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  1. ABSTRACT

    Marine mammals have undergone a dramatic series of morphological transformations throughout their evolutionary history that facilitated their ecological transition to life in the water. Pinnipeds are a diverse clade of marine mammals that evolved from terrestrial carnivorans in the Oligocene (∼27 million years ago). However, pinnipeds have secondarily lost the dental innovations emblematic of mammalian and carnivoran feeding, such as a talonid basin or shearing carnassials. Modern pinnipeds do not masticate their prey, but can reduce prey size through chopping behavior. Typically, small prey are swallowed whole. Nevertheless, pinnipeds display a wide breadth of morphology of the post-canine teeth. We investigated the relationship between dental morphology and pinniped feeding by measuring the puncture performance of the cheek-teeth of seven extant pinniped genera. Puncture performance was measured as the maximum force and the maximum energy required to puncture a standardized prey item (Loligo sp.). We report significant differences in the puncture performance values across the seven genera, and identify three distinct categories based on cheek-teeth morphology and puncture performance: effective, ineffective and moderate puncturers. In addition, we measured the overall complexity of the tooth row using two different metrics, orientation patch count rotated (OPCR) and relief index (RFI). Neither metric of complexity predicted puncture performance. Finally, we discuss these results in the broader context of known pinniped feeding strategies and lay the groundwork for subsequent efforts to explore the ecological variation of specific dental morphologies.

     
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  2. Abstract

    New imaging and biomechanical approaches have heralded a renaissance in our understanding of crocodylian anatomy. Here, we review a series of approaches in the preparation, imaging, and functional analysis of the jaw muscles of crocodylians. Iodine‐contrast microCT approaches are enabling new insights into the anatomy of muscles, nerves, and other soft tissues of embryonic as well as adult specimens of alligators. These imaging data and other muscle modeling methods offer increased accuracy of muscle sizes and attachments without destructive methods like dissection. 3D modeling approaches and imaging data together now enable us to see and reconstruct 3D muscle architecture which then allows us to estimate 3D muscle resultants, but also measurements of pennation in ways not seen before. These methods have already revealed new information on the ontogeny, diversity, and function of jaw muscles and the heads of alligators and other crocodylians. Such approaches will lead to enhanced and accurate analyses of form, function, and evolution of crocodylians, their fossil ancestors and vertebrates in general.

     
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  3. ABSTRACT

    Metriorhynchoid thalattosuchians were a marine clade of Mesozoic crocodylomorphs that evolved from semi‐aquatic, “gharial”‐like species into the obligately pelagic subclade Metriorhynchidae. To explore whether the sensory and physiological demands of underwater life necessitates a shift in rostral anatomy, both in neurology and vasculature, we investigate the trigeminal innervation and potential somatosensory abilities of metriorhynchoids by digitally segmenting the rostral neurovascular canals in CT scans of 10 extant and extinct crocodyliforms. The dataset includes the terrestrial, basal crocodyliformProtosuchus haughtoni, two semi‐aquatic basal metriorhynchoids, four pelagic metriorhynchids and three extant, semi‐aquatic crocodylians. In the crocodylian and basal metriorhynchoid taxa, we find three main neurovascular channels running parallel to one another posteroanteriorly down the length of the snout, whereas in metriorhynchids there are two, and inP. haughtonionly one. Crocodylians appear to be unique in their extensive trigeminal innervation, which is used to supply the integumentary sensory organs (ISOs) involved with their facial somatosensory abilities. Crocodylians have a far higher number of foramina on the maxillary bones than either metriorhynchoids orP. haughtoni, suggesting that the fossil taxa lacked the somatosensory abilities seen in extant species. We posit that the lack of ISO osteological correlates in metriorhynchoids is due to their basal position in Crocodyliformes, rather than a pelagic adaptation. This is reinforced by the hypothesis that extant crocodyliforms, and possibly some neosuchian clades, underwent a long “nocturnal bottleneck”—hinting that their complex network of ISOs evolved in Neosuchia, as a sensory trade‐off to compensate for poorer eyesight.

