New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism

Abstract

BackgroundDuring much of the Late Cretaceous, a shallow, epeiric sea divided North America into eastern and western landmasses. The western landmass, known as Laramidia, although diminutive in size, witnessed a major evolutionary radiation of dinosaurs. Other than hadrosaurs (duck-billed dinosaurs), the most common dinosaurs were ceratopsids (large-bodied horned dinosaurs), currently known only from Laramidia and Asia. Remarkably, previous studies have postulated the occurrence of latitudinally arrayed dinosaur “provinces,” or “biomes,” on Laramidia. Yet this hypothesis has been challenged on multiple fronts and has remained poorly tested.Methodology/Principal FindingsHere we describe two new, co-occurring ceratopsids from the Upper Cretaceous Kaiparowits Formation of Utah that provide the strongest support to date for the dinosaur provincialism hypothesis. Both pertain to the clade of ceratopsids known as Chasmosaurinae, dramatically increasing representation of this group from the southern portion of the Western Interior Basin of North America. Utahceratops gettyi gen. et sp. nov.—characterized by short, rounded, laterally projecting supraorbital horncores and an elongate frill with a deep median embayment—is recovered as the sister taxon to Pentaceratops sternbergii from the late Campanian of New Mexico. Kosmoceratops richardsoni gen. et sp. nov.—characterized by elongate, laterally projecting supraorbital horncores and a short, broad frill adorned with ten well developed hooks—has the most ornate skull of any known dinosaur and is closely allied to Chasmosaurus irvinensis from the late Campanian of Alberta.Conclusions/SignificanceConsidered in unison, the phylogenetic, stratigraphic, and biogeographic evidence documents distinct, co-occurring chasmosaurine taxa north and south on the diminutive landmass of Laramidia. The famous Triceratops and all other, more nested chasmosaurines are postulated as descendants of forms previously restricted to the southern portion of Laramidia. Results further suggest the presence of latitudinally arrayed evolutionary centers of endemism within chasmosaurine ceratopsids during the late Campanian, the first documented occurrence of intracontinental endemism within dinosaurs.

Dinosaur morphological diversity and the end-Cretaceous extinction

“…It may be, however, that the North American record represents a local anomaly. Extreme fluctuations of the inland Western Interior Sea, mountain building, and proposed biogeographic provincialism 31 may have affected the evolution of North American dinosaurs in distinct ways from species on other continents, meaning that the North American record may not be representative of a global pattern, if one exists. A renaissance in the discovery of Cretaceous Asian dinosaurs over the past two decades has enabled our calculations of separate disparity measures for Asian species, and future work should focus on Asia- specific diversity and abundance trends like those widely compiled and debated for North American dinosaurs 15 .…”

“…We calculated the terminal Cretaceous disparity patterns of every dinosaur subgroup that is represented by a reasonable number of Campanian-Maastrichtian specimens that could be scored for large matrices of discrete characters. Analysed clades include: non-avian coelurosaurian theropods 28 , tyrannosauroid theropods 29 , pachycephalosaurs 30 , ceratopsids (most of which are chasmosaurines, because there is no available global phylogeny of ceratopsids that includes comprehensive sampling of centrosaurines and chasmosaurines 31 ), hadrosauroids 32 , sauropods 33 , and ankylosaurs 34 . Other groups, such as abelisauroid theropods, non-hadrosauroid ornithopods, and non-ceratopsid ceratopsians, could not be analysed because large samples of reasonably complete CampanianMaastrichtian specimens scored for discrete character data are not yet available, almost entirely due to poor fossil sampling [41][42][43] .…”

“…The seven discrete character datasets record the presence, absence, number, and form of skeletal features in each species of the clade being analysed, and, therefore, comprise a database of overall morphological form for the group. Each of the seven discrete character datasets [28][29][30][31][32][33][34] was used to derive a Euclidean distance matrix, which quantifies the pairwise differences between the taxa in question, using the freeware program MATRIX. The seven distance matrices were next subjected to individual principal coordinates analyses (PCO), a multivariate statistical technique that assimilates information from the distance matrix into a more manageable set of coordinate axes, using the freeware program GINGKO (Universitat de Barcelona, http://biodiver.bio.ub.es/ginkgo/).…”

“…We calculated four measures of range and variance-based disparity for seven major Late Cretaceous non-avian dinosaur groups [28][29][30][31][32][33][34] , to test whether disparity increased, decreased, or stayed consistent during the final two stages of the Cretaceous (Campanian and Maastrichtian). Taxa were temporally binned using two schemes (equal-length late Campanian and Maastrichtian bins (Fig.…”

Theropod specimens from the Navesink Formation and their implications for the Diversity and Biogeography of Ornithomimosaurs and Tyrannosauroids on Appalachia

“…During the Cenomanian Stage of the Late Cretaceous, the Western Interior Seaway separated the terrestrial ecosystems of the eastern portion of North America with those of the American west, in turn affecting the faunas of both the east and west of the continent by subjecting each to isolation for millions of years. While in the west, it is known that the ancestors of the iconic Tyrannosaurus rex evolved reduced forelimbs and massive, powerful skulls, the tyrannosaurs of the eastern portion of North America, at that time a landmass called Appalachia (e.g., Sampson et al, 2010;Loewen et al, 2013), have remained more obscure, and rarely are associated or partial skeletons ever found. The two most complete tyrannosaurs from Late Cretaceous continent described so far are Dryptosaurus aquilunguis and Appalachiosaurus montgomeriensis, originally known from the Maastrichtian New Egypt Formation of New Jersey 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 and from the middle Campanian Demopolis Chalk Formation of Alabama, respectively (e.g., Cope, 1866;Marsh, 1877;Weishampel et al, 2004;Carr, Williamson & Schwimmer, 2005;Weishampel, 2006;Brusatte, Benson & Norell, 2011).…”