Commonality in Pelvic Anatomy among Three Fossorial, Scratch-Digging, Mammalian Species

Phylogenetic, Allometric, and Ecological Factors Affecting Morphological Variation in the Scapula and Humerus of Spiny Rats (Rodentia: Echimyidae)

Locomotion, as a fundamental function in mammals directly associated with the use of ecological resources, is expected to have anatomical structures functionally committed that evolved under intense selective pressure, possibly carrying specializations for different locomotor habits. Among caviomorph rodents, the family Echimyidae stands out for having the greatest species richness, with relatively well-resolved phylogenetic relationships, wide variation in body mass, and remarkable diversity of locomotor habits, including arboreal, scansorial, semi-aquatic, semifossorial, and terrestrial forms. Thus, Echimyidae constitutes a promising model for understanding how phylogenetic, allometric, and ecological factors affect the evolution of postcranial structures directly linked to locomotor function. We investigated the influence of these three factors on scapular and humeral morphological variation in 38 echimyid species using two-dimensional geometric morphometry and phylogenetically informed comparative methods. Scapular and humeral shape variation had a low correlation with body mass and structure size, conveying a small or negligible allometric effect. Conversely, a significant moderate to strong phylogenetic signal was detected in both structures, suggesting that an important part of their morphometric variation results from shared evolutionary history. Notably, morphological variation of the scapula was extensively structured by phylogeny, without the marked influence of locomotor habits, suggesting that its shape may be a suitable taxonomic marker. Finally, locomotor habits were important in structuring the morphological variation of the humerus. Our results suggest that the morphologies of the scapula and humerus, despite being anatomically and functionally interconnected, were differentially shaped by ecological factors associated with locomotor habits.

Ecological correlates of the morphology of the auditory bulla in rodents: Application to the fossil record

For rodents, hearing is essential to survival. It enables predator evasion, prey detection, and conspecific recognition; it is also likely to be constrained by the physical environment. The resulting hypothetical link between tympanic bulla morphology and ecology has never been investigated across a broad array of rodent species before. Such link may enable the determination of the ecological affinities of many fossil species only known from partial skulls. In this study, we used geometric morphometrics to quantify the shape of the auditory bulla of 197 specimens representing 91 species from 17 families of extant rodents across four different locomotory modes. We used landmarks and semi-landmarks on the ventral and lateral views of the skull to capture morphological characteristics of the bulla and external auditory meatus (EAM). Our results demonstrate an association between bullar morphology and locomotion in rodents. Bullar shape enables the correct classification of 76% of the species in our training set. Fossorial taxa, in particular, show a characteristic morphology including an asymmetric bulla with a dorsally located and laterally expanded EAM that has a small opening diameter. A phylogenetically informed flexible discriminant analysis shows a weak phylogenetic effect on tympanic morphology. There is no evidence for differences in bullar hypertrophy across locomotory categories. The application of this approach to select fossil rodents from the Oligo-Miocene shows broad agreements with prior studies and yields new locomotory inferences for 14 fossil species, including the first proposed locomotion for members of the family Florentiamyidae. Such results call for the timing of burrowing diversification in rodents to be reevaluated.

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.

Variación no alométrica en el cráneo del perezoso bayo

El perezoso bayo Bradypus variegatus (Schinz, 1825) es un mamífero monomórfico, de ontogenia craneal poco conocida. En este estudio analizamos la alometría estática entre tamaño y forma, utilizando 21 especímenes diferentes de edades diversas. El tamaño y la forma fueron determinados mediante técnicas de morfometría geométrica. De los resultados obtenidos se desprende que la variación en la forma craneal queda muy poco explicada por la variación en el tamaño. Muchos estudios han señalado características morfológicas únicas en los perezosos en relación al resto de mamíferos, características fenotípicas que van de la estructura esquelética a tejidos blandos. En este caso, el escalado no alométrico del cráneo debería ser visto como otra característica única de este grupo taxonómico.