     
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  4. Abstract

    Snakes—a subset of lizards—have traditionally been divided into two major groups based on feeding mechanics: “macrostomy,” involving the ingestion of proportionally large prey items; and “microstomy,” the lack of this ability. “Microstomy”—considered present in scolecophidian and early‐diverging alethinophidian snakes—is generally viewed as a symplesiomorphy shared with non‐snake lizards. However, this perspective of “microstomy” as plesiomorphic and morphologically homogenous fails to recognize the complexity of this condition and its evolution across “microstomatan” squamates. To challenge this problematic paradigm, we formalize a new framework for conceptualizing and testing the homology of overall character complexes, or “morphotypes,” which underlies our re‐assessment of “microstomy.” Using micro‐computed tomography (micro‐CT) scans, we analyze the morphology of the jaws and suspensorium across purported “microstomatan” squamates (scolecophidians, early‐diverging alethinophidians, and non‐snake lizards) and demonstrate that key components of the jaw complex are not homologous at the level of primary character state identity across these taxa. Therefore, rather than treating “microstomy” as a uniform condition, we instead propose that non‐snake lizards, early‐diverging alethinophidians, anomalepidids, leptotyphlopids, and typhlopoids each exhibit a unique and nonhomologous jaw morphotype: “minimal‐kinesis microstomy,” “snout‐shifting,” “axle‐brace maxillary raking,” “mandibular raking,” and “single‐axle maxillary raking,” respectively. The lack of synapomorphy among scolecophidians is inconsistent with the notion of scolecophidians representing an ancestral snake condition, and instead reflects a hypothesis of the independent evolution of fossoriality, miniaturization, and “microstomy” in each scolecophidian lineage. We ultimately emphasize that a rigorous approach to comparative anatomy is necessary in constructing evolutionary hypotheses that accurately reflect biological reality.

     
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  5. ABSTRACT

    Birds and their ornithodiran ancestors are unique among vertebrates in exhibiting air‐filled sinuses in their postcranial bones, a phenomenon called postcranial skeletal pneumaticity. The factors that account for serial and interspecific variation in postcranial skeletal pneumaticity are poorly understood, although body size, ecology, and bone biomechanics have all been implicated as influencing the extent to which pneumatizing epithelia invade the skeleton and induce bone resorption. Here, I use high‐resolution computed‐tomography to holistically quantify vertebral pneumaticity in members of the neognath family Ciconiidae (storks), with pneumaticity measured as the relative volume of internal air space. These data are used to describe serial variation in extent of pneumaticity and to assess whether and how pneumaticity varies with the size and shape of a vertebra. Pneumaticity increases dramatically from the middle of the neck onwards, contrary to previous predictions that cervical pneumaticity should decrease toward the thorax to maintain structural integrity as the mass and bending moments of the neck increase. Although the largest vertebrae sampled are also the most pneumatic, vertebral size cannot on its own account for serial or interspecific variation in extent of pneumaticity. Vertebral shape, as quantified by three‐dimensional geometric morphometrics, is found to be significantly correlated with extent of pneumaticity, with elongate vertebrae being less pneumatic than craniocaudally short and dorsoventrally tall vertebrae. Considered together, the results of this study are consistent with the hypothesis that shape‐ and position‐specific biomechanics influence the amount of bone loss that can be safely tolerated. These results have potentially important implications for the evolution of vertebral morphology in birds and their extinct relatives.

     
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  6. A mammalian-wide pattern of allometry connects micro- to macro-evolution in the skulls of ruminant artiodactyls. 
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  7. Early dinosaurs evolved diverse tooth shapes and mechanics as adaptations to different diets, so ensuring their success. 
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  8. Abstract The labyrinth of the vertebrate inner ear is a sensory system that governs the perception of head rotations. Central hypotheses predict that labyrinth shape and size are related to ecological adaptations, but this is under debate and has rarely been tested outside of mammals. We analyze the evolution of labyrinth morphology and its ecological drivers in living and fossil turtles, an understudied group that underwent multiple locomotory transitions during 230 million years of evolution. We show that turtles have unexpectedly large labyrinths that evolved during the origin of aquatic habits. Turtle labyrinths are relatively larger than those of mammals, and comparable to many birds, undermining the hypothesis that labyrinth size correlates directly with agility across vertebrates. We also find that labyrinth shape variation does not correlate with ecology in turtles, undermining the widespread expectation that reptilian labyrinth shapes convey behavioral signal, and demonstrating the importance of understudied groups, like turtles. 
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