Samantha S. B. Hopkins

Tempo of trophic evolution and its impact on mammalian diversification

Mammals are characterized by the complex adaptations of their dentition, which are an indication that diet has played a critical role in their evolutionary history. Although much attention has focused on diet and the adaptations of specific taxa, the role of diet in large-scale diversification patterns remains unresolved. Contradictory hypotheses have been proposed, making prediction of the expected relationship difficult. We show that net diversification rate (the cumulative effect of speciation and extinction), differs significantly among living mammals, depending upon trophic strategy. Herbivores diversify fastest, carnivores are intermediate, and omnivores are slowest. The tempo of transitions between the trophic strategies is also highly biased: the fastest rates occur into omnivory from herbivory and carnivory and the lowest transition rates are between herbivory and carnivory. Extant herbivore and carnivore diversity arose primarily through diversification within lineages, whereas omnivore diversity evolved by transitions into the strategy. The ability to specialize and subdivide the trophic niche allowed herbivores and carnivores to evolve greater diversity than omnivores.

QUANTITATIVE MORPHOLOGICAL PROXIES FOR FOSSORIALITY IN SMALL MAMMALS

Abstract Burrowing behavior is widespread among mammals and has generated a diverse array of adaptive responses to the physical demands of this lifestyle. While extensive research has been devoted to the morphological, ecological, and evolutionary implications of burrowing, it remains difficult to compare burrowing adaptations between mammals of widely divergent ancestry. A reliable quantitative proxy for fossoriality (burrowing) is necessary for such comparisons as well as for detailed descriptions of ecology from specimens of rare, extinct, and fossil mammals. This study presents several quantitative indices of the morphology of burrowing mammals based on 20 measurements of skull and skeletal morphology taken from 123 different mammalian species, both burrowing and nonburrowing. Discriminant analyses revealed that these quantitative characters successfully distinguish nonburrowing taxa from those that are adapted to a burrowing lifestyle. Additionally, more subtle distinctions between subterranean taxa (which rarely emerge above ground) and other burrowers as well as between mammals using different methods of burrow excavation were identified from these characters. A test of these indices using 6 extinct species yielded results consistent with more-detailed descriptions of the functional morphology of these taxa, indicating that our quantitative proxies provide an important basis for comparisons of fossorial adaptations across divergent mammalian clades.

The evolution of fossoriality and the adaptive role of horns in the Mylagaulidae (Mammalia: Rodentia)

Ceratogaulus, a member of the extinct fossorial rodent clade Mylagaulidae, is the only known rodent with horns and the smallest known horned mammal. The function of the large, dorsally projecting nasal horns on this burrowing animal has been the subject of wide speculation among palaeontologists; suggested uses range from sexual combat to burrowing. Mammals have evolved adaptations for digging repeatedly; horns and other cranial appendages have also evolved numerous times. These two adaptations co-occur in mammals extremely rarely: only two fossil genera (Ceratogaulus and the xenarthran Peltephilus) and no extant mammals are both horned and fossorial. Tracing the evolution of fossoriality in aplodontoid rodents (the larger clade to which Ceratogaulus belongs) reveals that Ceratogaulus descended from ancestors who dug by head-lifting. Whereas this suggests an obvious explanation for the horns of this rodent, evidence from functional morphology, anatomy, phylogeny and geologic context indicates that the horns in Ceratogaulus were used for defence, rather than digging, and evolved to offset increased predation costs associated with spending more time foraging above ground as body size increased.

BIOSTRATIGRAPHY AND MAGNETOSTRATIGRAPHY OF THE MID-MIOCENE RAILROAD CANYON SEQUENCE, MONTANA AND IDAHO, AND AGE OF THE MID-TERTIARY UNCONFORMITY WEST OF THE CONTINENTAL DIVIDE

