Matthew C. Mihlbachler

Dietary change and evolution of horses in North America.

A survey of horse-teeth wear during the past 55 million years implies that evolutionary pressures were generally low. The evolution of high-crowned molars among horses (Family Equidae) is thought to be an adaptation for abrasive diets associated with the spread of grasslands. The sharpness and relief of the worn cusp apices of teeth (mesowear) are a measure of dietary abrasion. We collected mesowear data for North American Equidae for the past 55.5 million years to test the association of molar height and dietary abrasion. Mesowear trends in horses are reflective of global cooling and associated vegetation changes. There is a strong correlation between mesowear and crown height in horses; however, most horse paleopopulations had highly variable amounts of dietary abrasion, suggesting that selective pressures for crown height may have been weak much of the time. However, instances of higher abrasion were observed in some paleopopulations, suggesting intervals of stronger selection for the evolution of dentitions, including the early Miocene shortly before the first appearance of Equinae, the horse subfamily in which high-crowned dentitions evolved.

Coevolution of Tooth Crown Height and Diet in Oreodonts (Merycoidodontidae, Artiodactyla) Examined with Phylogenetically Independent Contrasts

The evolution of increased tooth crown height is considered to be an adaptation for coping with excessive rates of dental wear associated with abrasive herbivorous diets, such as grazing and(or high levels of exogenous grit (e.g. dust, sand, ash). Evolutionary trends in the crown heights of North American ungulates are grossly consistent with a transition from closed forests in the early Eocene to open grasslands in the late Miocene. However, the evolutionary proliferation of hypsodonty (high crowned teeth) in the early and middle Miocene occurs later than the apparent origin of open grassland habitats in North America. The paleoecology of species from the interval between the appearance of grasslands and the evolutionary proliferation of hypsodonty is critical to understanding the role of Cenozoic climate change in mammalian evolution. The paleodiets of late Eocene to middle Miocene oreodonts (Merycoidodontidae) were reconstructed by examining the relative facet development of molars (mesowear). A two-phase diet trend was discovered. Phase 1 suggests either an average reduction in the amount of exogenous grit from the late Eocene to early Oligocene or a decrease in fruit consumption related to the disappearance of more wooded habitats. Phase 2 is a gradual transition from early Oligocene low-abrasion browsing to high abrasion diets similar to mixed feeding and grazing in the Miocene. According to mesowear data, oreodont diets similar to those of modern grazers in terms of abrasion are not seen until the early Miocene (early Hemingfordian land mammal age). The coevolutionary relationship of molar crown height and diet, as represented by mesowear, was examined using phylogenetically independent contrasts. No significant coevolutionary relationship was found. In several instances, diet was found to shift over time despite morphological stasis (i.e. within a single species). These results do not clearly indicate that the overall trend of increasing dietary abrasion imposed sufficient selection to drive crown height evolution in oreodonts. Therefore, direct fossil evidence of dietary abrasion as a causal factor in the evolution of crown height, at least in this clade, is elusive.

Demography of late Miocene rhinoceroses (Teleoceras proterum and Aphelops malacorhinus) from Florida: linking mortality and sociality in fossil assemblages

Abstract Among polygynous mammals, a heightened risk of mortality is linked to the intensity of intragender competition and life-history stages, such as sexual maturity, where inexperienced individuals are vulnerable to the aggressive behaviors of dominant individuals. In this respect, the age- and sex-specific mortality patterns found in fossil assemblages could be informative of sociality in extinct species. This possibility was explored by comparing the age- and sex-specific demography of attritional rhinoceros assemblages, Teleoceras proterum (n = 2) and Aphelops malacorhinus (n = 1), from pond and fluvial sedimentary facies of the late Miocene of Florida, with modern skeletal assemblages of extant rhinos and other large mammals. Subadult and young adult males (between 15–40% of potential life span) numerically dominate the Teleoceras assemblages, indicating a disproportionately high frequency of localized young male mortality. The estimated age-specific mortality rates indicate elevated mortality risks among males at an age equivalent to the years encompassing male physiological and social maturity in modern rhinos, a pattern that suggests a high frequency of socially mediated mortality. Age-specific mortality rate curves of modern black rhino populations are essentially identical. A high frequency of intraspecific fight-related mortality characterizes modern rhinos and strongly suggests that elevated Teleoceras mortality was influenced by intragender competition. Although Teleoceras is widely believed to have been the analog of extant Hippopotamus, mortality rates of young males are not elevated in a modern Hippopotamus population. The Aphelops assemblage is not significantly male-biased and does not indicate elevated mortality rates of young males, suggesting that aspects of Aphelops sociality differed from modern rhinos. Although the nature of Aphelops sociality is not clear, aggression toward young males may have been less extreme or less frequent in Aphelops populations.

