Elsevier

Cretaceous Research

Volume 32, Issue 3, June 2011, Pages 264-276
Cretaceous Research

Titanoceratops ouranos, a giant horned dinosaur from the late Campanian of New Mexico

https://doi.org/10.1016/j.cretres.2010.12.007Get rights and content

Abstract

At the end of the Cretaceous, 65.5 million years ago, the giant ceratopsids Triceratops and Torosaurus dominated North America’s dinosaur fauna. The origins of these giant ceratopsids, the Triceratopsini, are poorly understood. This paper describes Titanoceratops ouranos, a giant ceratopsid from the late Campanian (73–74 Ma) of New Mexico, and the earliest known triceratopsin. The holotype was previously interpreted as an aberrant and exceptionally large specimen of Pentaceratops sternbergi, but the animal does not show the diagnostic features of Pentaceratops. Instead, cladistic analysis shows that Titanoceratops is the sister taxon of a clade formed by Eotriceratops, Triceratops, and Torosaurus. With an estimated mass of 6.5 tons, Titanoceratops is among the largest dinosaurs known from the Campanian of North America, and rivaled Triceratops in size. The recognition of Titanoceratops suggests that giant chasmosaurines evolved once, among the Triceratopsini, and that the group evolved large size five million years earlier than previously thought. The giant horned dinosaurs probably originated in the southern part of the North American continent during the Campanian but only became widespread during the Maastrichtian.

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

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