Titanoceratops ouranos, a giant horned dinosaur from the late Campanian of New Mexico
Introduction
In the late Maastrichtian, North America’s dinosaur community underwent a major restructuring, as the hadrosaur-dominated faunas of the Campanian (Currie and Russell, 2005, Ryan and Evans, 2005) were replaced by a fauna dominated by chasmosaurine ceratopsids (Dodson et al., 2004). Indeed, ceratopsids are so abundant in the late Maastrichtian of western North America that Marsh (1891b) named these strata the “Ceratops beds”. Two genera are currently recognized from the Late Maastrichtian: Triceratops (Marsh, 1889) and the closely related Torosaurus (Marsh, 1891a). These were enormous animals. Triceratops weighed well in excess of 5000 kg (Paul, 1997), comparable to an African elephant (Loxodonta africana), the largest extant land animal (Nowak, 1991), and Torosaurus was similar in size.
The origins of Triceratops and its relatives are poorly understood. Until now, the oldest known member of the Triceratops lineage, the Triceratopsini, was the 68-million year old Eotriceratops xerinsularis from the uppermost beds of the Maastrichtian Horseshoe Canyon Formation of Alberta (Wu et al., 2007). Triceratopsins from the Maastrichtian North Horn Formation of Utah (Gilmore, 1946, Sullivan et al., 2005) and the Javelina Formation of Texas (Lawson, 1976, Hunt and Lehman, 2008) may be older, but their ages are not known with certainty, and therefore they provide little information about the timing of this group’s appearance. Triceratops-like dinosaurs have never been described from the Campanian, and so it would seem that the giant triceratopsins appeared and diversified, almost out of nowhere, in the Maastrichtian.
This paper describes a giant triceratopsin from the late Campanian of New Mexico (Fig. 1). This specimen, OMNH 10165, includes a partial skull and associated postcrania (Lehman, 1998). The skeleton was collected in 1941 in San Juan County, New Mexico, but lay untouched until 1995, when the jackets were opened and the specimen was prepared and mounted. The animal was identified as Pentaceratops sternbergi (Lehman, 1998) which is common in these beds (Osborn, 1923, Wiman, 1930, Rowe et al., 1981, Lehman, 1993) and so the frill was reconstructed after Pentaceratops. However, it is shown here that this identification is in error. A rediagnosis of Pentaceratops and a redescription of OMNH 10165 show that it does not conform to the diagnosis of Pentaceratops. Instead, the skeleton exhibits numerous derived features that are shared with Triceratops and its close relatives. OMNH 10165 represents the earliest known member of Triceratopsini, and it demonstrates that these giant dinosaurs appeared and evolved large size much earlier than previously believed.
Institutional abbreviations: AMNH, American Museum of Natural History, LACM, Los Angeles County Museum, Los Angeles; New York; MNA, Museum of Northern Arizona, Flagstaff; OMNH, Oklahoma Museum of Natural History, Norman, OK; PMU, Paleontological Museum of Uppsala, Uppsala; UALP, University of Arizona Laboratory of Paleontology; UKVP, University of Kansas Museum of Natural History; UNM, University of New Mexico (now New Mexico Museum of Natural History and Science), Albuquerque; USNM, Smithsonian United States National Museum, Washington.
Section snippets
Geology
The holotype was collected in San Juan County, New Mexico, from the upper Fruitland Formation or the lower Kirtland Formation (Fig. 1, Fig. 2). The Fruitland Formation consists of approximately 100 m of sandstones, mudstones, and thick coals deposited in a coastal floodplain setting (Bauer, 1916, Lucas et al., 2006a, Sullivan and Lucas, 2006a). It is conformably overlain by the Kirtland Formation, which consists of up to 600 m of sandstones, siltstones, mudstones and shales (Bauer, 1916,
Systematic paleontology
Ornithischia Seeley 1888
Ceratopsia Marsh 1890
Ceratopsidae Marsh 1888
Chasmosaurinae Lambe 1915
PentaceratopsOsborn, 1923
Pentaceratops sternbergi, Osborn 1923
Holotype. AMNH 6325, skull.
Separation of OMNH 10165 from Pentaceratops
The phylogenetic analysis presented here strongly supports the placement of OMNH 10165 in the Triceratopsini. By contrast, the original referral of OMNH 10165 to Pentaceratops was made on the basis of three characters: 1, a narrow median bar of the parietal; 2, a large, curved epijugal horncore, and 3, a finger-like process of maxillary ramus of the premaxilla (Lehman, 1998). None of these are unique to Pentaceratops, and the degree to which Pentaceratops and OMNH 10165 resemble each other in
Acknowledgments
Thanks to Wann Langston for information about the excavation of the holotype, Kyle Davies for invaluable information on the mounted skeleton, Thomas Williamson for information on the stratigraphy of the San Juan Basin, Jennifer Larsen for assistance, Matt Carrano, Jacques Gauthier, and Mark Norell for access to specimens, and Andy Farke and Jacques Gauthier for discussion. Thanks also to ReBecca Hunt-Foster for invaluable assistance with this project, including measurements of OMNH 10165 and
References (65)
- et al.
Albertonykus borealis, a new alvarezsaur (Dinosauria: Theropoda) from the Early Maastrichtian of Alberta, Canada: implications for the systematics and ecology of the Alvarezsauridae
Cretaceous Research
(2009) - et al.
Craniofacial ontogeny in centrosaurine dinosaurs (Ornithischia: Ceratopsidae): taxonomic and behavioral implications
Zooogical Journal of the Linnean Society
(1997) Contributions to the geology and paleontology of San Juan County, New Mexico: 1. Stratigraphy of a part of the Chaco River Valley
United States Geological Survey Professional Paper
(1916)The Cretaceous Ojo Alamo beds of New Mexico with a description of the new dinosaur genus Kritosaurus
Bulletin of the American Museum of Natural History
(1910)Anchiceratops, a new genus of horned dinosaur from the Edmonton Cretaceous of Alberta
Bulletin of the American Museum of Natural History
(1914)- et al.
The structure and relationships of Protoceratops
Annals of the New York Academy of Sciences
(1940) - et al.
Dinosaurs, dragons, and dwarfs: the evolution of maximal body size
Proceedings of the National Academy of Sciences
(2001) - et al.
Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea
Journal of Vertebrate Paleontology
(2010) - et al.
The geographic and stratigraphic distribution of articulated and associated dinosaur remains
- et al.
Ceratopsidae