Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA
Introduction
Big Bend National Park, Texas contains the southernmost Upper Cretaceous terrestrial deposits known from the United States. Although the region has been studied for many years, vertebrate fossils from these beds tend to be rare and poorly preserved, and as a result, comparatively little is known about the dinosaur fauna of the region. Pachycephalosaurs, a group of ornithischian dinosaurs characterized by a massive, domed skull roof (Maryañska et al., 2004), have been among the rarest and most poorly understood members of the dinosaur assemblage. Until now, only a single, highly fragmentary frontoparietal dome (TMM 42010–1) has been reported from the Aguja Formation of Big Bend, and three poorly preserved domes (TMM 42532–1,–2,–3) are known from the San Carlos Formation, a lateral equivalent to the Aguja that is exposed to the west of Big Bend (Lehman, 1985). The only other specimens of Pachycephalosauridae that are known from Big Bend are isolated teeth (Sankey, 2001).
In January of 2008, the WPA (Works Progress Administrative) quarries near Talley Mountain were revisited as part of ongoing fieldwork in Big Bend National Park (Fig. 1). One of us (Tanke) discovered a frontoparietal dome (LSUMNS 20010) on a large ironstone concretionary mass exposed on the floor of WPA-1, a multitaxon bonebed dominated by Agujaceratops. Following another visit to the area in December of 2008, a second specimen was recovered from beds above WPA-1, again in association with Agujaceratops. Although initial studies suggested that the Aguja pachycephalosaur might be referable to Stegoceras, upon closer examination the Aguja skull was found to differ from any previously described pachycephalosaurid taxon. Despite the limited nature of the material, and despite the fact that the holotype and referred specimens exhibit predepositional wear, the material is sufficiently diagnostic to warrant the creation of a new taxon.
LSUMNS 20010 is significant because it represents the most complete pachycephalosaur specimen known from Texas, which allows for a detailed description and permits comparisons with pachycephalosaurs found to the North. The second specimen, LSUMNS 20012, is fragmentary, but agrees well with LSUMNS 20010 in the preserved features, providing further evidence that the Texan specimens are distinct from previously known taxa. The systematics of the Aguja pachycephalosaur are of particular interest in light of the observation that the late Campanian of Big Bend supported dinosaur species that are not found to the north (Lehman, 1997, Sankey, 2001).
Section snippets
Geological setting
The Aguja Formation (Fig. 2) is an eastward thinning deposit that ranges from 135 to 285 meters in thickness. The formation is composed of sandstones with interbedded shales. Marine, paralic, and floodplain depositional environments are present in the formation (Lehman, 1985). The lower part of the upper shale member of the Aguja contains sandstones, carbonaceous mudstones, thin lignite beds, and occasional ironstone concretions. These sediments represent a range of depositional environments
Systematic palaeontology
Institutional abbreviations. AMNH, American Museum of Natural History, New York, New York; CCM, Carter County Museum, Ekalaka, Montana; LSUMNS, Louisiana State University Museum of Natural Science; Baton Rouge; TMP, Royal Tyrrell Museum of Palaeontogy, Drumheller, Alberta; NMC/CMN, National Museum of Canada (Canadian Museum of Nature), Ottawa;, ROM, Royal Ontario Museum, Toronto; TMM, Texas Memorial Museum, Austin; UCMP, University of California Museum of Paleontology, Berkeley.
Dinosauria Owen,
Description
The holotype specimen LSUMNS 20010 consists of the fused frontals and parietals (Fig. 3, Fig. 4) (except where noted, the description refers to the holotype). The holotype is comparable in size to Stegoceras validum (UA2), with the frontoparietal dome measuring 115 mm long in LSUMNS 20010, versus 125 mm in UA2. The referred specimen (Fig. 5) is slightly larger: while the holotype measures 69 mm across the orbital lobes, the referred specimen measures 81 mm here (Table 2). The frontoparietals are
Phylogenetic analysis
Phylogenetic analysis was conducted using a matrix of 90 characters and 20 taxa. Following first-hand examination of the fossils and study of the literature, it was found that the data matrix presented by Sereno (2000) has a relatively large number of miscoded characters: e.g. in this matrix, Stegoceras is miscoded for characters 5, 22, 30, and 37 (NRL, pers. obs. of UA2); 24 appears to be miscoded for Goyocephale (Perle et al., 1982) and cannot be coded for Wannanosaurus (Butler and Zhao, 2008
Biogeography and biodiversity
As the southernmost known pachycephalosaurid, Texacephale is important for understanding Late Cretaceous dinosaur biogeography. Previously, it has been argued that the paleocommunity of the Aguja is distinct from the assemblages found in more northern locales such as Alberta and Montana (Rowe et al., 1992, Lehman, 1997, Sankey, 2001). Vertebrates found in the Aguja, but not in contemporary northern assemblages include the dinosaurs Agujaceratops mariscalensis (Lehman, 1989, Forster et al., 1993
Acknowledgments
Fossils were collected under research permit BIBE-2007-SCI-0001 to Sankey. Thanks to D. Corrick (Science and Natural Resources Division) for his support of this research and for use of research housing in Big Bend National Park. Thanks to David Evans (ROM), Tim Rowe and Wann Langston (TMM) and Judith Schiebout (LSUMNS) for discussions. Thanks also to Brandon Strilisky (TMP), Phil Currie (UALVP) and Kieran Shepard (CMN) for collections access. Clive Coy assisted in preparing the holotype.
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