Were There two Species of Agujaceratops in Cretaceous West Texas?

New specimens of horned dinosaurs from the Aguja Formation of West Texas, and a revision of Agujaceratops

Authors:

Lehman et al

Abstract:

Most horned dinosaur remains recovered from the Aguja Formation in West Texas are referable to the endemic chasmosaurine Agujaceratops mariscalensis. One specimen, however, differs sufficiently to justify its designation as the holotype of a new species, Agujaceratops mavericus sp. nov. This specimen and an isolated postorbital horncore from the same vicinity are stratigraphically the highest found in the Aguja Formation. A well-preserved juvenile specimen exhibits some unique features, and others compatible with A. mavericus, but due to its immature condition cannot be identified with certainty. A parietal referred to A. mariscalensis is the most complete thus far known, and shows that the frill of this taxon is more elaborately ornamented than previously believed, bearing a set of large horn-like spikes at the posterolateral corners. These two species share features of the premaxilla and squamosal, which warrant their inclusion in the same genus. However, characters thought to distinguish the two species vary in a manner similar to that found in other chasmosaurines, where debate persists as to their taxonomic significance. A consensus species concept has yet to be adopted for ceratopsid genera, of which most are monotypic. As a result, the two Agujaceratops species could be interpreted as arbitrary anagenetic stages in a single lineage, end-members in a spectrum of ontogenetic and sex-associated variation in that lineage, or two sympatric lineages that occupied separate niches in the same range.

New Torosaurus Specimen Found in Maastrichtian Cretaceous Hell Creek Formation

A New Specimen of the Controversial Chasmosaurine Torosaurus latus (Dinosauria: Ceratopsidae) from the Upper Cretaceous Hell Creek Formation of Montana

Authors:

MacDonald et al

Abstract:

Torosaurus latus is an uncommon and contentious taxon of chasmosaurine ceratopsid known from several upper Maastrichtian units in western North America. We describe a partial parietal of To. latus from the Hell Creek Formation of Montana. Although the specimen’s ontogenetic maturity means that it cannot inform the ongoing debate over whether To. latus is the old adult form of the contemporary Triceratops, the specimen is one of the best-preserved To. latus parietals and supplements previous descriptions.

Kosmoceratops was NOT Present in Campanian Cretaceous Alberta & Saskatchewan

A Re-Evaluation of the Chasmosaurine Ceratopsid Genus Chasmosaurus (Dinosauria: Ornithischia) from the Upper Cretaceous (Campanian) Dinosaur Park Formation of Western Canada

Authors:

Campbell et al

Abstract:

Background

The chasmosaurine ceratopsid Chasmosaurus is known from the Upper Cretaceous (Campanian) Dinosaur Park Formation of southern Alberta and Saskatchewan. Two valid species, Chasmosaurus belli and C. russelli, have been diagnosed by differences in cranial ornamentation. Their validity has been supported, in part, by the reported stratigraphic segregation of chasmosaurines in the Dinosaur Park Formation, with C. belli and C. russelli occurring in discrete, successive zones within the formation.

Results/Conclusions

An analysis of every potentially taxonomically informative chasmosaurine specimen from the Dinosaur Park Formation indicates that C. belli and C. russelli have indistinguishable ontogenetic histories and overlapping stratigraphic intervals. Neither taxon exhibits autapomorphies, nor a unique set of apomorphies, but they can be separated and diagnosed by a single phylogenetically informative character—the embayment angle formed by the posterior parietal bars relative to the parietal midline. Although relatively deeply embayed specimens (C. russelli) generally have relatively longer postorbital horncores than specimens with more shallow embayments (C. belli), neither this horncore character nor epiparietal morphology can be used to consistently distinguish every specimen of C. belli from C. russelli.

Status of Kosmoceratops in the Dinosaur Park Formation

Kosmoceratops is purportedly represented in the Dinosaur Park Formation by a specimen previously referred to Chasmosaurus. The reassignment of this specimen to Kosmoceratops is unsupported here, as it is based on features that are either influenced by taphonomy or within the realm of individual variation for Chasmosaurus. Therefore, we conclude that Kosmoceratops is not present in the Dinosaur Park Formation, but is instead restricted to southern Laramidia, as originally posited.

