V. Louise Roth

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.

Homology and hierarchies: Problems solved and unresolved

Homology as a topic in phylogenetic analysis has to do with what is conserved in evolution. The problem of homology in systematics — to find homologues, and in so doing, to identify taxa — is distinct from the problem of identifying what kinds of features tend to be conserved, how and why. The two sets of issues are fundamentally interdependent at the point that one selects the appropriate taxonomic units, identifies the characters one wishes to study, or decides what constitutes a single character.

HOW ELEPHANTS GROW: HETEROCHRONY AND THE CALIBRATION OF DEVELOPMENTAL STAGES IN SOME LIVING AND FOSSIL SPECIES

ABSTRACT I present 1) baseline information on ontogeny in Recent elephants, and 2) some preliminary comparisons with fossil specimens. Relationships between size, state of skeletal fusion, and dental stage are similar in the two Recent species, Loxodonta africana and Elephas maximus. Limb bones that complete epiphyseal fusion late in ontogeny also grow more in absolute terms. Growth and dental progression are linearly related and probably gradual with respect to time, whereas skeletal fusion appears to be concentrated within a short period. Females are small and advanced in skeletal fusion for their dental stages. Limb proportions in elephants change with increasing size: the scapula becomes longer relative to other bones, and the femur becomes longer relative to the tibia. Among L. africana, E. maximus, and Mammuthus columbi, animals similar in size have similar limb proportions; the largest individuals, of L. africana and M. columbi, have exaggerated proportions and are therefore peramorphic. One might ...

Scaling of the mandible in squirrels

We compared the shape of the mandible among New World tree squirrels and selected outgroup taxa using linear measurements and areas defined by the median axis and conventional anatomical landmarks. We modified the median axis technique to define novel measurements, which proved complementary to those obtained from conventional landmarks. Allometric analyses showed that the scaling of the mandible among the New World tree squirrels is generally isometric (as has been observed in other groups of mamimals), but diverges from isometry in a tendency in smaller animals for the masseteric ridge to be displaced anteriorly, the condylar process and posterior portion of the ascending ramus to be relatively elongated, and the coronoid process to be shortened. Allometric analyses also revealed the ways and extent that outgroup taxa deviated from the scaling pattern observed for the New World tree squirrels. A flying squirrel (subfamily Pteromyinae), a moderate‐sized callosciurine squirrel, and three species of pygmy tree squirrels from Asia and Africa show mandibular proportions very similar to those predicted for New World tree squirrels of corresponding size. Ground squirrels (tribe Marmotini) and successively more distant relatives such as Aplodontia, two myomorph rodents, and a rabbit show greater differences from the New World tree squirrels in their mandibular proportions. Combining the use of median‐axis and conventional measurements makes it possible to examine changing relationships between locations of anatomically homologous landmarks and the geometry of the form. J Morphol 232:107–132, 1997.

The impact of large terrestrial carnivores on Pleistocene ecosystems

Significance At very high densities, populations of the largest herbivores, such as elephants, have devastating effects on the environment. What prevented widespread habitat destruction in the Pleistocene, when the ecosystem sustained many species of huge herbivores? We use data on predator–prey body mass relationships to predict the prey size ranges of large extinct mammalian carnivores, which were more diverse and much larger than living species. We then compare these prey size ranges with estimates of young mammoth sizes and show that juvenile mammoths and mastodons were within predicted prey size ranges of many of the Pleistocene carnivores. From this and other fossil evidence we argue that, by limiting population sizes of megaherbivores, large carnivores had a major impact on Pleistocene ecosystems. Large mammalian terrestrial herbivores, such as elephants, have dramatic effects on the ecosystems they inhabit and at high population densities their environmental impacts can be devastating. Pleistocene terrestrial ecosystems included a much greater diversity of megaherbivores (e.g., mammoths, mastodons, giant ground sloths) and thus a greater potential for widespread habitat degradation if population sizes were not limited. Nevertheless, based on modern observations, it is generally believed that populations of megaherbivores (>800 kg) are largely immune to the effects of predation and this perception has been extended into the Pleistocene. However, as shown here, the species richness of big carnivores was greater in the Pleistocene and many of them were significantly larger than their modern counterparts. Fossil evidence suggests that interspecific competition among carnivores was relatively intense and reveals that some individuals specialized in consuming megaherbivores. To estimate the potential impact of Pleistocene large carnivores, we use both historic and modern data on predator–prey body mass relationships to predict size ranges of their typical and maximum prey when hunting as individuals and in groups. These prey size ranges are then compared with estimates of juvenile and subadult proboscidean body sizes derived from extant elephant growth data. Young proboscideans at their most vulnerable age fall within the predicted prey size ranges of many of the Pleistocene carnivores. Predation on juveniles can have a greater impact on megaherbivores because of their long interbirth intervals, and consequently, we argue that Pleistocene carnivores had the capacity to, and likely did, limit megaherbivore population sizes.

