Muscular-hydrostats, muscular organs which lack typical systems of skeletal support, include the tongues of mammals and lizards, the arms and tentacles of cephalopod molluscs and the trunks of elephants. In this paper the means by which such organs produce elongation, shortening, bending and torsion are discussed. The most important biomechanical feature of muscular-hydrostats is that their volume is constant, so that any decrease in one dimension will cause a compensatory increase in at least one other dimension. Elongation of a muscular-hydrostat is produced by contraction of transverse, circular or radial muscles which decrease the cross-section. Shortening is produced by rontraction of longitudinal muscles. The relation between length and width of a constant volume structure allows amplification of muscle force or displacement in muscular-hydrostats and other hydrostatic systems. Bending requires simultaneous contraction of longitudinal and antagonistic circular, transverse or radial muscles. In bending, one muscle mass acts as an effector of movement while the alternate muscle mass provides support for that movement. Torsion is produced by contraction of muscles which wrap the muscular-hydrostat in a helical fashion.

We report new evidence that bears decisively on a long-standing controversy in primate systematics. DNA sequence data for the complete cytochrome b gene, combined with an expanded morphological data set, confirm the results of a previous study and again indicate that all extant Malagasy lemurs originated from a single common ancestor. These results, as well as those from other genetic studies, call for a revision of primate classifications in which the dwarf and mouse lemurs are placed within the Afro-Asian lorisiforms. The phylogenetic results, in agreement with paleocontinental data, indicate an African origin for the common ancestor of lemurs and lorises (the Strepsirrhini). The molecular data further suggest the surprising conclusion that lemurs began evolving independently by the early Eocene at the latest. This indicates that the Malagasy primate lineage is more ancient than generally thought and places the split between the two strepsirrhine lineages well before the appearance of known Eocene fossil primates. We conclude that primate origins were marked by rapid speciation and diversification sometime before the late Paleocene.Although strepsirrhine (we use the term strepsirrhine to define the living tooth-combed primates, their immediate ancestor, and all of its descendants) primates comprise more than one-third of the living members of the primate order, there is no current consensus concerning their phylogeny, classification, or time of divergence. Phylogenetic debate centers around two groups of Malagasy lemurs, the mouse and dwarf lemur group (family Cheirogaleidae) and the aye-aye (family Daubentoniidae). Morphologists inferred from the basicranial anatomy of the cheirogaleids that these animals are actually members of the Afro-Asian loris group (1, 2). This hypothesis was widely accepted and reflected in the majority of modern primate classifications (3-6). Cladistic studies of DNA sequences (7-9) have failed to support the lorisiform association, however, and have found instead that cheirogaleids belong within a Malagasy primate clade, thereby agreeing with an early synthetic view (10) and with genetic distance studies (11-13). The unusual morphological specializations of the aye-aye (e.g., ever-growing rodent-like incisors, clawed digits, and an extremely elongated middle finger) have made it difficult to place within strepsirrhine phylogeny also. One morphology-based hypothesis claims that the aye-aye comprises a monotypic sister group to all remaining strepsirrhines (14, 15); another holds that the phylogenetic position of the aye-aye is indeterminate relative to all other primates (16). DNA sequence studies have likewise given conflicting results. A study of mitochondrial DNA placed the aye-aye at the base of the strepsirrhine clade (7), whereas a study of nuclear DNA placed it securely with the other Malagasy primates (9).The resolution of these controversies is important for our understanding of both primate phylogeny and historical bio- Another area of debate concerns strepsirrhine ...

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. macroevolution | ecological specialization | character evolution L iving mammals are remarkably diverse: they span eight orders of magnitude in mass, occupy a variety of habitats across the globe, and exploit subterranean, aquatic, terrestrial, arboreal, and aerial niches. Living mammals also show striking differences in diversity between lineages of similar age, from the more than 2,200 species of rodent to the single species of aardvark (1, 2). Early mammals were small, homoeothermic endotherms with tribosphenic molars. Homoeothermic endothermy enabled mammals to survive in a wider range of ambient temperatures and achieve higher sustained activity levels, but it also increased energy demands (3). These increased energetic demands necessitated adaptations or behaviors that either allowed more efficient extraction of energy from the food consumed, entailed consumption of more energy rich foods, or required an increase in the time spent foraging and eating. The tribosphenic molar, which combines shearing and crushing functions in the precisely occluding teeth, is considered to be a key innovation that promoted more effective carnivory and omnivory in early mammalian lineages (4). This type of tooth is also frequently cited as facilitating the diversification of therian mammals (4-6). The tribosphenic molar is an evolutionarily and functionally highly versatile structure (4, 7) that, in combination with heterodonty (different tooth types within the jaw), enabled mammals to evolve a disparate array of specialized dentitions and thus adapt to a broad variety of niches. Indeed, the extraordinary dental diversity of mammals-to the extent that many species can be identified by the morphology of their molars alone (8)-is a testament to the importance of diet to mammalian evolution.Although the adaptations of individual mammalian lineages to d...