Regaliceratops peterhewsi: A Maastrichtian Cretaceous Chasmosaurine Ceratopsid Nicknamed 'Hellboy'

A New Horned Dinosaur Reveals Convergent Evolution in Cranial Ornamentation in Ceratopsidae

Authors:

Brown et al

Abstract:

Ceratopsid (horned) dinosaurs are an iconic group of large-bodied, quadrupedal, herbivorous dinosaurs that evolved in the Late Cretaceous and were largely restricted to western North America. Ceratopsids are easily recognized by their cranial ornamentation in the form of nasal and postorbital horns and frill (capped by epiossifications); these structures show high morphological disparity and also represent the largest cranial display structures known to have evolved. Despite their restricted occurrence in time and space, this group has one of the best fossil records within Dinosauria, showing a rapid diversification in horn and frill morphology. Here a new genus and species of chasmosaurine ceratopsid is described based on a nearly complete and three-dimensionally preserved cranium recovered from the uppermost St. Mary River Formation (Maastrichtian) of southwestern Alberta. Regaliceratops peterhewsi gen. et sp. nov. exhibits many unique characters of the frill and is characterized by a large nasal horncore, small postorbital horncores, and massive parietal epiossifications. Cranial morphology, particularly the epiossifications, suggests close affinity with the late Campanian/early Maastrichian taxon Anchiceratops, as well as with the late Maastrichtian taxon Triceratops. A median epiparietal necessitates a reassessment of epiossification homology and results in a more resolved phylogeny. Most surprisingly, Regaliceratops exhibits a suite of cranial ornamentations that are superficially similar to Campanian centrosaurines, indicating both exploration of novel display morphospace in Chasmosaurinae, especially Maastrichtian forms, and convergent evolution in horn morphology with the recently extinct Centrosaurinae. This marks the first time that evolutionary convergence in horn-like display structures has been demonstrated between dinosaur clades, similar to those seen in fossil and extant mammals.

A Reassessment of Chasmosaurine Ceratopsid Judiceratops tigris From Campanian Cretaceous Montana

A reassessment of the horned dinosaur Judiceratops tigris (Ornithischia: Ceratopsidae) from the Upper Cretaceous (Campanian) of Montana, U.S.A.

Author:

Campbell

Abstract:

udiceratops tigris is a recently described ceratopsid, collected from sediments of the Upper Cretaceous (middle Campanian) Judith River Formation (JRF) of northern Montana, U.S.A. The current diagnosis for this taxon is tentative, as it is based on four fragmentary cranial specimens that have limited anatomical overlap. Although these four specimens were not found associated, they were all collected from the same localized area (Kennedy Coulee). New observations, interpretations, and reconstructions of J. tigris are presented in this study, based on the original assumption that these specimens represent the same taxon. Based on these findings, J. tigris had: elongate postorbital horncores with an autapomorphic ovate (egg-shaped) cross section; squamosals with non-imbricated episquamosals, and an anterolaterally-oriented anteriormost episquamosal; a wide medial parietal bar with an autapomorphic meniscus-shaped cross section; an anteroposteriorly wide posterior parietal bar, and, consequently, reduced parietal fenestrae; and at least four small, low-lying epiparietals on each side of the frill. A phylogenetic analysis incorporating the revised diagnosis of J. tigris supports the previous referral of this taxon to Chasmosaurinae, but missing anatomy prevents an understanding of how J. tigris is related to other chasmosaurines. Judiceratops tigris is retained here as a distinct taxon, and, hence, the oldest known chasmosaurine, but has more in common with other chasmosaurines than was previously thought. Further fossil collecting in the JRF of Montana may produce more complete specimens of J. tigris and provide a means of testing the new reconstructions presented here.

