North America (Jacobs and Winkler 1998).

Until the late twentieth century, most Early Cretaceous North American sauropod material was referred to Pleurocoelus or Astrodon. The topotypic material for two species of Pleurocoelus and the only species of Astrodon is from the Lower Cretaceous Arundel Formation of Maryland. Astrodon johnstoni was the first sauropod described from North America and was named based upon isolated teeth (Leidy 1865). Marsh (1888) later described two species of the taxon Pleurocoelus, P. nanus and P. altus, on the basis of isolated, fragmentary, and mostly juvenile remains. The type material for P. nanus consists of four vertebrae, comprising a cervical, dorsal, sacral, and a caudal. Isolated sauropod bones from the Arundel Formation were referred to Pleurocoelus largely based on proximity of the localities and the size of the bones. Langston (1974) and Gallup (1989) previously referred sauropod specimens from the Lower Cretaceous of Texas to Pleurocoelus.

Nearly all of the vertebrae known from the Arundel sauropod lack neural arches, and evidence suggests that neurocentral fusion was absent, indicating they are from juvenile individuals. Consequently, the Arundel vertebrae lack diagnostic features and are not easily distinguishable from those of other sauropods, particularly taxa for which juvenile vertebrae are not known. Serial morphological variability in sauropod vertebral columns makes direct comparisons of vertebrae from different positions difficult.

Isometric growth has been documented in the appendicular skeleton of some sauropods (Carpenter and McIntosh 1994, Wilhite 1999, 2005, Tidwell and Wilhite 2005). According to Wilhite (1999), with few exceptions, during ontogeny sauropod limb breadth measurements grow at a constant rate of approximately one-third the rate of limb length. Thus, limb proportions and morphology change little between juveniles and adults of the same taxon, and there is good potential that well-preserved, isolated sauropod appendicular elements can be confidently identified to family or genus level (Wilhite 2005). Unlike the type specimens of Pleurocoelus, the juvenile appendicular elements, referred to the latter taxon, can be differentiated from several other sauropod genera based on their morphology and limb proportions (personal observations; Appendix 1, Appendix 2).

Ostrom (1970) described sauropod remains from the Cloverly Formation of Wyoming and Montana. Until the last decade, the Cloverly sauropod and the Arundel taxon remained the only Early Cretaceous sauropods in North America known from more than a few isolated skeletal elements. In recent years, however, a number of sauropod discoveries from the Early Cretaceous of North America have emerged, with new specimens described from Utah and Oklahoma (Tidwell et al. 1999, Tidwell et al. 2001, Wedel et al. 2000a).

In this study, a large sample of sauropod bones from an Early Cretaceous locality in central Texas is described. The Texas sauropod is demonstrated to be different from other Early Cretaceous sauropods. A new genus is created to include this distinctive species. Finally, a cladistic analysis is performed to investigate the phylogenetic position of this new taxon and the significance of this taxon to the diversity and geographic distribution of Early Cretaceous North American sauropods is discussed.

DMNH, Denver Museum of Science and Nature, Denver; FMNH, Field Museum of Natural History, Chicago; FWMSH, Fort Worth Museum of Science and History, Fort Worth; OMNH, Oklahoma Museum of Natural History, University of Oklahoma, Norman; SMU, Department of Geological Sciences, Southern Methodist University, Dallas; TMM, Texas Memorial Museum, Austin; USNM, National Museum of Natural History, Washington D.C.; YPM, Yale Peabody Museum, Yale University, New Haven.

acdl, anterior centrodiapophyseal lamina; acpl, anterior centroparapophyseal lamina; cpol, centropostzygapophyseal lamina; cprl, centroprezygapophyseal lamina; EI, elongation index; hyp, hyposphene; nc, neural canal; ns, neural spine; pcdl, posterior centrodiapophyseal lamina; pcpl, posterior centroparapophyseal lamina; podl, postzygodiapophyseal lamina; posl, postspinal lamina; ppdl, paradiapophyseal lamina; pl, pleurocoel; prdl, prezygodiapophyseal lamina; prpl, prezygoparapophyseal lamina; prsl, prespinal lamina; spdl, spinodiapophyseal lamina; sprl, spinoprezygapophyseal lamina; spol, spinopostzygapophyseal lamina; tpol, intrapostzygapophyseal lamina; tprl, intraprezygapophyseal lamina.

JP, Jeffrey Pittman (specimen field number).

