Animals have evolved limb proportions adapted to different environments, but it is not yet clear to what extent these proportions are directly influenced by the environment during prenatal development. The developing skeleton experiences mechanical loading resulting from embryo movement. We tested the hypothesis that environmentally-induced changes in prenatal movement influence embryonic limb growth to alter proportions. We show that incubation temperature influences motility and limb bone growth in West African Dwarf crocodiles, producing altered limb proportions which may, influence post-hatching performance. Pharmacological immobilisation of embryonic chickens revealed that altered motility, independent of temperature, may underpin this growth regulation. Use of the chick also allowed us to merge histological, immunochemical and cell proliferation labelling studies to evaluate changes in growth plate organisation, and unbiased array profiling to identify specific cellular and transcriptional targets of embryo movement. This disclosed that movement alters limb proportions and regulates chondrocyte proliferation in only specific growth plates. This selective targeting is related to intrinsic mTOR (mechanistic target of rapamycin) pathway activity in individual growth plates. Our findings provide new insights into how environmental factors can be integrated to influence cellular activity in growing bones and ultimately gross limb morphology, to generate phenotypic variation during prenatal development.

Tyrannosaurs Revisited: Tyrannosaurs represent some of the most successful and largest carnivores in Earth's history. An expanding fossil record has allowed studies of their evolution and behavior that now allow broader comparisons with other groups, not just dinosaurs. Brusatte et al. (p. 1481 ) review the biology and evolutionary history of tyrannosaurs and update their phylogenetic relations to include several new fossils. The analysis suggests that tyrannosaurs remained relatively small (less than about 5 meters long) until the Late Cretaceous (about 80 million years ago).

Living crocodilians (Crocodylia) and birds (Neornithes) differ in many aspects of hindlimb anatomy and locomotor function. How did this disparity evolve? We integrate information from fossils with functional descriptions of locomotion in living crocodilians and birds, using a phylogenetic perspective. We then outline the major changes in three-dimensional control of the hip joint along the line from the ancestral archosaur to birds. Our analysis reveals that most aspects of hip morphology and function in Alligator are ancestral for Archosauria. Femoral protractors and retractors are located cranial and caudal to the hip, respectively. Similarly, femoral adductors and abductors are located ventral and dorsal to the hip. Transformations of this ancestral pattern on the line to birds involved modifications in osteology, myology, and neural control. In some cases, homologous muscles changed function by acquiring new activity patterns. In others, activity was conserved, but origins and/or insertions were altered. Fossil theropods document the stepwise evolution of a novel mechanism of limb adduction/abduction involving long-axis rotation of the femur. This mechanism accounts for the conspicuous absence of significant musculature ventral and dorsal to the hip joint in extant birds.