Femoral Shape in Procyonids (Carnivora, Procyonidae): Morphofunctional Implications, Size and Phylogenetic Signal

Functional and phylogenetic interpretation of the forelimb myology of two South American carnivorans, the ring-tailed coati (Nasua nasua) and crab-eating raccoon (Procyon cancrivorus)

A comparative analysis of the forelimb myology of two neotropical procyonids (Nasua nasua and Procyon cancrivorus) was performed to assess how observed differences in their myological configuration would be related to their diverse ecological behaviors and phylogeny. Although both species are associated with the arboreal substrate, N. nasua is a more agile climber that usually digs; whereas P. cancrivorus spends most of its time on the ground foraging, climbing on the trees as shelter and is a good swimmer. Here, myological descriptions, muscle maps, phylogenetic optimizations, and muscle mass data of the forelimb of these two procyonids are presented. The main functional muscular groups are discussed in a comparative framework with other carnivorans that present a wide ecological diversity. Also, muscular characters were mapped onto a phylogeny to explore their evolution and to obtain ancestral state reconstructions. Results indicate clear myological differences among the two neotropical procyonids associated with their ecological preferences. One of the most remarkable anatomical differences is the arrangement and relative mass of the extrinsic musculature, mainly the musculus rhomboideus and the delto-pectoral complexes. In Nasua nasua, these suggested a greater stability in their shoulder girdle for climbing and digging and probably would provide stronger neck and head movements when they use them for foraging on the ground. Conversely, P. cancrivorus has a different extrinsic muscular configuration, which would allow an increment on the stride length and faster movements of the forelimb associated with more frequent terrestrial gaits. Also, significant differences are observed in the distal musculature, associated with strong movements of forepaws when climbing and digging in N. nasua; whereas, P. cancrivorus configuration suggested precise forearm and digits movements, related to manipulation of food items when they are catching prey or feeding. Most of the codified features of P. cancrivorus would reflect retention of plesiomorphies acquired in the common ancestor of caniforms or arctoids, whereas N. nasua shows derived traits, particularly in the proximal forelimb region. The present work increases the information available on the myology of these particular taxa and extant generalized arctoid models in general. The analyses presented here will be useful both for other comparative myological studies (morpho-functional and phylogenetic) and for muscular reconstruction in extinct procyonids, as well as other carnivorans.

Effects of size, phylogeny and locomotor habits on the pelvic and femoral morphology of South American spiny rats (Rodentia: Echimyidae)

The rodent family Echimyidae (spiny rats, hutias and coypu) is notable for its high phylogenetic and ecological diversity, encompassing ~100 living species with body mass ranging from 70 to 4500 g, including arboreal, epigean (non-arboreal or scansorial), fossorial and semi-aquatic taxa. In view of this diversity, it was hypothesized that echimyid morphological variation in the pelvis and femur should reflect: (1) allometric association with body mass; (2) morphofunctional specializations for the different locomotor habits; and (3) phylogenetic history. To test these propositions, we examined 30 echimyid species, in addition to eight species of two other octodontoid families, Abrocomidae and Octodontidae. Pelvic and femoral variation was assessed with linear morphometry, using bivariate and multivariate statistical methods, part of which was phylogenetically informed. Approximately 80% of the total variation among echimyids was explained by body mass, and some univariate measurements were found potentially to be effective as body mass estimators after simple allometric procedures, notably in the pelvis. Even considering the significant phylogenetic signal, variation in shape was largely structured by locomotor habits, mainly in the pelvis, suggesting that the echimyid hindlimb diversification was driven, in part, by selective pressures related to locomotor habits. Finally, echimyid femoral disparity was considerably greater than in other octodontoids, contrasting with their relatively modest cranial variation. Thus, this study suggests that hindlimb diversity constitutes a key factor for the exceptional echimyid ecological and phyletic diversification.

Postcranial elements of small mammals as indicators of locomotion and habitat

Many studies have shown a correlation between postcranial anatomy and locomotor behavior in mammals, but the postcrania of small mammals (<5 kg) is often considered to be uninformative of their mode of locomotion due to their more generalized overall anatomy. Such small body size was true of all mammals during the Mesozoic. Anatomical correlates of locomotor behavior are easier to determine in larger mammals, but useful information can be obtained from the smaller ones. Limb bone proportions (e.g., brachial index) can be useful locomotor indicators; but complete skeletons, or even complete long bones, are rare for Mesozoic mammals, although isolated articular surfaces are often preserved. Here we examine the correlation of the morphology of long bone joint anatomy (specifically articular surfaces) and locomotor behavior in extant small mammals and demonstrate that such anatomy may be useful for determining the locomotor mode of Mesozoic mammals, at least for the therian mammals.