The evolution of pelvic limb muscle moment arms in bird-line archosaurs.

Bipedal locomotion evolved along the archosaurian lineage to birds, shifting from "hip-based" to "knee-based" mechanisms. However, the roles of individual muscles in these changes and their evolutionary timings remain obscure. Using 13 three-dimensional musculoskeletal models of the hindlimbs of bird-line archosaurs, we quantify how the moment arms (i.e., leverages) of 35 locomotor muscles evolved. Our results support two hypotheses: From early theropod dinosaurs to birds, knee flexors' moment arms decreased relative to knee extensors', and medial long-axis rotator moment arms for the hip increased (trading off with decreased hip abductor moment arms). Our results reveal how, from the Triassic Period, bipedal theropod dinosaurs gradually modified their hindlimb form and function, shifting more from hip-based to knee-based locomotion and hip-abductor to hip-rotator balancing mechanisms inherited by birds. Yet, we also discover unexpected ancestral specializations in larger Jurassic theropods, lost later in the bird-line, complicating the paradigm of gradual transformation.

Functional Morphology and Morphological Diversification of Hind Limb Cross-Sectional Traits in Mustelid Mammals.

Locomotor habits in mammals are strongly tied to limb bones' lengths, diameters, and proportions. By comparison, fewer studies have examined how limb bone cross-sectional traits relate to locomotor habit. Here, we tested whether climbing, digging, and swimming locomotor habits reflect biomechanically meaningful differences in three cross-sectional traits rendered dimensionless- cross-sectional area (CSA), second moments of area (SMA), and section modulus (MOD)-using femora, tibiae, and fibulae of 28 species of mustelid. CSA and SMA represent resistance to axial compression and bending, respectively, whereas MOD represents structural strength. Given the need to counteract buoyancy in aquatic environments and soil's high density, we predicted that natatorial and fossorial mustelids have higher values of cross-sectional traits. For all three traits, we found that natatorial mustelids have the highest values, followed by fossorial mustelids, with both of these groups significantly differing from scansorial mustelids. However, phylogenetic relatedness strongly influences diversity in cross-sectional morphology, as locomotor habit strongly correlates with phylogeny. Testing whether hind limb bone cross-sectional traits have evolved adaptively, we fit Ornstein-Uhlenbeck (OU) and Brownian motion (BM) models of trait diversification to cross-sectional traits. The cross-sectional traits of the femur, tibia, and fibula appear to have, respectively, diversified under a multi-rate BM model, a single rate BM model, and a multi-optima OU model. In light of recent studies on mustelid body size and elongation, our findings suggest that the mustelid body plan-and perhaps that of other mammals-is likely the sum of a suite of traits evolving under different models of trait diversification.

Morfologia funcional e diversificação morfológica dos tratos seccionais ósseos dos membros posteriores de mamíferos mustelídeos (Functional morphology and morphological diversification of hindlimb bone cross-sectional traits in mustelid mammals)Os hábitos locomotores em mamíferos são fortemente associados ao comprimento, diâmetro e proporções dos ossos que formam os membros. Comparativamente, poucos estudos examinaram como os tratos seccionais dos ossos dos membros são relacionadas com o hábito locomotor. Aqui, testamos se os hábitos locomotores de escalada, escavação e natação refletem significantemente nas diferenças biomecanicas em três tratos seccionais obtidos sem dimensão ­ área de seção transversal (CSA), segundo momento de área (SMA) e módulo de seção (MOD) - usando o femur, tíbia e fíbula de 28 espécies de mustelídeos. CSA e SMA representam resistência à compressão e flexão axial, respectivamente, enquanto MOD representa resistência estrutural. Dada a necessidade de neutralizar a flutuabilidade em ambientes aquáticos e a alta densidade do solo, previmos que as mustelídeos natatoriais e fossoriais tenham maiores valores de tratos seccionais. Para os três tratos, descobrimos que os mustelídeos natatórios têm os valores mais altos, seguidos pelos mustelídeos fossoriais, com ambos grupos diferindo significativamente dos mustelídeos escaladores. No entanto, relações filogenéticas influenciam fortemente a diversidade na morfologia seccional, uma vez que o hábito locomotor se correlaciona fortemente com a filogenia. Testando se os traços seccionais dos ossos dos membros posteriores evoluíram adaptativamente, ajustamos os modelos de diversificação de traços de Ornstein-Uhlenbeck (OU) e movimento browniano (BM) aos traços seccionais. As características seccionais do fêmur, tíbia e fíbula parecem ter se diversificado, respectivamente, sob um modelo de BM com múltiplas taxas, um modelo de BM com taxa única e um modelo de UO multi-optimo. À luz de estudos recentes sobre tamanho e alongamento do corpo de mustelídeos, nossas descobertas sugerem que o plano de corpo de mustelídeos­e talvez o de outros mamíferos­é provavelmente a soma de um conjunto de características que evoluem sob diferentes modelos de traços de diversificação.Translated to Portuguese by Diego Vaz (dbistonvaz@vims.edu).