Quantitative variation in elephant dentitions: implications for the delimitation of fossil species

Dental measurements are commonly used in the diagnosis of fossil elephant species, yet elephant teeth develop slowly, within a highly dynamic context that enhances opportunities for physical deformation (or its subtler manifestation, quantitative phenotypic variation). This paper examines intraspecific variation in elephant teeth and compares it with variability in other mammals (83 species in 7 orders). I conclude that (1) male elephants tend to have slightly larger cheek teeth than females, though the difference is not marked; (2) of the full complement of six cheek teeth per jaw quadrant, no single tooth consistently varies less than the others (so on this basis, for taxonomic decisions no tooth is preferable to the customarily used M3 = tooth VI); (3) single- population samples vary less than more inclusive, geographically heterogeneous samples of elephant teeth; (4) although differential wear and eruption are important sources of variation in dental measurements, complete elephant teeth are consistently more variable in length and width than the cheek teeth of other mammals; (5) variability in dental dimensions of recognized fossil species of elephants is in general not grossly inconsistent with variability noted in modern elephants, but there are some exceptions. With the information and guidelines presented here, consideration of variability, and assessment of the statistical power permitted by available samples, can enhance confidence and precision in the delimitation of species and provide a firmer basis for macroevo- lutionary inferences.

Fabricational noise in elephant dentitions

A marked retardation of dental ontogeny characterizes the family Elephantidae. As a consequence of this retardation, elephant teeth are subject to the forces of mastication, eruption, and progression while still in a developing and pliant stage. As specimens described here illustrate, the mechanical forces are often sufficient to deform the gross morphology of dentitions. Morphological variation in elephant teeth can be regarded as “fabricational noise”—revealing information about the dynamic context in which the teeth develop. Accordingly, dental variation is less species-specific in elephants than in other mammals. The fossil record may comprise fewer species of elephants than is generally believed, and trends inferred to reflect rapid evolution within this family may in fact reflect phenotypic plasticity.

Noninvasive histological comparison of bone growth patterns among fossil and extant neonatal elephantids using synchrotron radiation X-ray microtomography

ABSTRACT How is the bone tissue in skeletal supports of a neonatal elephant organized, and how does this histological structure differ among the neonates of modern species, mammoths, and insular dwarfs? We used synchrotron X-ray microtomography (SR-µCT) to obtain high-resolution image-‘slices’ noninvasively, from the femoral and tibial diapbyses of neonatal African elephants, a young juvenile Asian elephant, Columbian mammoths, and California Channel Island pygmy mammoths. The results compared favorably in level of detail with histological sectioning, but without the shrinkage, distortion, or loss of tissue inevitable with histology. From the tomography images we were able to rank by ontogenetic stage specimens of taxa that are otherwise difficult to categorize because they vary greatly in size; from these images we observed that laminar fibrolamellar bone predominated and were able to quantify vascular patterns. Bones of the Columbian mammoth typically had the thickest and largest number of laminae, whereas the insular dwarfs were notable in their variability. A distinct change in tissue microstructure marks the boundary between prenatal and postnatal periosteal bone deposition. Qualitatively, patterns of early bone growth of elephantids resemble those in many young tetrapods that grow into large adults, including sauropod dinosaurs.

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.

Maternal effects on body size of large insular Peromyscus maniculatus: evidence from embryo transfer experiments

Adult Peromyscus maniculatus gambelii from mainland California are significantly smaller than P. m. santacruzae from Santa Cruz Island, California, whether they are captured in the wild or raised from birth in captivity. The two subspecies also differ significantly in weight as neonates. Results from the experimental transfer of 4-day old P. m. gambelii embryos into P. m. santacruzae foster mothers demonstrate that the average neonatal weight of P. m. gambelii offspring increases significantly if they are borne and delivered by P. m. santacruzae foster mothers. This apparent prenatal non-genetic maternal influence on neonatal body size does not appear to be caused by differences in gestation period or average litter size, or by a genetic bias in the donor sample. Although the transferred offspring were large at birth, and were nursed by the large P. m. santacruzae foster mothers, they were relatively small (equivalent to other P. m. gambelii ) at maturity. The results of our experimental studies are concordant with predictions from quantitative genetic studies on laboratory mice.