New horned dinosaurs from Utah provide evidence for intracontinental dinosaur endemism

Abstract

Background: During 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 findings: Here 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/significance: Considered 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.

Figure 1. Paleogeography of North America during the late Campanian Stage of the Late Cretaceous (∼75 Ma).

Modified after Blakey .

Figure 2. Skeletal elements recovered for Utahceratops gettyi n. gen et n. sp. and Kosmoceratops richardsoni n. gen et n. sp.

Utahceratops gettyi is known from six specimens, including two partial skulls, which together preserve about 96% of the skull and 70% of the postcranial skeleton. Highlighted elements are preserved. Kosmoceratops richardsoni is known from four specimens, one of which preserves a nearly complete skull and 45% of the postcranium. Scale bar represents one meter.

Figure 3. Select craniofacial elements of Utahceratops gettyi n. gen et n. sp.

A. Various cranial elements in dorsal view. B. Craniofacial skeleton in lateral view. The orbital region has been photo-reversed for consistency. C. Cast of restored skull in oblique view. Scale bar represents one meter. Abbreviations: aaof, accessory antorbital fossa; ab, antorbital buttress; aof, antorbital fenestra; D, dentary; dpp, dorsal parietal process; cp, coronoid process; ej, epijugal horn; ep, epiparietal position 1–3; es, episquamosal; ff, frontal fontanelle; J, jugal; L, lacrimal; ltf, laterotemporal fenestra; M, maxilla; N, nasal; na, naris; nh, nasal horncore; ns, narial strut; o, orbit; P, parietal; PD, predentary; pfe, parietal fenestra; PM, premaxilla; PO, postorbital; poh, postorbital horncore; pp, premaxillary process of nasal; R, rostrum; sfe, squamosal fenestra; SQ, squamosal.

Figure 4. Skull reconstruction of Utahceratops gettyi n. gen. et n. sp.

In dorsal (A) and lateral (B) views.

Figure 5. UMNH VP 17000, articulated holotype skull of Kosmoceratops richardsoni n. gen et n. sp.

In oblique (A), dorsal (B) and right lateral (C) views. Scale bar represents one meter. Abbreviations: D, dentary; ep, epiparietal position 1–3; eps, epiparietosquamosal; es, episquamosal; ff, frontal fontanelle; J, jugal; L, lacrimal; ltf, laterotemporal fenestra; M, maxilla; N, nasal; na, naris; nh, nasal horncore; o, orbit; P, parietal; pfe, parietal fenestra; PM, premaxilla; PO, postorbital; poh, postorbital horncore; R, rostrum; sq, squamosal; stf, supratemporal fenestra.

Figure 6. Skull reconstruction of Kosmoceratops richardsoni n. gen. et n. sp.

In dorsal (A) and lateral (B) views.

Figure 7. Phylogenetic relationships of Utahceratops gettyi n. gen et n. sp., and Kosmoceratops richardsoni n. gen. et n. sp. within Ceratopsidae.

Strict consensus of 3 most parsimonious trees (tree length  = 263; CI = 0.669; RI = 0.790; RC = 0.529) of an analysis of 148 characters across 7 non-chasmosaurines and 18 chasmosaurines (more than doubling number of characters and taxa relative to any previous analysis of clade). Outgroup taxa and centrosaurines have been collapsed for clarity. Species durations based on first and last documented stratigraphic occurrences correlated where possible to radiometric dates. Solid bars denote species durations known with high degree of confidence. Striped bars denote species durations known with lesser degree of confidence. Solid bars overlaying striped bars indicate that the stratigraphic context of some specimens of the indicated taxon is well established (solid bar) whereas that of others is not (striped bar). Taxa listed in dark green were recovered from the northern portion of the Western Interior; taxa listed in light green are from the southern WIB. See Text S1 for further results of the phylogenetic analysis, including Bremer support and bootstrap values. Stratigraphic data based on Roberts et al. and Sampson and Loewen .

Figure 8. Paleogeography of North America during the Late Cretaceous (∼75 Ma), showing biogeographic distribution of chasmosaurine ceratopsid dinosaurs on the western landmass, Laramidia, during the late Campanian (∼76–73 Ma).

Green represents coastal and alluvial plain habitats and reddish brown represents highlands. Present day boundaries of states and provinces are noted, as are the locations of key dinosaur-bearing geologic formations. Abbreviations: A, Aguja Formation, Texas; D, Dinosaur Park Formation, Alberta; F, Fruitland-Kirtland Formations; K, Kaiparowits Formation, Utah). Modified after Blakey .