Abstract The Barstovian of the northern Rocky Mountains, U.S.A., is known mainly from deposits east of the continental divide; this article provides new information from west of the divide. The biostratigraphic, geologic, magnetostratigraphic, and lithostratigraphic setting is reported for an unusually complete Arikareean, Hemingfordian, and Barstovian stratigraphic section known as the Railroad Canyon Sequence in Montana and Idaho. At least 35 taxa of fossil vertebrates collected from 50 different localities are placed in stratigraphic context. Lithostratigraphic attributes indicate the presence of a freshwater lake during the Arikareean, intermittent saline lakes through much of the Hemingfordian, a late Hemingfordian unconformity (the Mid-Tertiary Unconformity), and absence of persistent lakes through the Barstovian. The sequence records the development of increasingly arid conditions in the depositional basin. A change in bedding attitude and generally coarser sediments above the Mid-Tertiary Unconformity indicate uplift of the region during the Barstovian, possibly accompanied by slight structural tilting. The analysis helps date Barstovian faunas, and provides a useful tie point for correlating the Barstovian Land-Mammal Age to the magnetostratigraphic and radioisotopic scale in the western Rockies, and ultimately to the Global Polarity Time Scale (GPTS). It also suggests that the Mid-Tertiary unconformity is approximately coeval on the eastern and western flanks of the mountains, supporting tectonic models that require regional uplift between ca. 16.8 and 17.5 Ma.

Biodiversity and Topographic Complexity: Modern and Geohistorical Perspectives

Topographically complex regions on land and in the oceans feature hotspots of biodiversity that reflect geological influences on ecological and evolutionary processes. Over geologic time, topographic diversity gradients wax and wane over millions of years, tracking tectonic or climatic history. Topographic diversity gradients from the present day and the past can result from the generation of species by vicariance or from the accumulation of species from dispersal into a region with strong environmental gradients. Biological and geological approaches must be integrated to test alternative models of diversification along topographic gradients. Reciprocal illumination among phylogenetic, phylogeographic, ecological, paleontological, tectonic, and climatic perspectives is an emerging frontier of biogeographic research.

PHYLOGENY AND BIOGEOGRAPHY OF THE GENUS ANSOMYS QIU, 1987 (MAMMALIA: RODENTIA: APLODONTIDAE) AND DESCRIPTION OF A NEW SPECIES FROM THE BARSTOVIAN (MID-MIOCENE) OF MONTANA

Abstract New aplodontid material recovered from Hepburns Mesa, Montana, stimulated reexamination of Ansomys, a genus of aplodontid previously known only from Asia. A cladistic analysis of the known species of Ansomys, as well as new material from Hepburns Mesa and a few other morphologically similar species, prompted reconstruction of the biogeographic history of the genus. One new species, A. hepburnensis, from the Barstovian of Montana, is described, and two other species, A. nexodens and A. descendens, are placed in Ansomys rather than in Pseudallomys and Plesispermophilus, respectively. Addition of these three species to Ansomys extends its distribution throughout the Holarctic in the mid-Miocene. Stratigraphic ranges combined with phylogenetic relationships between species suggest this wide distribution as early as the late Oligocene, which is unique among aplodontid genera. The distribution, the rarity, the unusually small size, and the complex cusp morphology of Ansomys suggest a very specialized ecology for members of this clade.

New material of Alphagaulus pristinus (Mammalia, Rodentia, Mylagaulidae) from the Deep River Formation (Montana, U.S.A.): Implications for ecology, ontogeny, and phylogeny

ABSTRACT The Mylagaulidae are a family of extinct fossorial rodents that are common in the North American Miocene. Alphagaulus pristinus, from the Barstovian of Montana, was previously known only from partial dentaries and isolated teeth, with no well-described skull or postcranial material. Alphagaulus is important to understanding the evolution of burrowing in mylagaulids, because it is within this genus that the most dramatic evolution of fossorial morphology within mylagaulids occurs. The other three species of the genus are all known from partial skulls, rare isolated postcrania for Alphagaulus vetus, and, in the case of Alphagaulus tedfordi, a nearly complete skeleton. We describe new material housed at the University of Washington Burke Museum, including three well-preserved skulls and partial skeletons showing different ontogenetic stages from juveniles to an adult. The description of this new material gives insights into the ontogeny of these burrowing mammals. In particular, it is apparent that the developmental trajectory mimics evolution in the increase of fossorial modifications of the skeleton with time. The description of these specimens also allows reanalysis of the phylogenetic relationships within the Mylagaulidae, confirming that the genus Alphagaulus is paraphyletic. Though the phylogenetic positions of species within the genus are different, our results are more stable than those of previous analyses, allowing greater confidence in the reconstructed relationships among mylagaulid species and genera.

