Weighing in on miniaturization: New body mass estimates for Triassic eucynodonts and analyses of body size evolution during the cynodont-mammal transition

Body size influences most aspects of an animal's biology, consequently, evolutionary diversification is often accompanied by differentiation of body sizes within a lineage. It is accepted that miniaturization, or the evolution of extremely small body sizes, played a key role in the origin and early evolution of different mammalian characters in non-mammaliaform cynodonts. However, while there are multiple studies on the biomechanical, behavioral, and physiological consequences of smaller sizes, few explore the evolutionary processes that lead to them. Here, we use body mass as a universal size measurement in phylogenetic comparative analyses to explore aspects of body size evolution in Cynodontia, focusing on the cynodont-mammal transition, and test the miniaturization hypothesis for the origin of Mammaliaformes. We estimated the body masses of 29 species, ranging from Theriocephalia to Mammaliaformes, providing the largest collection of Triassic cynodont body mass estimates that we know of, and used these estimates in analyses of disparity through time and RRphylo . Unexpectedly, our results did not support the miniaturization hypothesis. Even though cynodont body size disparity fell during the Late Triassic, and remained lower than expected under a purely Brownian motion model of evolution up until the Early Jurassic, we found that rates of body size evolution were significantly lower in prozostrodontians leading to the first Mammaliaformes than in other lineages. Evolution rates were higher in medium and large-sized taxa, indicating that size was changing more rapidly in those lineages and that small sizes were probably a persistent plesiomorphic character-state in Cynodontia.

Hidden diversity of the genus Trinomys (Rodentia: Echimyidae): phylogenetic and populational structure analyses uncover putative new lineages

Trinomys, one of the most species-rich spiny rat genera in Brazil, is widely distributed in Caatinga, Cerrado and Atlantic Forest biomes, and currently includes ten recognized species, three of which are polytypic. Although some studies employing molecular data have been conducted to better characterize phylogenetic relationships among species, 19 nominal taxa have been suggested, implying considerable incongruence regarding species boundaries. We addressed this incongruence by intensively sampling all species across the geographic distribution of the genus. In addition to publicly available data, we generated 182 mt-Cytb gene sequences, and employed phylogenetic and computational species delimitation methods to obtain a clearer picture of the genus diversity. Moreover, we evaluated populational diversity within each accepted species, considering their geographical distribution and a timescale for the evolution of the genus. Beyond confirming the general patterns described for the evolution of the group, this new analysis suggests that Trinomys is comprised of at least 16 evolutionary lineages, 13 of them recognized as species or subspecies, and three never before characterized. This study highlights the importance of increased sample sizes and computational species delimitation methods in uncovering hidden diversity in Trinomys.

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.

Historical, allometric and ecological effects on the shape of the lumbar vertebrae of spiny rats (Rodentia: Echimyidae)

In mammals, the lumbar vertebrae are important for sustaining the trunk, for allowing the trunk to flex and extend, and, during locomotion, for transferring forces from the sacroiliac region to the anterior region of the body. The Echimyidae is a group that comprises spiny rats, the coypu and hutias. It is the caviomorph rodent family with the greatest ecological diversity and species richness, as well as having a wide variation in body mass. Thus, echimyid rodents provide a promising model for understanding how phylogenetic, allometric and ecological factors associated with locomotion affect the evolution of the post-cranial skeleton. To assess the effect of these three factors on the morphology of the lumbar vertebrae, the penultimate lumbar vertebra of 26 echimyid species was photographed under five views and submitted to phylogenetically informed comparative analysis using 2D geometric morphometrics. Vertebral shape variation showed a low correlation with body mass and vertebral size, and a low to moderate phylogenetic signal. Remarkably, locomotory habit had a strong influence on lumbar morphology, particularly when analysed in lateral view. Our results indicate that the echimyid penultimate lumbar vertebra is potentially useful for future ecomorphological studies on living and fossil small mammals.