Morfología funcional y diversificación morfológica de los rasgos transversales del hueso de la extremidad posterior en mamíferos mustélidos (Functional morphology and morphological diversification of hindlimb bone cross-sectional traits in mustelid mammals)Los hábitos locomotores en los mamíferos están fuertemente ligados a las longitudes, diámetros y proporciones de los huesos de las extremidades. Comparativamente, menos estudios han examinado cómo los rasgos de la sección transversal del hueso de la extremidad se relacionan con el hábito locomotor. Aquí, probamos si los hábitos locomotores de escalada, excavación y natación reflejan diferencias biomecánicamente significativas en tres rasgos transversales que se convierten en un área de sección transversal adimensional (CSA), segundo momento de área (SMA) y módulo de sección (MOD). Esto usando fémures, tibias y peronés de 28 especies de mustélidos. CSA y SMA representan resistencia a la compresión axial y la flexión, respectivamente, mientras que MOD representa la resistencia estructural. Dada la necesidad de contrarrestar la flotabilidad en ambientes acuáticos y la alta densidad del suelo, predijimos que los mustélidos natatorios y fosforiales tienen valores más altos de rasgos de sección transversal. Para los tres rasgos, encontramos que los mustélidos nadadores tienen los valores más altos, seguidos por los mustélidos fosoriales, y ambos grupos difieren significativamente de los mustélidos trepadores. Sin embargo, la relación filogenética influye fuertemente en la diversidad de la morfología transversal, ya que el hábito locomotor se correlaciona fuertemente con la filogenia. Al probar si los rasgos de la sección transversal del hueso de la extremidad posterior han evolucionado de forma adaptativa, ajustamos los modelos de diversificación de rasgos de Ornstein-Uhlenbeck (OU) y movimiento Browniano (BM) a los rasgos de sección transversal. Los rasgos de sección transversal del fémur, la tibia y el peroné parecen haberse diversificado respectivamente bajo un modelo de BM de tasa múltiple, un modelo de BM de tasa única y un modelo de OU multi-optimo. A la luz de los estudios recientes sobre el tamaño y el alargamiento del cuerpo de los mustélidos, nuestros hallazgos sugieren que el plan del cuerpo de los mustélidos, y tal vez el de otros mamíferos, es probablemente la suma de un conjunto de rasgos que evolucionan bajo diferentes modelos de diversificación de rasgos.Translated to Spanish by J Heras (herasj01@gmail.com).

Trabecular bone architecture in the stylopod epiphyses of mustelids (Mammalia, Carnivora).

Mustelidae, a carnivoran clade that includes for instance weasels, badgers, otters and martens, has undergone several evolutionary transitions of lifestyle, resulting in specializations for fossorial, natatorial and scansorial locomotion, in addition to more generalized species. The family is therefore regarded as offering an adequate framework for morpho-functional analyses. However, the architecture of the epiphyseal trabecular bone, which is argued to be particularly responsive to the biomechanical environment, has never been studied. Here, we quantify trabecular bone parameters of the proximal and distal epiphyses of the humerus and femur in 29 species of mustelids and assess the differences of these parameters among groups defined a priori based on the aforementioned locomotor types. The parameters are assessed in a phylogenetic framework, taking into account the potential effect on an individual's body mass. The range of variation described by the acquired parameters is relatively restricted when compared to that of other clades. Generalists, however, are featuring a wider range of variation than the other types. While clear discrimination of locomotor types is difficult, some differences were highlighted by our analysis, such as a greater bone fraction associated with the natatorial taxa, which we discuss in a functional context.

Automated approximation of center of mass position in X-ray sequences of animal locomotion.

A crucial aspect of comparative biomechanical research is the center of mass (CoM) estimation in animal locomotion scenarios. Important applications include the parameter estimation of locomotion models, the discrimination of gaits, or the calculation of mechanical work during locomotion. Several methods exist to approximate the CoM position, e.g. force-plate-based approaches, kinematic approaches, or the reaction board method. However, they all share the drawback of not being suitable for large scale studies, as detailed initial conditions from kinematics are required (force-plates), manual interaction is necessary (kinematic approach), or only static settings can be analyzed (reaction board). For the increasingly popular case of X-ray-based animal locomotion analysis, we present an alternative approach for CoM estimation which overcomes these shortcomings. The main idea is to only use the recorded X-ray images, and to map each pixel to the mass of matter it represents. As a consequence, our approach is surgically noninvasive, independent of animal species and locomotion characteristics, and neither requires prior knowledge nor any kind of user interaction. To assess the quality of our approach, we conducted a comparison to highly accurate reaction board experiments for lapwing and rat cadavers, and achieved an average accuracy of 2.6mm (less than 2% of the animal body length). We additionally verified the practical applicability of the algorithm by comparison to a previously published CoM study which is based on the kinematic method, yielding comparable results.