A new goniopholidid from the Upper Jurassic Morrison Formation, USA: novel insight into aquatic adaptation toward modern crocodylians

Goniopholididae is a group of basal neosuchian crocodyliforms closely related to Paralligatoridae and Eusuchia that lived during the Jurassic and Early Cretaceous. Goniopholidids have the long, flat snout and secondary palate of modern crocodylians, the acquisition of which is regarded as a key feature in the early evolution of crocodylian body plan and their aquatic adaptation. Here, we report a new species, Amphicotylus milesi , with the description from the best-preserved specimen to date of Goniopholididae from Wyoming, USA. Its posterior extension of the nasopharyngeal passage (pterygoid secondary palate) and the shortening and dorsal deflection of the ceratobranchial suggest that basal neosuchians could raise their gular valve to separate oral and pharyngeal cavities as in modern crocodylians. The anatomy of Amphicotylus milesi sheds light on the acquisition of this new respiratory system in the crocodyliform evolution and their early aquatic adaptation, leading to modern crocodylians.

Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

Middle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; south­western United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposi­tion with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this prove­nance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local base­ment uplifts were largely buried by Middle to Late Jurassic time. This study aims to delineate spatial and tem­poral patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrog­raphy, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth ele­mental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in west­ern Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anom­alously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochem­istry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identifica­tion of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wana­kah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin sug­gests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.

The promise of taphonomy as a nomothetic discipline: taphonomic bias in two dinosaur-bearing faunas in North America

The fossil record of dinosaurs is a rich, if biased, one with nearly complete skeletons, partial skeletons, and isolated parts found in diverse, well-studied faunal assemblages around the world. Among the recognized biases are the preferential preservation of large dinosaurs and the systematic underrepresentation of small dinosaurs. Such biases have been quantitatively described in the Upper Cretaceous (Campanian) Dinosaur Park Formation of Alberta, where large, nearly complete dinosaurs were found and described early in collecting history and small, very incomplete dinosaurs were found and described later. This pattern, apparently replicated in the Maastrichtian Hell Creek Formation of Montana, is so striking that it begs the question of whether this is a nomothetic principle for the preservation of dinosaur faunas elsewhere. We tested this hypothesis by analyzing the very well-studied dinosaur fauna of the Upper Jurassic (Kimmeridgian) Morrison Formation of the western United States. The Morrison Formation fails to show any correlation between body size and completeness, order of discovery, or order of description. Both large and small dinosaurs of the Morrison include highly complete as well as highly incomplete taxa, and both large and small dinosaurs were discovered and described early in collecting history as well as more recently. The differences in preservation between the Dinosaur Park Formation and the Morrison Formation are so striking that we posit a Dinosaur Park model of dinosaur fossil preservation and a Morrison model. Future study will show whether either or both represent durable nomothetic models for dinosaur fossil preservation.

Anatomy and systematics of the diplodocoid Amphicoelias altus supports high sauropod dinosaur diversity in the Upper Jurassic Morrison Formation of the USA

Sauropod dinosaurs were an abundant and diverse component of the Upper Jurassic Morrison Formation of the USA, with 24 currently recognized species. However, some authors consider this high diversity to have been ecologically unviable and the validity of some species has been questioned, with suggestions that they represent growth series (ontogimorphs) of other species. Under this scenario, high sauropod diversity in the Late Jurassic of North America is greatly overestimated. One putative ontogimorph is the enigmatic diplodocoid Amphicoelias altus , which has been suggested to be synonymous with Diplodocus . Given that Amphicoelias was named first, it has priority and thus Diplodocus would become its junior synonym. Here, we provide a detailed re-description of A. altus in which we restrict it to the holotype individual and support its validity, based on three autapomorphies. Constraint analyses demonstrate that its phylogenetic position within Diplodocoidea is labile, but it seems unlikely that Amphicoelias is synonymous with Diplodocus . As such, our re-evaluation also leads us to retain Diplodocus as a distinct genus. There is no evidence to support the view that any of the currently recognized Morrison sauropod species are ontogimorphs. Available data indicate that sauropod anatomy did not dramatically alter once individuals approached maturity. Furthermore, subadult sauropod individuals are not prone to stemward slippage in phylogenetic analyses, casting doubt on the possibility that their taxonomic affinities are substantially misinterpreted. An anatomical feature can have both an ontogenetic and phylogenetic signature, but the former does not outweigh the latter when other characters overwhelmingly support the affinities of a taxon. Many Morrison Formation sauropods were spatio-temporally and/or ecologically separated from one another. Combined with the biases that cloud our reading of the fossil record, we contend that the number of sauropod dinosaur species in the Morrison Formation is currently likely to be underestimated, not overestimated.