Dinosaur Paleontology

In the lead-up to the Cretaceous/Paleogene mass extinction, dinosaur diversity is argued to have been either in long-term decline, or thriving until their sudden demise. The latest Cretaceous (Campanian–Maastrichtian [83–66 Ma]) of North America provides the best record to address this debate, but even here diversity reconstructions are biased by uneven sampling. Here we combine fossil occurrences with climatic and environmental modelling to quantify latest Cretaceous North American dinosaur habitat. Ecological niche modelling shows a Campanian-to-Maastrichtian habitability decrease in areas with present-day rock-outcrop. However, a continent-wide projection demonstrates habitat stability, or even a Campanian-to-Maastrichtian increase, that is not preserved. This reduction of the spatial sampling window resulted from formation of the proto-Rocky Mountains and sea-level regression. We suggest that Maastrichtian North American dinosaur diversity is therefore likely to be underestimated, with the apparent decline a product of sampling bias, and not due to a climatically-driven decrease in habitability as previously hypothesised.

Stable isotopes are powerful tools for elucidating ecological trends in extant vertebrate communities, though their application to Mesozoic ecosystems is complicated by a lack of extant isotope data from comparable environments/ecosystems (e.g. coastal floodplain forest environments, lacking significant C4 plant components). We sampled 20 taxa across a broad phylogenetic, body size, and physiological scope from the Atchafalaya River Basin of Louisiana as an environmental analogue to the Late Cretaceous coastal floodplains of North America. Samples were analysed for stable carbon, oxygen and nitrogen isotope compositions from bioapatite and keratin tissues to test the degree of ecological resolution that can be determined in a system with similar environmental conditions, and using similar constraints, as those in many Mesozoic assemblages. Isotopic results suggest a broad overlap in resource use among taxa and considerable terrestrial–aquatic interchange, highlighting the challenges of ecological interpretation in C3 systems, particularly when lacking observational data for comparison. We also propose a modified oxygen isotope-temperature equation that uses mean endotherm and mean ectotherm isotope data to more precisely predict temperature when compared with measured Atchafalaya River water data. These results provide a critical isotopic baseline for coastal floodplain forests, and act as a framework for future studies of Mesozoic palaeoecology.

The last two decades have seen a remarkable increase in the known diversity of basal avialans and their paravian relatives. The lack of resolution in the relationships of these groups combined with attributing the behavior of specialized taxa to the base of Paraves has clouded interpretations of the origin of avialan flight. Here, we describe Hesperornithoides miessleri gen. et sp. nov., a new paravian theropod from the Morrison Formation (Late Jurassic) of Wyoming, USA, represented by a single adult or subadult specimen comprising a partial, well-preserved skull and postcranial skeleton. Limb proportions firmly establish Hesperornithoides as occupying a terrestrial, non-volant lifestyle. Our phylogenetic analysis emphasizes extensive taxonomic sampling and robust character construction, recovering the new taxon most parsimoniously as a troodontid close to Daliansaurus, Xixiasaurus, and Sinusonasus. Multiple alternative paravian topologies have similar degrees of support, but proposals of basal paravian archaeopterygids, avialan microraptorians, and Rahonavis being closer to Pygostylia than archaeopterygids or unenlagiines are strongly rejected. All parsimonious results support the hypothesis that each early paravian clade was plesiomorphically flightless, raising the possibility that avian flight originated as late as the Late Jurassic or Early Cretaceous.

Here we report new hadrosaur remains recovered from the Cerro del Pueblo Formation (Upper Cretaceous: Campanian) at La Parrita locality, northeastern Mexico. Although the material is fragmentary, the identification of dental microwear in three out of the four teeth collected is notable. This sample allows for a preliminary assessment of hadrosaur dental microwear from this geographic region and time frame. The variables studied were the orientation and average count of scratches, average count of pits, and average microwear feature width in an area of 400 X 400 µm at 35X magnification. The results show a predominance of dorsodistally-ventromesially inclined scratches in all three teeth indicating that the mandible primarily moved in an orthopalinal direction during the power stroke. The pattern of scratch orientation we recorded is comparable to that reported for different hadrosaur species from Alberta, Canada, and Wyoming, U.S.A.; suggesting similar jaw mechanics in the hadrosaurs from Coahuila. In contrast, there are significant differences in the average number of pits and microwear feature width between the Alberta and the Coahuila hadrosaurs samples, with the latter presenting more pits and finer microwear features. Given the similar jaw mechanics between the Alberta and Coahuila hadrosaurs, suggested by our analysis, the difference in average number of pits and microwear feature width is probably due to differences in diet and/or the type and amount of grit ingested during feeding. The greater number of pits in the hadrosaurs from Coahuila might indicate that these individuals ingested a greater proportion of hard food items than the Alberta hadrosaurs. Resumen Aquí reportamos nuevos restos de hadrosaurio recuperados de la Formación Cerro del Pueblo (Cretácico Superior: Campaniano) provenientes de la localidad de La Parrita, noreste de México. A pesar de que el material es fragmentario, la identificación de microdesgaste dental en tres de los cuatro dientes recolectados es notable. Esta muestra permite una evaluación preliminar del microdesgaste dental en hadrosaurios de esta región geográfica y durante este marco de tiempo. Las variables estudiadas fueron la orientación y el número promedio de rayones, el número promedio de fosas y el ancho promedio de los rasgos de microdesgaste en un área de 400 X 400 µm con un aumento de 35X. Los resultados muestran que la mayoria de los rayones estan inclinados dorsodistal-ventromesialmente en los tres dientes, lo que indica que la mandíbula se movió principalmente en dirección ortopalinal durante el golpe masticatorio. El patrón de orientación de los rayones que registramos es comparable al reportado para diferentes especies Paleontología Mexicana Volumen 8, núm. 1

This study represents the first description and analysis of insect damage on ornithomimid bones (Dinosauria: Ornithomimidae) from the Late Cretaceous Corral de Enmedio Formation of the state of Sonora, Mexico. The insect borings correspond to Cuniculichnus variabilis, Cubiculum atsintli and cf. Cubiculum cooperi traces that were likely produced by dermestid beetle larvae on a dry ornithomimid corpse prior to burial. In addition, the taphonomic signature of the specimen suggests that once disarticulated, the bones were transported far away from the original site of death by fluvial currents and deposited in a floodplain environment.