Divergent evolution of terrestrial locomotor abilities in extant Crocodylia.

Extant Crocodylia are exceptional because they employ almost the full range of quadrupedal footfall patterns ("gaits") used by mammals; including asymmetrical gaits such as galloping and bounding. Perhaps this capacity evolved in stem Crocodylomorpha, during the Triassic when taxa were smaller, terrestrial, and long-legged. However, confusion about which Crocodylia use asymmetrical gaits and why persists, impeding reconstructions of locomotor evolution. Our experimental gait analysis of locomotor kinematics across 42 individuals from 15 species of Crocodylia obtained 184 data points for a wide velocity range (0.15-4.35 ms-1). Our results suggest either that asymmetrical gaits are ancestral for Crocodylia and lost in the alligator lineage, or that asymmetrical gaits evolved within Crocodylia at the base of the crocodile line. Regardless, we recorded usage of asymmetrical gaits in 7 species of Crocodyloidea (crocodiles); including novel documentation of these behaviours in 5 species (3 critically endangered). Larger Crocodylia use relatively less extreme gait kinematics consistent with steeply decreasing athletic ability with size. We found differences between asymmetrical and symmetrical gaits in Crocodylia: asymmetrical gaits involved greater size-normalized stride frequencies and smaller duty factors (relative ground contact times), consistent with increased mechanical demands. Remarkably, these gaits did not differ in maximal velocities obtained: whether in Alligatoroidea or Crocodyloidea, trotting or bounding achieved similar velocities, revealing that the alligator lineage is capable of hitherto unappreciated extreme locomotor performance despite a lack of asymmetrical gait usage. Hence asymmetrical gaits have benefits other than velocity capacity that explain their prevalence in Crocodyloidea and absence in Alligatoroidea-and their broader evolution.

Crocodylian Head Width Allometry and Phylogenetic Prediction of Body Size in Extinct Crocodyliforms.

Body size and body-size shifts broadly impact life-history parameters of all animals, which has made accurate body-size estimates for extinct taxa an important component of understanding their paleobiology. Among extinct crocodylians and their precursors (e.g., suchians), several methods have been developed to predict body size from suites of hard-tissue proxies. Nevertheless, many have limited applications due to the disparity of some major suchian groups and biases in the fossil record. Here, we test the utility of head width (HW) as a broadly applicable body-size estimator in living and fossil suchians. We use a dataset of sexually mature male and female individuals (n = 76) from a comprehensive sample of extant suchian species encompassing nearly all known taxa (n = 22) to develop a Bayesian phylogenetic model for predicting three conventional metrics for size: body mass, snout-vent length, and total length. We then use the model to estimate size parameters for a select series of extinct suchians with known phylogenetic affinity (Montsechosuchus, Diplocynodon, and Sarcosuchus). We then compare our results to sizes reported in the literature to exemplify the utility of our approach for a broad array of fossil suchians. Our results show that HW is highly correlated with all other metrics (all R 2≥0.85) and is commensurate with femoral dimensions for its reliably as a body-size predictor. We provide the R code in order to enable other researchers to employ the model in their own research.

Alometría del Ancho de la Cabeza de Cocodrilo y Predicción Filogenética del Tamaño Corporal en Cocodrilos Extintos (Crocodylian Head Width Allometry and Phylogenetic Prediction of Body Size in Extinct Crocodyliforms)El tamaño corporal y los cambios de tamaño corporal afectan ampliamente los parámetros de la historia de vida de todos los animales, lo que ha hecho que las estimaciones precisas del tamaño corporal de los taxones extintos sean un componente importante para comprender su paleobiología. Entre los crocodilianos extintos y sus precursores (por ejemplo, los suquios), se han desarrollado varios métodos para predecir el tamaño corporal a partir de conjuntos de indicadores de tejido duro. Sin embargo, muchos tienen aplicaciones limitadas debido a la disparidad de algunos grupos importantes de crocodiliformes y sesgos en el registro fósil. Aquí, probamos la utilidad del ancho de la cabeza como un estimador de tamaño corporal ampliamente aplicable en crocodiliformes vivos y fósiles. Utilizamos un conjunto de datos de individuos machos y hembras sexualmente maduros (n=76) de una muestra exhaustiva de especies existentes de cocodrilos que abarcan casi todos los taxones conocidos (n=22) para desarrollar un modelo filogenético bayesiano y predecir tres métricas convencionales para el tamaño: masa corporal, longitud del orificio de ventilación y longitud total. Luego usamos el modelo para estimar los parámetros de tamaño para una serie selecta de crocodiliformes extintos con afinidad filogenética conocida (Montsechosuchus, Diplocynodon, y Sarcosuchus). Luego comparamos nuestros resultados con los tamaños reportados en la literatura para demostrar la utilidad de nuestro enfoque en una gama amplia de tales fósiles. Nuestros resultados muestran que el ancho de la cabeza está altamente correlacionado con todas las otras métricas (todo R 2≥0.85) y es conmensurable a las dimensiones femorales debido a su confiabilidad como predictor del tamaño corporal. Proporcionamos el código R para permitir que otros investigadores empleen el modelo en su propia investigación.Translated to Spanish by C.A. Alfonso (calfonsoc@vt.edu).

Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation.

BACKGROUND: Crocodilians have dominated predatory niches at the water-land interface for over 85 million years. Like their ancestors, living species show substantial variation in their jaw proportions, dental form and body size. These differences are often assumed to reflect anatomical specialization related to feeding and niche occupation, but quantified data are scant. How these factors relate to biomechanical performance during feeding and their relevance to crocodilian evolutionary success are not known. METHODOLOGY/PRINCIPAL FINDINGS: We measured adult bite forces and tooth pressures in all 23 extant crocodilian species and analyzed the results in ecological and phylogenetic contexts. We demonstrate that these reptiles generate the highest bite forces and tooth pressures known for any living animals. Bite forces strongly correlate with body size, and size changes are a major mechanism of feeding evolution in this group. Jaw shape demonstrates surprisingly little correlation to bite force and pressures. Bite forces can now be predicted in fossil crocodilians using the regression equations generated in this research. CONCLUSIONS/SIGNIFICANCE: Critical to crocodilian long-term success was the evolution of a high bite-force generating musculo-skeletal architecture. Once achieved, the relative force capacities of this system went essentially unmodified throughout subsequent diversification. Rampant changes in body size and concurrent changes in bite force served as a mechanism to allow access to differing prey types and sizes. Further access to the diversity of near-shore prey was gained primarily through changes in tooth pressure via the evolution of dental form and distributions of the teeth within the jaws. Rostral proportions changed substantially throughout crocodilian evolution, but not in correspondence with bite forces. The biomechanical and ecological ramifications of such changes need further examination.