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We describe a new, small-bodied rhynchocephalian reptile, Opisthiamimus gregori gen. et sp. nov., from the Upper Jurassic Morrison Formation of Wyoming, USA. Whereas many fossil rhynchocephalians are based on isolated incomplete jaws, the holotype of O. gregori includes most of the skull and postcranium and therefore represents one of the most complete specimens of Rhynchocephalia known from North America. We used micro-computed tomography to examine its skeletal anatomy in detail and to develop a three-dimensional reconstruction of the skull. The skull of O. gregori is similar to that of several non-neosphenodontian rhynchocephalians such as Planocephalosaurus (e.g. large orbits) and Clevosaurus (e.g. parietal parasagittal crests) yet exhibits a suite of other features related to the proal shearing mechanism that becomes increasingly elaborated among more phylogenetically nested taxa such as Sphenodon (e.g. lateral palatine tooth row parallels maxillary tooth row along its entire length, pyramidal dentary teeth with mesial shearing crests). The postcranial skeleton of O. gregori exhibits characteristics typical of a terrestrial rhynchocephalian. Our phylogenetic analyses use a substantially updated data set of 118 characters and 46 taxa, and both maximum parsimony and Bayesian frameworks. Results place O. gregori inside Eusphenodontia but outside Neosphenodontia, and therefore in a key position for contributing to character polarity for more deeply nested clades such as Clevosauridae, Sphenodontidae and Pleurosauridae. We also erect Leptorhynchia taxon nov., composed primarily of aquatically adapted taxa (e.g. Pleurosaurus, Sapheosaurus), which is supported by both cranial and postcranial characters. Because O. gregori is not particularly closely related to the other named Morrison rhynchocephalians (e.g. Opisthias rarus), it increases both the alpha and beta taxonomic diversities within the formation. Similarly, major differences in body size and inferred diet of the Morrison taxa imply considerable concomitant palaeoecological diversity just prior to a major global decline in rhynchocephalian diversity around the close of the Jurassic. [ABSTRACT FROM AUTHOR]

We present the first empirical measurements of bite-force performance from adult Sphenodon (Rhynchocephalia), the only extant non-squamate lepidosaur. Using raw bite-force data, we calculated maximum bite forces at the anterior and posterior extremes of the lower tooth row: 81.8 N and 163.5 N (female) and 119.1 N and 238.3 N (male). Combining our results with published data from juvenile animals, we calculated scaling coefficients of bite force on linear morphometrics of body and head size as c. 2.7 (anterior) and c. 3.5+ (posterior). These exceed isometric scaling predictions (2.0), yet are similar to those for other non-avian reptiles. This supports previous views that Sphenodon cannot bite as hard as agamid lizards. We discuss the role of bite force in the behavioural ecology of Sphenodon, propose that the lower temporal bar, unique among extant lepidosaurs, does not necessarily constrain bite force, and evaluate possible effects of other morphological characteristics on bite-force performance. [ABSTRACT FROM AUTHOR]