Oligo-Miocene climate change and mammal body-size evolution in the northwest United States: a test of Bergmann's Rule

Abstract Whether or not climate plays a causal role in mammal body-size evolution is one of the longest-standing debates in ecology. Bergmanns Rule, the longest-standing modeladdressing this topic, posits that geographic body-mass patterns are driven by temperature, whereas subsequent research has suggested that other ecological variables, particularly precipitation and seasonality, may be the major drivers of body-size evolution. While paleoecological data provide a unique and crucial perspective on this debate, paleontological tests of Bergmanns rule and its corollaries have been scarce. We present a study of body-size evolution in three ecologically distinct families of mammal (equids, canids, and sciurids) during the Oligo-Miocene of the northwest United States, an ideal natural laboratory for such studies because of its rich fossil and paleoclimatic records. Body-size trends are different in all three groups, and in no case is a significant relationship observed between body size and any climatic variable, counter to what has been observed in modern ecosystems. We suggest that for most of the Cenozoic, at least in the Northwest, body mass has not been driven by any one climatic factor but instead has been the product of complex interactions between organisms and their environments, though the nature of these interactions varies from taxon to taxon. The relationship that exists between climate and body size in many groups of modern mammals, therefore, is the exception to the rule and may be the product of an exceptionally cool and volatile global climate. As anthropogenic global warming continues and ushers in climatic conditions more comparable to earlier intervals of the Cenozoic than to the modern day, models of corresponding biotic variables such as body size may lose predictive power if they do not incorporate paleoecological data.

Osteopathology in Rhinocerotidae from 50 Million Years to the Present

Individual elements of many extinct and extant North American rhinocerotids display osteopathologies, particularly exostoses, abnormal textures, and joint margin porosity, that are commonly associated with localized bone trauma. When we evaluated six extinct rhinocerotid species spanning 50 million years (Ma), we found the incidence of osteopathology increases from 28% of all elements of Eocene Hyrachyus eximius to 65–80% of all elements in more derived species. The only extant species in this study, Diceros bicornis, displayed less osteopathologies (50%) than the more derived extinct taxa. To get a finer-grained picture, we scored each fossil for seven pathological indicators on a scale of 1–4. We estimated the average mass of each taxon using M1-3 length and compared mass to average pathological score for each category. We found that with increasing mass, osteopathology also significantly increases. We then ran a phylogenetically-controlled regression analysis using a time-calibrated phylogeny of our study taxa. Mass estimates were found to significantly covary with abnormal foramen shape and abnormal bone textures. This pattern in osteopathological expression may reflect a part of the complex system of adaptations in the Rhinocerotidae over millions of years, where increased mass, cursoriality, and/or increased life span are selected for, to the detriment of long-term bone health. This work has important implications for the future health of hoofed animals and humans alike.

Are Hypsodonty and Occlusal Enamel Complexity Evolutionarily Correlated in Ungulates

The spread of grasslands and cooling climate in the Miocene contributed to an increasingly abrasive diet for ungulates. This increase in abrasiveness is proposed to select for both hypsodonty and increasing complexity of occlusal enamel bands. If these traits evolved in response to strong selection to resist tooth wear while feeding in grassland habitats, we might expect them to have evolved in a correlated fashion. If, on the other hand, there was a developmental or physiological constraint, or if selection was not strong on total enamel production, we would expect species to have evolved one or the other of these traits at a time, producing an uncorrelated, or even inversely correlated, pattern of trait evolution. To test these hypotheses, we examined the Occlusal Enamel Index (OEI) and Hypsodonty Index (HI) of 773 ungulate teeth. We tested the dependence of OEI on HI for the orders Artiodactyla and Perissodactyla using phylogenetic generalized least squares regression (PGLS). The two traits are not significantly correlated in the PGLS, for Artiodactyla and Perissodactyla. Despite their physical proximity, close functional utility, and conventional correlation, our results reject the hypothesis that HI and OEI are evolutionarily linked in these lineages, suggesting that selection to resist tooth wear was not so strong as to drive the overall evolutionary trajectory of both these traits at the same time.