Body Size in Amblyrhiza inundata (Rodentia, Caviomorpha), an Extinct Megafaunal Rodent From the Anguilla Bank, West Indies: Estimates and Implications

Rodent species typically evolve larger mean body sizes when isolated on islands, but the extinct caviomorph Amblyrhiza inundata, known only from Quaternary cave deposits on the islands of Anguilla and St. Martin (northern Lesser Antilles), provides an unusually dramatic example of insular gigantism. Here we report on a series of body mass estimates for Amblyrhiza using predictive equations based on anteroposterior diameters and cortical cross-sectional areas of humeral and femoral diaphyses. Analyses of 14 isolated specimens (5 femoral, 9 humeral), all representing adult or near adult animals, yield body mass estimates ranging from slightly less than 50 kg to more than 200 kg. Body size estimates derived from humeral measurements are lower than those derived from femoral measurements, but the significance of this will remain unclear until matched limb bones (i.e., specimens from the same animal) are recovered. Incisor measurements are also highly variable, but in this case the distribution is demonstrably bimodal. Presence of multiple coeval species, temporal variation, limb heterogeneity, and sexual dimorphism all qualify as possible explanations of the variation encountered in Amblyrhiza data sets, but available samples are not adequate for making a robust choice among them. Body size affects many life history variables, including demography. Population estimates derived from empirical data and predictive equations suggest that only a few thousand individuals of Amblyrhiza could have occupied the islands of the Anguilla Bank at any one time during the Late Quaternary. At certain times-for example, during the last interglacial (Sangamonian) highstand-population numbers might have sunk to only a few hundred. Absolutely small population sizes of Amblyrhiza and severe fluctuations in island area during the late Quaternary surely affected its susceptibility to extinction, whether or not humans were ultimately responsible for the event (for which there is as yet no direct evidence)

Hindlimb function in the alligator: integrating movements, motor patterns, ground reaction forces and bone strain of terrestrial locomotion

Alligator hindlimbs show high torsional loads during terrestrial locomotion, in sharp contrast to the bending or axial compressive loads that predominate in animals that use parasagittal limb movements. The present study integrates new data on hindlimb muscle function with previously obtained data on hindlimb kinematics, motor patterns, ground reaction forces and bone strain in order to (1) assess mechanisms underlying limb bone torsion during non-parasagittal locomotion in alligators and (2) improve understanding of hindlimb dynamics during terrestrial locomotion. Three dynamic stance phase periods were recognized: limb-loading, support-and-propulsion, and limb-unloading phases. Shear stresses due to torsion were maximized during the limb-loading phase, during which the ground reaction force (GRF) and caudofemoralis (CFL) muscles generated opposing moments about the femur. Hindlimb retraction during the subsequent stance-and-propulsion phase involves substantial medial rotation of the femur, powered largely by coordinated action of the GRF and CFL. Several muscles that actively shorten to flex and extend limb joints during stance phase in sprawling and erect quadrupeds act in isometric or even eccentric contraction in alligators, stabilizing the knee and ankle during the support-and-propulsion phase. Motor patterns in alligators reveal the presence of local and temporal segregation of muscle functions during locomotion with muscles that lie side by side dedicated to performing different functions and only one of 16 muscles showing clear bursts of activity during both stance and swing phases. Data from alligators add to other recent discoveries that homologous muscles across quadrupeds often do not move joints the same way as is commonly assumed. Although alligators are commonly considered models for early semi-erect tetrapod locomotion, many aspects of hindlimb kinematics, muscle activity patterns, and femoral loading patterns in alligators appear to be derived in alligators rather than reflecting an ancestral semi-erect condition