During locomotion, lizards and crocodilians generally use a more sprawling limb posture than most mammals and experience substantial axial rotation of the femur. Consequently, the limb bones of most mammals are loaded predominantly in bending, but the limb bones of lizards and crocodilians are loaded primarily in torsion. As body size increases, torsional shear stress in limb bones is expected to increase more than bending stress; therefore, limb bone diameters of lizards and crocodilians might be expected to scale with relatively greater positive allometry than limb bone diameters of mammals that use upright posture. To test this hypothesis, scaling patterns of the femur and tibia in lizards (iguanians and varanids) and crocodilians were compared with patterns in felid and canid mammals, using both non-phylogenetic statistical methods and phylogenetically independent contrasts. Comparisons with theoretical models indicate that size-related changes in limb bone geometry do not completely compensate for size-related increases in limb bone stress in the lizard or crocodilian lineages examined. Unless lizards and crocodilians compensate for size-related increases in limb bone stress through other mechanisms (e.g. changes in limb kinematics or the mechanical properties of limb bones), limb bone stresses are predicted to be relatively greater among larger species of these lineages. However, limb bone diameters appear to scale with greater positive allometry (relative to body mass) in varanids than in iguanians, suggesting that larger lizard lineages might compensate for increased stress through changes in bone geometry to a greater degree than smaller lineages. Allometric scaling patterns for many limb bone diameters among iguanians are more similar to those of felids and canids than to those of varanids; thus, sprawling locomotor habits do not correlate clearly with a particular pattern of limb bone scaling. This suggests that similarity of interspecific scaling patterns of limb bone lengths and diameters is not sufficient to justify inferences of similar locomotor function.

The Triassic reptile Euparkeria has been frequently given a pivotal position in interpretations of the evolution of archosaurs. Most recently, Welman (1995) has argued from braincase data that Euparkeria is more closely related to birds than are either theropod dinosaurs or crocodilians – a conclusion clearly at odds with the current orthodoxy. The braincase of a single specimen of Euparkeria is described in detail and compared with previous descriptions and with the braincases of other diapsids. Variations among the known specimens are documented. The homology of various braincase structures are reassessed in light of the study by Welman (1995). We argue that the braincase of Euparkeria has an undivided metotic fissure, an incompletely ossified medial wall of the otic capsule, a well-defined ‘semilunar depression’, and posteroventrally positioned foramina in the parabasisphenoid for the entrance of the cerebral branches of the internal carotid arteries. It lacks enclosure of the Eustachian system in bone, well-developed tympanic sinuses, or a well-defined recess for the lagena. A review of braincase morphology in extinct and extant diapsids suggests that braincase features of Euparkeria are largely plesiomorphic for Archosauria. The evolutionary relationships between Euparkeria and extant archosaurs (birds and crocodilians) are considered by reviewing braincase morphology in extant and extinct diapsids. No shared derived characters could be found that support the resolutions (crocodilians (Euparkeria+birds)) or (birds (Euparkeria+crocodilians)). Three derived characters shared by extant archosaurs support the resolution (Euparkeria (crocodilians+birds)), but only the presence of laterally positioned foramina in the parabasisphenoid for the entrance of the cerebral branches of the internal carotid arteries appears to represent strong evidence. The other two features are a degree of ossification (of the medial wall of the otic capsule) that exhibits some homoplasy among archosaurs, and an absence (of the ‘semilunar depression’), and therefore do not represent particularly robust hypotheses of homology. Our interpretation of the braincase of Euparkeria is fully congruent with the consensus among recent explicit phylogenetic analyses that this taxon is close to, but not a member of, the archosaur crown group. Birds and crocodilians share a number of other derived similarities (subdivided metotic fissure, elongated and tubular cochlear recess, enclosed Eustachian system, extensive tympanic sinuses, quadrate-prootic articulation) that are probably not homologous because of their absence in a number of non-avian dinosaurs and crocodilian-line crown-group archosaurs.