Lehman T.M.,Texas Tech University |
Barnes K.R.,HC 65
Journal of Systematic Palaeontology | Year: 2016
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. http://zoobank.org/urn:lsid:zoobank.org:pub:1846D524-AC7F-4126-8787-33B26D80CF52 © The Trustees of the Natural History Museum, London 2016. All rights reserved.
Longrich N.R.,Yale University |
Barnes K.,110 CR 322 |
Clark S.,424 Riverside Blvd. |
Millar L.,HC 65
Bulletin of the Peabody Museum of Natural History | Year: 2013
Caenagnathid theropods are a relatively common part of the theropod fauna in the Late Cretaceous of Asia and North America, but have not previously been described from the southernmost United States. Here, we describe caenagnathid fossils from the late Campanian Aguja Formation of West Texas, and revise the systematics of caenagnathids from the Campanian of North America. Caenagnathids from the late Campanian of Canada represent three species in three genera: Caenagnathus collinsi, Chirostenotes pergracilis and Leptorhynchos elegans gen. nov. Leptorhynchos is diagnosed by its small size, its short, deep mandible, and the upturned tip of the beak. A single caenagnathid is known from the late Campanian of Utah, Hagryphus giganteus. Two caenagnathid species occur in the Aguja Formation, ?Chirostenotes sp. and Leptorhynchos gaddisi sp. nov. L. gaddisi differs from L. elegans in that the tip of the beak is narrower and less upturned. Phylogenetic analysis recovers Caenagnathidae and Oviraptoridae as monophyletic sister taxa. Within Caenagnathidae, the North American species seem to form a monophyletic assemblage, the Caenagnathinae, within which Chirostenotes and Caenagnathus form a clade to the exclusion of Leptorhynchos. The discovery of Chirostenotes gaddisi provides more evidence for the existence of a distinct dinosaurian fauna in southern North America during the Campanian. Furthermore, the Aguja fossils show that caenagnathids were widespread and highly diverse in the Late Cretaceous of North America. This diversity was maintained in two ways. First, variation in body size and beak shape suggests that diversity within formations is maintained by niche partitioning, in a way analogous to Darwin's finches. Second, diversity is maintained by high degree of endemism, with different species of caenagnathids occurring in different habitats. © 2013 Peabody Museum of Natural History, Yale University. All rights reserved.
Lehman T.M.,Texas Tech University |
Barnes K.,HC 65
Journal of Paleontology | Year: 2010
A specimen of the aquatic reptile Champsosaurus sp. from the Paleocene Black Peaks Formation in southwestern Texas is the southernmost yet known. The fragmentary specimen exhibits some unusual features, such as a great anterior extent of the quadratojugal on the lower temporal arch, and cannot be attributed with confidence to any of the named species. Champsosaurus appears to have been tolerant of temperate climates and had a northern latitudinal range exceeding that of crocodylians. It seems likely that the brief southward extension in range of Champsosaurus during early Paleocene time resulted from a decrease in mean annual temperature, comparable to over 10° of paleolatitude. © 2010 The Paleontological Society.