Jaw morphology is closely linked to both diet and biomechanical performance, and jaws are one of the most common Mesozoic mammal fossil elements. Knowledge of the dietary and functional diversity of early mammals informs on the ecological structure of palaeocommunities throughout the longest era of mammalian evolution: the Mesozoic. Here, we analyse how jaw shape and mechanical advantage of the masseter (MAM) and temporalis (MAT) muscles relate to diet in 70 extant and 45 extinct mammals spanning the Late Triassic-Late Cretaceous. In extant mammals, jaw shape discriminates well between dietary groups: insectivores have long jaws, carnivores intermediate to short jaws, and herbivores have short jaws. Insectivores have low MAM and MAT, carnivores have low MAM and high MAT, and herbivores have high MAM and MAT. These traits are also informative of diet among Mesozoic mammals (based on previous independent determinations of diet) and set the basis for future ecomorphological studies.

Finite-element (FE) analysis has been used in palaeobiology to assess the mechanical performance of the jaw. It uses two types of models: tomography-based three-dimensional (3D) models (very accurate, not always accessible) and two-dimensional (2D) models (quick and easy to build, good for broad-scale studies, cannot obtain absolute stress and strain values). Here, we introduce extruded FE models, which provide fairly accurate mechanical performance results, while remaining low-cost, quick and easy to build. These are simplified 3D models built from lateral outlines of a relatively flat jaw and extruded to its average width. There are two types: extruded (flat mediolaterally) and enhanced extruded (accounts for width differences in the ascending ramus). Here, we compare mechanical performance values resulting from four types of FE models (i.e. tomography-based 3D, extruded, enhanced extruded and 2D) in Morganucodon and Kuehneotherium. In terms of absolute values, both types of extruded model perform well in comparison to the tomography-based 3D models, but enhanced extruded models perform better. In terms of overall patterns, all models produce similar results. Extruded FE models constitute a viable alternative to the use of tomography-based 3D models, particularly in relatively flat bones.

Savanna-like ecosystems were present at high latitudes in North America during much of the Neogene. Present-day African savannas, like the Serengeti, have been proposed to be modern analogs of these paleosavannas, particularly those from the middle Miocene of the Great Plains region of the United States. Both these extant and extinct savannas contain a preponderance of artiodactyl (even-toed ungulate) species; however, the taxonomic composition of each fauna is different. While present-day African savannas are dominated by ruminants (primarily bovids), the Neogene savannas of North America were dominated by a diversity of both camelid and non-bovid ruminant families. This study provides a quantitative test of the similarity of the artiodactyl faunas of the North American Neogene paleosavannas to those of the modern-day African savannas. A correspondence analysis of ecomorphological features revealed considerable overlap between modern and fossil faunas. The morphospace occupation of the extinct North American ruminants falls within that of the African bovids. Some of the extinct camelids also fall within this same morphospace, but many do not, perhaps indicating an environmental difference such as greater aridity in Neogene North America. The diversity and disparity of artiodactyl faunas through the Neogene of North America changed along with changing temperatures and precipitation regimes. The taxonomic and ecomorphological diversity of the Serengeti ruminant fauna is statistically comparable to those North American paleofaunas occurring during or immediately after the Middle Miocene Climatic Optimum (MMCO), but the later, more depauperate faunas are no longer comparable. This study quantitatively analyzes artiodactyl communities as they changed with the cooling and drying trend seen during the Neogene.

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Fossil specimens of American mastodons (Mammut americanum) that have been recovered from fluvial sedimentary deposits of the late Pleistocene of southeastern Hidalgo, central Mexico, are formally described. The sample includes cranial and postcranial remains of an adult male that represents one of the most complete mastodons that have been found in Mexico, as well as a molar belonging to an old individual. The material from Hidalgo shows the following diagnostic features of M. americanum: well-developed tusks that curved upward; upper and lower molars with a simple occlusal pattern; last molars with four to five lophs/ids; a medial sulcus between the lophs/ids; a scapula with a straight caudal border, a short and expanded neck, and a glenoid cavity that is oval in outline; a short and robust humerus with a prominent epicondylar crest; a radius with a wide distal end; and an ulna with a well-developed olecranon process and a deep trochlear notch. The dental size of the mastodon from Hidalgo is comparable to that of mastodons from the Gulf Coastal Plain and the Great Lakes region, whereas the limb dimensions are similar to those of the Watkins Glen Mastodon from the late Pleistocene of New York. In order to characterize the diet and habitat of the mastodons from Hidalgo, a stable carbon and oxygen isotope analysis in tooth enamel apatite was performed. The results indicate that studied specimens were mainly C3 browsers (mean δ 13 C isotope value of -10.06 ‰) that thrived in closed habitats covered by high vegetation, such as trees (mean δ 18 O isotope value of -4.31 ‰). This information suggests the presence of closed habitats in the southeastern region of Hidalgo, during the second half of the Pleistocene.Keywords: mastodons, taxonomy, palaeoecology, Hidalgo, central Mexico. Resumen Se describen formalmente ejemplares de mastodontes Americanos (Mammut americanum) recuperados de depósitos sedimentarios