Even after 200 years of study, some details of the cranial anatomy of ichthyosaurs, one of the most successful groups of marine vertebrates in the Mesozoic, are still unclear. New information on the braincase, palate and occiput are provided from three-dimensional scans of an exceptionally preserved ichthyosaur ('Hauffiopteryx' typicus) skull from the Toarcian (183-174 Ma, Lower Jurassic) of Strawberry Bank, England. This ichthyosaur has unusual, hollow, tubular hyoid bars. The occipital and braincase region is fully reconstructed, creating the first digital cranial endocast of an ichthyosaur. Enlarged optic lobes and an enlarged cerebellum suggest neuroanatomical adaptations that allowed it to be a highly mobile, visual predator. The olfactory region also appears to be enlarged, suggesting that olfaction was more important for ichthyosaurs than has been assumed. Phylogenetic analysis suggests this ichthyosaur is closely related to, but distinct from, Hauffiopteryx, and positioned within Thunnosauria, a more derived position than previously recovered. These results further our knowledge of ichthyosaur cranial anatomy in three dimensions and provide a platform in which to study the anatomical adaptations that allowed ichthyosaurs to dominate the marine realm during the Mesozoic.

Bird necks display unparalleled levels of morphological diversity compared to other vertebrates, yet it is unclear what factors have structured this variation. Using three-dimensional geometric morphometrics and multivariate statistics, we show that the avian cervical column is a hierarchical morpho-functional appendage, with varying magnitudes of ecologically driven osteological variation at different scales of organization. Contrary to expectations given the widely varying ecological functions of necks in different species, we find that regional modularity of the avian neck is highly conserved, with an overall structural blueprint that is significantly altered only by the most mechanically demanding ecological functions. Nevertheless, the morphologies of vertebrae within subregions of the neck show more prominent signals of adaptation to ecological pressures. We also find that both neck length allometry and the nature of neck elongation in birds are different from other vertebrates. In contrast with mammals, neck length scales isometrically with head mass and, contrary to previous work, we show that neck elongation in birds is achieved predominantly by increasing vertebral lengths rather than counts. Birds therefore possess a cervical spine that may be unique in its versatility among extant vertebrates, one that, since the origin of flight, has adapted to function as a surrogate forelimb in varied ecological niches.

Dental morphology and patterns of tooth replacement in representatives of the clade Pliosauridae (Reptilia, Sauropterygia) are evaluated in detail. The jaws of one basal (Thalassiodracon hawkinsii) and two derived species (Pliosaurus carpenteri, Pliosaurus kevani) were visualized by μCT scans, and the ontogenetic patterns, or ‘movement paths’, of replacement teeth could be mapped. Other specimens (Peloneustes philarchus and Pliosaurus westbuyensis) with well-preserved jaws containing functional and replacement teeth in situ were also examined directly, and waves of tooth replacement could be inferred from the degree of in situ tooth development and the fusion between functional and replacement alveoli. The analysis revealed symmetrical tooth eruption over the medial axis throughout the length of the jaw in the basal pliosaurid Thalassiodracon. By contrast, symmetrical tooth eruption patterns occur only along the anterior sections of the jaws of derived pliosaurids. In Pliosaurus, replacement schedules differ in the anterior and posterior portions of the jaws and appear to correlate with differences in tooth morphology and symmetrical replacement. The anterior teeth exhibit longer replacement cycle periods and symmetrical replacement, while shorter cycle periods and asymmetry are seen posteriorly. A longer period suggests slower replacement and is characteristic of large, specialized caniniform teeth in the longer snouted Late Jurassic taxa. Smaller posterior teeth have a shorter period and therefore a faster replacement cycle. The transition from long to short replacement period over the length of the jaw is thought to account for the loss of symmetry. This differentiation could relate to differential tooth function and a type of heterodonty. We therefore propose a new model of pliosaurid tooth replacement patterns and present it in a phylogenetic context.

It is accepted that non-avian theropod dinosaurs, with their long muscular tails and small forelimbs, had a centre-of-mass close to the hip, while extant birds, with their reduced tails and enlarged wings have their mass centred more cranially. Transition between these states is considered crucial to two key innovations in the avian locomotor system: crouched bipedalism and powered flight. Here we use image-based models to challenge this dichotomy. Rather than a phylogenetic distinction between ‘dinosaurian’ and ‘avian’ conditions, we find terrestrial versus volant taxa occupy distinct regions of centre-of-mass morphospace consistent with the disparate demands of terrestrial bipedalism and flight. We track this decoupled evolution of body shape and mass distribution through bird evolution, including the origin of centre-of-mass positions more advantageous for flight and major reversions coincident with terrestriality. We recover modularity in the evolution of limb proportions and centre-of-mass that suggests fully crouched bipedalism evolved after powered flight.

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