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The evolution of jaws is hypothesized to have fueled radiations among vertebrates, contributing to their overwhelming success in the present day. Past work shows rapid early expansion of diversity in jaw structure in many lineages; however, the evolutionary dynamics underlying this pattern are unclear and hindered by the lack of a robust comparative framework. Here, using a macroevolutionary approach, we explore the diversification of lower jaws in early bony fishes, a major contributor to this initial radiation. Using newly generated three-dimensional mandibular shape data from 86 species, we find evidence of adaptive radiation in jaws during the earliest interval of bony fish evolutionary history (423–359 Ma). These patterns are principally driven by early lungfishes and coelacanths, which display high rates of jaw diversification, rapid shifts into novel functional regions of trait space, and substantial innovation in jaw morphology and feeding ecology, standing in contrast to their ‘‘living fossil’’ descendants of today. Conversely, ray-finned fishes and tetrapodomorphs, morphologically diverse groups in the present day, show little indication of their future success, possessing slow rates of jaw evolution and low functional diversity. This profound inversion of patterns in modern taxa highlights the significance of paleontological data in understanding drivers of evolutionary diversification and the limitations of approaches using only living species. Overall, our findings provide insight into the evolutionary dynamics associated with the evolution of jaws and provide context for the role of jaws in vertebrate success. 

Abstract Actinopterygii is a major extant vertebrate group, but limited data are available for its earliest members. Here we investigate the morphology of Devonian actinopterygians, focusing on the lower jaw. We use X‐ray computed tomography (XCT) to provide comprehensive descriptions of the mandibles of 19 species, which span the whole of the Devonian and represent roughly two‐thirds of all taxa known from more than isolated or fragmentary material. Our findings corroborate previous reports in part but reveal considerable new anatomical data and represent the first detailed description for roughly half of these taxa. The mandibles display substantial variation in size, spanning more than an order of magnitude. Although most conform to a generalized pattern of a large dentary and one or two smaller infradentaries, XCT data reveal significant differences in the structure of the jaw and arrangement of teeth that may be of functional relevance. We report the presence of a rudimentary coronoid process in several taxa, contributed to by the dentary and/or infradentaries, as well a raised articular region, resulting in a mandible with an offset bite and that functions as a bent level arm. Among the most striking variation is that of tooth morphology: several taxa have heterodont dentary teeth that vary in size and orientation, and multiple variations on enlarged, whorl‐like and posteriorly‐oriented anterior coronoid dentition are observed. We use these new data to revise morphological characters that may be of phylogenetic significance and consider the possible functional implicationds of these traits. The observed variation in mandible form and structure suggests previously unappreciated functional diversity among otherwise morphologically homogenous Devonian ray‐finned fishes. 

Abstract This paper is the second in a two‐part series that charts the evolution of appendicular musculature along the mammalian stem lineage, drawing upon the exceptional fossil record of extinct synapsids. Here, attention is focused on muscles of the hindlimb. Although the hindlimb skeleton did not undergo as marked a transformation on the line to mammals as did the forelimb skeleton, the anatomy of extant tetrapods indicates that major changes to musculature have nonetheless occurred. To better understand these changes, this study surveyed the osteological evidence for muscular attachments in extinct mammalian and nonmammalian synapsids, two extinct amniote outgroups, and a large selection of extant mammals, saurians, and salamanders. Observations were integrated into an explicit phylogenetic framework, comprising 80 character–state complexes covering all muscles crossing the hip, knee, and ankle joints. These were coded for 33 operational taxonomic units spanning >330 Ma of tetrapod evolution, and ancestral state reconstruction was used to evaluate the sequence of muscular evolution along the stem lineage from Amniota to Theria. The evolutionary history of mammalian hindlimb musculature was complex, nonlinear, and protracted, with several instances of convergence and pulses of anatomical transformation that continued well into the crown group. Numerous traits typically regarded as characteristically “mammalian” have much greater antiquity than previously recognized, and for some traits, most synapsids are probably more reflective of the ancestral amniote condition than are extant saurians. More broadly, this study highlights the utility of the fossil record in interpreting the evolutionary appearance of distinctive anatomies. 

Abstract This paper is the first in a two‐part series that charts the evolution of appendicular musculature along the mammalian stem lineage, drawing upon the exceptional fossil record of extinct synapsids. Here, attention is focused on muscles of the forelimb. Understanding forelimb muscular anatomy in extinct synapsids, and how this changed on the line to mammals, can provide important perspective for interpreting skeletal and functional evolution in this lineage, and how the diversity of forelimb functions in extant mammals arose. This study surveyed the osteological evidence for muscular attachments in extinct mammalian and nonmammalian synapsids, two extinct amniote outgroups, and a large selection of extant mammals, saurians, and salamanders. Observations were integrated into an explicit phylogenetic framework, comprising 73 character–state complexes covering all muscles crossing the shoulder, elbow, and wrist joints. These were coded for 33 operational taxonomic units spanning >330 Ma of tetrapod evolution, and ancestral state reconstruction was used to evaluate the sequence of muscular evolution along the stem lineage from Amniota to Theria. In addition to producing a comprehensive documentation of osteological evidence for muscle attachments in extinct synapsids, this work has clarified homology hypotheses across disparate taxa and helped resolve competing hypotheses of muscular anatomy in extinct species. The evolutionary history of mammalian forelimb musculature was a complex and nonlinear narrative, punctuated by multiple instances of convergence and concentrated phases of anatomical transformation. More broadly, this study highlights the great insight that a fossil‐based perspective can provide for understanding the assembly of novel body plans. 