Langston (1974) referred a series of 21 caudal vertebrae, fragmentary remains of some cervical and dorsal vertebrae, a chevron, dorsal ribs, and a distal scapula of a sauropod from Wise County, Texas (SMU 61732), to the genus Pleurocoelus. Langston (1974) also noted that the same species (TMM 40435) was found in the Glen Rose Formation of Blanco County, Texas. Langston (1974, p.86) identified the Wise County specimen as Pleurocoelus based on the morphology of the caudal vertebrae, claiming that, "the elevated, forwardly-placed neural arch atop a slender, spool-shaped, amphiplatyan centrum is characteristic." However, distal caudals of the Late Jurassic sauropod Brachiosaurus also fit this description (Janensch 1950).

Salgado et al. (1995) argued that, considering the available evidence, SMU 61732 from Wise County, Texas, could not be attributed to the same genus as any of the Arundel specimens. In their discussion of sauropods from Utah, Tidwell et al. (1999) reached the conclusion that SMU 61732 is distinctly different from Pleurocoelus. Gomani et al. (1999) compared SMU 61732 and sauropod bones from Jones Ranch, Texas, to Brachiosaurus, Euhelopus, and Malawisaurus, all titanosauriform taxa whose phylogenetic positions are reasonably well established. They determined that the Texas sauropod fossils share only two derived characters with titanosaurians, while the other 10 characters they analyzed were either plesiomorphic for titanosauriforms or were synapomorphies for the Texas sauropod and Brachiosaurus. Gomani et al. (1999) assumed SMU 61732 and the Jones Ranch material belonged to the same species but provided no supporting evidence.

An isolated, articulated hindfoot was found north of the Wise County locality that produced SMU 61732 and described as Pleurocoelus sp. Gallup 1989. According to Gallup (1989), the morphology of the foot is consistent with sauropod trackways and footprints found in Texas Lower Cretaceous rocks. The abundant sauropod footprints preserved in the Glen Rose Formation, given the ichnogenus Brontopodus Farlow et al. 1989, were attributed to Pleurocoelus sp. (Langston 1974, Gallup 1989).

The sauropod material in this study comes from the W.W. Jones Ranch, SMU Locality 282, Hood County, Texas (Figure 1). A group of students from the University of Texas at Austin discovered the site in the mid-1980s. Jeffrey G. Pittman worked the quarry for three field seasons beginning in 1985, discontinuing work in 1987. In 1993 researchers from Southern Methodist University, the Fort Worth Museum of Science and History, and Tarleton State University re-opened the quarry and have worked the site ever since.

Winkler et al. (2000) provided a description of the quarry, including certain aspects of the taphonomy, bone distribution, and depositional environment. The bone-bed occurs in the Twin Mountains Formation of the Trinity Group. At Jones Ranch the Trinity Group comprises the Twin Mountains, Glen Rose, and Paluxy formations, from bottom to top. The Twin Mountains and Paluxy formations are terrestrially derived, whereas the Glen Rose Formation is a shallow marine limestone that represents a significant marine transgression. The conformable contact between the Twin Mountains Formation and the Glen Rose limestone lies approximately 10 m above the sauropod-producing layers at Jones Ranch (Winkler et al. 2000). Biostratigraphic correlations using ammonites suggest that the base of the Glen Rose Formation in central Texas is no older than the Aptian-Albian boundary (Young 1974). The proximity of the Jones Ranch locality to the base of the Glen Rose Formation suggests an age near this boundary, or approximately 112 Ma (sensu Gradstein et al. 1995; Figure 2).

The depositional environment at the Jones Ranch quarry is fluvial. Here the Twin Mountains Formation consists of loosely consolidated channel sands and muds with pockets of hard, calcite-cemented sandstone concretions that are typically fossiliferous. All of the bones from Jones Ranch were found within an area of 400 square meters. The majority of the large sauropod bones from the quarry were found closely associated or articulated with other elements from the skeleton. Only three bones were found with no other bone within a 1 m radius (Figure 3). The quarry has so far produced representative elements of at least four individual sauropods (minimum number of individuals based on the number of preserved femora) all comparatively similar in size. Approximately 90% of the quarry assemblage has been removed from the field and preparation of collected specimens is close to two-thirds complete. All of the sauropod bones from the quarry can be attributed to the same species based on their close association and because duplicate elements do not show appreciable variation in size, proportion, or morphology beyond what can be expected from individual variation.