Microwear—mesowear congruence and mortality bias in rhinoceros mass-death assemblages

Abstract. n Although we do not know the cause of death of most fossil animals, mortality is often associated with ecological stress due to seasonality and other stochastic events (droughts, storms, volcanism) that may have caused shifts in feeding ecology preceding death. In these instances, dental microwear, which reflects feeding ecology in a narrow window of time, may provide a biased view of diet. Mesowear, another dental-wear proxy based on the morphology of worn cusps, requires macroscopic amounts of dental wear and reflects diet for a longer interval and may be less prone to bias from near-death ecological stress. We compared congruence between microwear and mesowear of North American, fossil rhinocerotid mass-death assemblages and collections of hunted modern rhinocerotids to test the hypothesis that fossil assemblages yield more incongruous microwear and mesowear data as a result of near-death ecological disturbances. In extant rhinos, both mesowear and microwear are associated with diet and height of the feeding environment. Mesowear and microwear in the modern rhinocerotid collections are statistically correlated, with strong relationships between average mesowear scores and labially distributed dental microwear. In contrast, a relationship between mesowear and microwear was not observed among the fossil rhinocerotid assemblages. Mesowear suggests that the fossil rhinos had low-abrasion diets, suggesting that they fed from clean, possibly tall vegetation. Some, but not all, mass-death assemblages produce microwear data with excessive scratches and/or pits compared with expectations based on mesowear results, suggesting that dental microwear was altered shortly before death in some but not all of the fossil assemblages. The dental-wear proxies available to paleoecologists provide amosaic of dietary evidence reflecting diet over long (mesowear) and more abbreviated (microwear) periods of time that, together, provide a richer understanding of feeding ecology and its relationship to environment, seasonal change, and other ecological disturbances.

The Brachial Plexus of the Sumatran Rhino (Dicerorhinus sumatrensis) and Application of Brachial Plexus Anatomy Toward Mammal Phylogeny

The peripheral nervous system is a promising resource for testing phylogeny although the branching patterns of peripheral nerves are not well documented outside of Homo sapiens. Here we describe the brachial plexus of the rare Sumatran rhinoceros (Dicerorhinus sumatrensis). We compare its brachial plexus to that of another perissodactyl (Equus asinus), an artiodactyl (Odocoileus virginianus), two carnivorans (Felis catus and Neovison vison), and one primate (Homo sapiens) and examine the phylogenetic structure of the resulting data. Brachial plexuses exhibit high rates of intraspecific polymorphism, but polymorphisms cannot be recognized from one specimen. To address concerns of error due to polymorphism, we dissected 52 mink brachial plexuses and compared them to human brachial plexus variation. Both species have numerous types of brachial plexus polymorphisms. Although most individual polymorphisms occur infrequently and unilaterally, because there are numerous types of polymorphisms, most humans and mink exhibit at least one polymorphism per brachial plexus. Parsimony analysis of 15 characters compiled from the brachial plexus data produced a tree that positions Artiodactyla and Perissodactyla as sister taxa, a result consistent with other analyses. Despite a high rate of polymorphism, the peripheral nervous system seems to carry a phylogenetic signal consistent with other morphological data. With a higher rate of taxon sampling, we suggest the brachial plexus will contribute valuable data for phylogenetic testing.

A small-bodied species of Brontotheriidae from the middle Eocene Nut Beds of the Clarno Formation, John Day Basin, Oregon

Abstract n We diagnose a new species of Brontotheriidae from a middle Eocene locality, the Clarno Nut Beds, from the Clarno Formation, John Day Basin, Central Oregon. Though renowned for its richness in fossil flora, fossil vertebrates are rare in the Clarno Nut Beds and this new species is the most abundantly represented mammal. Radiometric dating constrains the age of the Nut Beds fauna to about 43.76 Ma within the Uintan North American Land Mammal Age. This new taxon, represented by numerous cranial, mandibular, and dental specimens, is comparatively small for a brontothere and notable for its cranio-caudally shorted nasal incision, a trait shared with three larger-bodied middle Eocene species, Metatelmatherium ultimum. Wickia brevirhinus, and Sthenodectes incisivum. Phylogenetic analysis suggests the sister taxon of the Nut Beds brontothere could be one of two species—Wickia brevirhinus from the Sand Wash Basin of Colorado and Washakie Formation of Wyoming, or Metatelmatherium ultimum, a pan-Beringian species known from the Uintan Formation of Utah (and other Uintan age deposits) and the “Irdin Manha” Formation of Inner Mongolia, China. Phylogenetic results also indicate that the Nut Beds brontothere is a dwarf taxon. Though brontotheres are renowned for having evolved very large body sizes, this new brontothere is one of several discovered in recent decades that suggest evolutionary reductions in body size may have been relatively common in Brontotheriidae.