The evolutionary transition from early synapsids to therian mammals involved profound reorganization in locomotor anatomy and function, centered around a shift from “sprawled” to “erect” limb postures. When and how this functional shift was accomplished has remained difficult to decipher from the fossil record alone. Through biomechanical modeling of hindlimb force-generating performance in eight exemplar fossil synapsids, we demonstrate that the erect locomotor regime typifying modern therians did not evolve until just before crown Theria. Modeling also identifies a transient phase of increased performance in therapsids and early cynodonts, before crown mammals. Further, quantifying the global actions of major hip muscle groups indicates a protracted juxtaposition of functional redeployment and conservatism, highlighting the intricate interplay between anatomical reorganization and function across postural transitions. We infer a complex history of synapsid locomotor evolution and suggest that major evolutionary transitions between contrasting locomotor behaviors may follow highly nonlinear trajectories. 

For the first 100+ million years of their evolutionary history, the majority of mammals were very small, and many exhibited relatively generalized locomotor ecologies. Among extant mammals, small-bodied, generalist species share similar hindlimb bone morphology and locomotor mechanics, but details of their musculature have not been investigated. To examine whether hindlimb muscle architecture properties are also similar, we dissected hindlimb muscles of the gray short-tailed opossum (Monodelphis domestica) and aggregated muscle properties from the literature for three other small-bodied mammals (Mus musculus, Rattus norvegicus, Cavia porcellus). We then studied hindlimb musculature from a whole-limb perspective and by separating the limb into nine anatomical regions. The region analysis explained substantially more variance in the data (r2: 0.601 > 0.074) but only detected six statistically significant pairwise species differences in muscle architecture properties. This finding suggests either deep conservation of therian hindlimb muscle properties or, more likely, a biomechanical constraint imposed by small body size. In addition, we find specialization for either large force production (i.e., PCSA) or longer active working ranges (i.e. long muscle fascicles) in proximal limb regions but neither specialization in more distal limb regions. This functional pattern may be key for small mammals to traverse across uneven and shifting substrates, regardless of environment. These findings are particularly relevant for researchers seeking to reconstruct and model soft tissue properties of extinct mammals during the early evolutionary history of the clade. 

Thalattosuchians represent one of the several independent transitions into the marine realm among crocodylomorphs. The extent of their aquatic adaptations ranges from the semiaquatic teleosauroids, superficially resembling extant gharials, to the almost cetacean-like pelagic metriorhynchids. Understanding the suite of osteological, physiological, and sensory changes that accompanied this major transition has received increased attention, but is somewhat hindered by a dearth of complete three-dimensionally preserved crania. Here, we describe the cranial and endocranial anatomy of a well-preserved three-dimensional specimen of Macrospondylus bollensis from the Toarcian of Yorkshire, UK. The trigeminal fossa contains two similar-sized openings separated by a thin lamina of prootic, a configuration that appears unique to a subset of teleosauroids. Macrospondylus bollensis resembles other thalattosuchians in having pyramidal semicircular canals with elongate cochlear ducts, enlarged carotid canals leading to an enlarged pituitary fossa, enlarged orbital arteries, enlarged endocranial venous sinuses, reduced pharyngotympanic sinuses, and a relatively straight brain with a hemispherical cerebral expansion. We describe for the first time the olfactory region and paranasal sinuses of a teleosauroid. A relatively large olfactory region suggests greater capacity for airborne olfaction in teleosauroids than in the more aquatically adapted metriorhynchoids. Additionally, slight swellings in the olfactory region suggest the presence of small salt glands of lower secretory capacity than those of metriorhynchoids. The presence of osteological correlates for salt glands in a teleosauroid corroborates previous hypotheses that these glands originated in the common ancestor of Thalattosuchia, facilitating their rapid radiation into the marine realm. 

In evolutionary biomechanics, musculoskeletal computer models of extant and extinct taxa are often used to estimate joint range of motion (ROM) and muscle moment arms (MMAs), two parameters which form the basis of functional inferences. However, relatively few experimental studies have been performed to validate model outputs. Previously, we built a model of the short-beaked echidna ( Tachyglossus aculeatus ) forelimb using a traditional modelling workflow, and in this study we evaluate its behaviour and outputs using experimental data. The echidna is an unusual animal representing an edge-case for model validation: it uses a unique form of sprawling locomotion, and possesses a suite of derived anatomical features, in addition to other features reminiscent of extinct early relatives of mammals. Here we use diffusible iodine-based contrast-enhanced computed tomography (diceCT) alongside digital and traditional dissection to evaluate muscle attachments, modelled muscle paths, and the effects of model alterations on the MMA outputs. We use X-ray Reconstruction of Moving Morphology (XROMM) to compare ex vivo joint ROM to model estimates based on osteological limits predicted via single-axis rotation, and to calculate experimental MMAs from implanted muscles using a novel geometric method. We also add additional levels of model detail, in the form of muscle architecture, to evaluate how muscle torque might alter the inferences made from MMAs alone, as is typical in evolutionary studies. Our study identifies several key findings that can be applied to future models. 1) A light-touch approach to model building can generate reasonably accurate muscle paths, and small alterations in attachment site seem to have minimal effects on model output. 2) Simultaneous movement through multiple degrees of freedom, including rotations and translation at joints, are necessary to ensure full joint ROM is captured; however, single-axis ROM can provide a reasonable approximation of mobility depending on the modelling objectives. 3) Our geometric method of calculating MMAs is consistent with model-predicted MMAs calculated via partial velocity, and is a potentially useful tool for others to create and validate musculoskeletal models. 4) Inclusion of muscle architecture data can change some functional inferences, but in many cases reinforced conclusions based on MMA alone.