Mercuriceratops: A New Chasmosaurine Ceratopsid From Campanian Cretaceous North America

A new chasmosaurine from northern Laramidia expands frill disparity in ceratopsid dinosaurs

Authors:

Ryan et al

Abstract:

A new taxon of chasmosaurine ceratopsid demonstrates unexpected disparity in parietosquamosal frill shape among ceratopsid dinosaurs early in their evolutionary radiation. The new taxon is described based on two apomorphic squamosals collected from approximately time equivalent (approximately 77 million years old) sections of the upper Judith River Formation, Montana, and the lower Dinosaur Park Formation of Dinosaur Provincial Park, Alberta. It is referred to Chasmosaurinae based on the inferred elongate morphology. The typical chasmosaurine squamosal forms an obtuse triangle in dorsal view that tapers towards the posterolateral corner of the frill. In the dorsal view of the new taxon, the lateral margin of the squamosal is hatchet-shaped with the posterior portion modified into a constricted narrow bar that would have supported the lateral margin of a robust parietal. The new taxon represents the oldest chasmosaurine from Canada, and the first pre-Maastrichtian ceratopsid to have been collected on both sides of the Canada–US border, with a minimum north–south range of 380 km. This squamosal morphology would have given the frill of the new taxon a unique dorsal profile that represents evolutionary experimentation in frill signalling near the origin of chasmosaurine ceratopsids and reinforces biogeographic differences between northern and southern faunal provinces in the Campanian of North America.

Arrhinoceratops brachyops: a Campanian Cretaceous Chasmosaurine Ceratopsid Which Hints at a Deep Split Within Chasmosaurinae

NEW INFORMATION ON THE RARE HORNED DINOSAUR ARRHINOCERATOPS BRACHYOPS (ORNITHISCHIA: CERATOPSIDAE) FROM THE UPPER CRETACEOUS OF ALBERTA, CANADA

Authors:

Mallon et al

Abstract:

Arrhinoceratops brachyops is a poorly understood chasmosaurine ceratopsid from the Upper Cretaceous Horseshoe Canyon Formation of Alberta, previously described on the basis of only a single skull. Here, we report on a second specimen attributable to this species, including a relatively complete skull, syncervical, and partial left forelimb. This second specimen clarifies aspects of morphology not visible in the holotype, and also elucidates variation in A. brachyops. The species is distinguished by a square-shaped triangular process of the premaxilla, a steeply inclined triturating surface of the predentary, and a triangular nasal horncore in horizontal section. The dentary is also distinctive in bearing a bony lateral ridge similar to that of Anchiceratops ornatus, but more strongly developed. Phylogenetic analysis cannot resolve the relationships of Arrhinoceratops beyond the level of Chasmosaurinae, owing to both missing data and conflicting characters. However, we do find some support for a deep split within Chasmosaurinae, contrary to conventional topologies. We also report on other fragmentary specimens plausibly attributable to A. brachyops that suggest a minimum age range of approximately 750 Kyr for this species.

Judiceratops tigris: A Basal Chasmosaurine from the Campanian of North America

Judiceratops tigris, a New Horned Dinosaur from the Middle Campanian Judith River Formation of Montana

Author:

1. Nicholas R. Longrich

Affiliation:

a. Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven CT 06520-8109 USA

Abstract:

North America hosted a diverse assemblage of horned dinosaurs from the late Campanian until the end of the Cretaceous, but comparatively little is known about earlier horned dinosaurs. This paper reports on previously undescribed ceratopsian remains from the middle Campanian beds of the Judith River Formation of Montana, which represent the oldest known chasmosaurine. The Judith River chasmosaur shows a combination of characters not seen in any previously described ceratopsid. The parietal has a broad median bar, a rounded caudal margin, and highly reduced epiparietals. Episquamosals are enlarged anteriorly but decrease in size posteriorly, and imbricate as in centrosaurines. The postorbital horns are moderately elongate, inclined anterolaterally, and have an unusual teardrop-shaped cross section. The unique combination of characters seen in the Judith River chasmosaurine precludes referral to any previously known genus, and it is therefore described as a new genus and species, Judiceratops tigris. The addition of Judiceratops to the dinosaurian fauna of North America underscores the diversity of horned dinosaurs in the Late Cretaceous, which results from a combination of high diversity within faunas, a high degree of endemism, and rapid faunal turnover.