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27. Unraveling a Radiation: A Review of the Diversity, Stratigraphic Distribution, Biogeography, and Evolution of Horned Dinosaurs (Ornithischia: Ceratopsidae)
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405 27 Unraveling a Radiation: A Review of the Diversity, Stratigraphic Distribution, Biogeography, and Evolution of Horned Dinosaurs (Ornithischia: Ceratopsidae) SCOTT D. SAMPSON AND MARK A. LOEWEN recent discoveries have greatly expanded our knowledge of the horned dinosaur clade Ceratopsidae. Taxonomically , ceratopsids are currently represented by about 32 species—including 15 species of centrosaurines and 17 of chasmosaurines—an approximate doubling of known diversity in less than 5 years. Phylogenetically, general agreement exists as to the basic structural elements of the ceratopsid tree. Within Chasmosaurine, Chasmosaurus and Pentaceratops are regarded as basal branches, Anchiceratops and Arrhinoceratops as intermediate branches, and Triceratops and Torosaurus as among the most nested members of the clade. Within Centrosaurinae, Albertaceratops and other forms with long supraorbital horncores are most basal, Centrosaurus and Styracosaurus are united in a distinct clade, and Einiosaurus is allied with a series of more nested, ‘‘pachyrhinosaur’’-like forms bearing nasal and supraorbital bosses. Stratigraphically, most ceratopsid taxa can now be placed with considerable resolution into stratigraphic context, with a temporal range of approximately 79–65 Ma. In contrast to the conclusions of some previous studies, it appears that ceratopsid diversity within single ecosystems at any one time was generally low, typically consisting of a single species each of centrosaurine and chasmosaurine. Taking into account the high overall species diversity of ceratopsids (minimally 24 taxa during the final 8 million years of the Campanian) and the relatively low diversity at any one time, the clade appears to have experienced relatively rapid turnover of species, with species durations averaging considerably less than 1 million years. Biogeographically, ceratopsids appear to have had surprisingly small species ranges, with the great majority of taxa known from single geologic formations . Although a portion of this endemicity may well be due to inadequate sampling, there is strong evidence of north-south endemicity within the Western Interior Basin during the late Campanian. A corollary of this finding is that, despite their large-to-giant body sizes, most ceratopsids likely did not engage in long-distance migrations. Instead, the prevalent pattern of latitudinally disjunct biogeographic distributions provides strong support for the provincialism hypothesis previously postulated for Late Cretaceous terrestrial floras and faunas inhabiting the Western Interior Basin. Such diminutive species ranges are problematic for understanding the paleobiology of ceratopsids, suggestive of low-to-intermediate metabolic rates or abundant food supplies, or both. Given that virtually all ceratopsid species are currently known from small geographic ranges, that the current record includes several temporal gaps, and that most taxa are known from a restricted region in the northern region of the Western Interior Basin, it is likely that many additional species await discovery. 406 sampson & loewen Introduction Ceratopsid dinosaurs were a diverse group of Late Cretaceous, large-bodied (4–8 m long; 1–4 ton), quadrupedal herbivores that include some of the most remarkable vertebrates known. Their exceptionally derived skulls include edentulous, parrotlike beaks, robust dental batteries with unique shearing dentitions , hypertrophied narial regions, and a broad array of signature ornamentations—from nasal and supraorbital horns to expansive, elaborately adorned parietosquamosal frills (Figs. 27.1, 27.2). The longest of these skulls achieved lengths in excess of 3 m, the largest known for any terrestrial vertebrate (Colbert and Bump 1947; Lehman 1998). Ceratopsids are subdivided into two subclades (‘‘subfamilies’’): Centrosaurinae, typically with subcircular narial regions and relatively short, highly adorned frills (e.g., Styracosaurus albertensis); and Chasmosaurinae , typically with elongate narial regions and more elongate, less adorned frills (e.g., Triceratops horridus). Ceratopsidae are well suited for an analysis of vertebrate evolutionary radiation based on multiple features: (1) large and massive skulls that are relatively abundant in the fossil record (i.e., majority of taxa represented by mostly complete skull materials); (2) putative species-specific signaling structures preserved as bony outgrowths of the skull; (3) relatively brief temporal Late Cretaceous (Campanian and Maastrichtian ) distribution (currently approximately14 million years); and (4) a limited geographic distribution restricted to the Western Interior Basin (WIB) of North America (Dodson et al. 2004). (The lone possible exception is Turanoceratops tardabilis , a poorly known taxon erected on fragmentary elements —including double-rooted teeth—from the Late Cretaceous of Uzbekistan [Nessov et al. 1989]). As a result, ceratopsids arguably provide greater potential than any other major clade within Dinosauria to investigate the tempo and mode of evolution . This unique status is heightened by abundant recent discoveries that greatly increase the material basis of the clade. Although the first ceratopsid dinosaur was described well over a century ago (Marsh...