There have been a number of attempts to explain how crocodylian bite-force performance covaries with cranial form and diet. However, the mechanics and morphologies of crocodylian jaws have thus far remained incongruent with data on their performance and evolution. For example, it is largely assumed that the functional anatomy and performance of adults tightly fits the adult niche. At odds with this precept are groups with resource-dependent growth, whose juvenile stages undergo shifts in mass, morphology, and resource usage to overcome strong selection related to issues of small body size, as compared to adults. Crocodylians are an example of such a group. As living suchians, they also have a long and fossil-rich evolutionary history, characterized by analogous increases in body size, diversifications in rostrodental form, and shifts in diet. Here we use biomechanical and evolutionary modeling techniques to study the development and evolution of the suchian feeding apparatus and to formally assess the impact of potential ontogenetic-evolutionary parallels on clade dynamics. We show that patterns of ontogenetic and evolutionary bite-force changes exhibit inverted patterns of heterochrony, indicating that early ontogenetic trends are established as macroevolutionary patterns within Neosuchia, prior to the origin of Eusuchia. Although selection can act on any life-history stage, our findings suggest that selection on neonates and juveniles, in particular, can contribute to functionally important morphologies that aid individual and clade success without being strongly tied to their adult niche.

Synopsis 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 R2≥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.

Mammals with a carotid rete are capable of reducing brain temperature below that of carotid blood temperature, termed selective brain cooling. In artiodactyls, selective brain cooling conserves body water and may provide them with a selective advantage in conditions of increased temperature and aridity.

Synopsis Tyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods. Methods To compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a(p) to stringently test the null hypothesis of equal agility. Results Tyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration). Implications The capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.

Nearly all living artiodactyls (even-toed ungulates) possess a derived cranial arterial pattern that is highly distinctive from most other mammals. Foremost among a suite of atypical arterial configurations is the functional and anatomical replacement of the internal carotid artery with an extensive, subdural arterial meshwork called the carotid rete. This interdigitating network branches from the maxillary artery and is housed within the cavernous venous sinus. As the cavernous sinus receives cooled blood draining from the nasal mucosa, heat rapidly dissipates across the high surface area of the rete to be carried away from the brain by the venous system. This combination yields one of the most effective mechanisms of selective brain cooling. Although arterial development begins from the same embryonic scaffolding typical of mammals, possession of a rete is typically accompanied by obliteration of the internal carotid artery. Among taxa with available ontogenetic data, the point at which the internal carotid obliterates is variable throughout development. In small-bodied artiodactyls, the internal carotid typically obliterates prior to parturition, but in larger species, the vessel may remain patent for several years. In this study, we use digital anatomical data collection methods to describe the cranial arterial patterns for a growth series of giraffe (Giraffa camelopardalis), from parturition to senescence. Giraffes, in particular, have unique cardiovascular demands and adaptations owing to their exceptional body form and may not adhere to previously documented stages of cranial arterial development. We find the carotid arterial system to be conserved between developmental stages and that obliteration of the giraffe internal carotid artery occurs prior to parturition.