Roger B. J. Benson

Evolutionary shape changes of the vertebrate semicircular canals (SCCs) are often ascribed to evolutionary transitions in locomotion, including the origin of flight during the dinosaur–bird transition. However, the potential for transformations of braincase architecture to influence SCC shape independent of sensory-functional adaptation has generally been overlooked. We evaluate the influence of both factors using a large dataset of 146 living and recently extinct crown birds, including taxa representing ~24 independent evolutionary losses of flight, together with Mesozoic stem avialans, nonavialan dinosaurs, and pterosaurs. Among crown birds, evolutionary changes in locomotor capabilities, including the total loss of flight, have only weak effects on SCC shape, contrasting with large effects from changes in braincase geometry. Notably, rotation of the occiput toward a ventral orientation is associated with important changes to SCC shape that match those observed during the dinosaur–bird transition. This suggests that evolutionary transformations of SCC shape during the Mesozoic origin of avian flight resulted from evolutionary changes in the orientation of the craniocervical joint, and not from tuning of SCC sensory function. These findings illuminate the importance of integrated morphological changes to the head-neck system during bird origins and challenge the long-standing assumption that SCC morphology enables confident inferences of the locomotor capabilities of extinct vertebrates.

Tetrapods (limbed vertebrates) include lissamphibians and amniotes, and now comprise half of all vertebrate species, with particular importance on land. Limbs first appeared in stem tetrapods by the Late Devonian, 365 million years ago^1,2,3,4,5. However, many taxa either have limbs with fin-like functional anatomy or have secondarily lost them, raising questions about the ecological context and early evolution of the tetrapod limb⁶. Here we provide considerable new data on the enigmatic Early Carboniferous Westlothiana lizziae, widely cited as the earliest terrestrially adapted stem amniote^7,8. We find unexpected plesiomorphies of the skull roof, palate, braincase and mandible, alongside aquatic adaptations, including an ossified internal gill skeleton and extensive denticles on the palate and mandible. These traits return Westlothiana as a stem tetrapod in our phylogenetic analyses, demonstrating that terrestrial adaptations such as an amniote-like pedal formula, loss of fin-like forelimb function and superficially claw-like terminal phalanges evolved in a mosaic fashion, and that these traits evolved in a transitional, amphibious context before the origin of the tetrapod crown group. Our findings reinforce recent suggestions of a taxonomically inclusive tetrapod-stem lineage, but also illuminate substantial gaps in anatomical knowledge, especially for small-bodied taxa, that obstruct our understanding of early tetrapod evolution.

Vertebrates have varied body size distributions. Among these, non-avian dinosaurs have uniquely large minimum and maximum sizes, but the reasons remain unknown. Energetic fitness models provide a potential explanation, by describing how physiological scaling relationships should structure body size distributions under selection to maximize the conversion of energy into offspring. We show that the body size distributions of birds, mammals, and turtles fit the expectations of energetic fitness models, but snakes, lizards, and crocodylians do not. Moreover, no reasonable combination of physiological scaling relationships explains the up-shifted minimum body size of non-avian dinosaurs or flightless birds. We propose that energetic fitness models perform well when body size distributions are governed primarily by intrinsic energetic factors. However, extrinsic constraints imposed by ecological factors such as competition, predation, or ecological niche may also be needed to explain why species body size distributions differ from the model expectations in some groups. The abrupt macroevolutionary shift from non-avian dinosaurs with minimum adult body sizes of 400 g, to birds with modal body size of 30 g cannot be explained by changes in intrinsic physiological factors, and may instead have been caused by an abrupt release of ecological constraints, potentially related to the origin of flight.

Birds and mammals mostly produce clutches with small numbers of large individual offspring compared to non-bird reptiles, including dinosaurs on the bird-stem lineage. Existing hypotheses for this variation propose links between relative offspring size and large brains or high metabolic rates, but remain incompletely tested. We characterize the allometries of reproductive output traits (clutch mass, clutch size and individual offspring mass) and evaluate their correlates using phylogenetic regressions across 2857 living and extinct amniote species. Across amniotes, species with larger relative brain sizes have larger individual offspring, fewer offspring or both. This signifies a higher maternal investment per offspring in large-brained species, exemplified by birds and mammals. In dinosaurs, large egg sizes evolved shortly before the origin of the bird crown group, coinciding with an evolutionary increase in relative brain mass and suggesting that evolutionary increases in egg/neonate size evolved due to a requirement for greater investment per offspring with increasing brain size. These egg size increases may explain changes to dinosaur nest structure and pelvic anatomy along the bird-stem lineage. Our results provide insights into the evolutionary pressures shaping reproductive strategies and brain size across amniotes, underscoring the significance of these traits in the broader context of amniote diversification.

Gorgonopsian therapsids represent a transitional condition in the evolution of synapsid locomotion and postcranial structure. Most descriptions of gorgonopsians have focused on cranial material, however, limiting their usefulness for informing patterns of postcranial evolution on the mammal stem. While some recent work has begun to focus on postcrania, especially the pectoral girdle and forelimbs, comparatively little data are available on the pelvic girdle, hind limbs and tail. We report a new specimen of the late Permian gorgonopsian Aelurognathus tigriceps comprising a partial skull and well-preserved postcranial skeleton, including the near-complete series of dorsal vertebrae and ribs, complete pelvic girdle, hind limbs, feet, and a nearly complete tail. The tail is longer than any other published gorgonopsian. The new material presented here provides an opportunity to better establish broader patterns of morphology in the gorgonopsian postcranial skeleton.

Understanding how anatomical structures evolve requires disentangling the roles of integration and modularity in shaping morphological variation. The vertebral column, a serially repeated and regionally differentiated structure, provides a powerful system for investigating these processes. Here, we examine how vertebral morphology evolves in relation to whole-body elongation across the adaptive radiation of Lake Malawi cichlid fishes. We tested for evolutionary integration between the precaudal and caudal domains, as well as assessed the contributions of vertebral count, centrum shape, and intervertebral spacing on body elongation. We find strong evolutionary integration between the shapes of precaudal and caudal vertebrae, with both vertebral shapes varying along similar axes. Despite this, precaudal and caudal vertebral counts evolve independently, indicating a decoupling between the specification of identity and the development of their respective shapes. Whole-body elongation is significantly associated with coordinated changes in vertebral and rib morphology, including proportional increases in centrum size, posterior displacement of neural and haemal spines, and increased rib curvature. In contrast, centrum elongation and intervertebral spacing do not contribute to body elongation across the radiation. These results demonstrate that body elongation in cichlids necessitates integrated, multivariate changes in axial morphology. Our findings highlight the importance of morphological integration in facilitating coordinated evolutionary responses in anatomical systems.

Land vertebrates today comprise amniotes and lissamphibians, which have highly different modes of gas exchange, and distinct skull shapes and body size distributions. A central hypothesis of tetrapod evolution proposes a connection between these traits during their initial divergence, 335 million years ago. However, this has yet to be tested in a broad phylogenetic context. We investigate the evolution of body size, skull proportions, and respiratory traits among early land vertebrates, from the Middle Devonian to Early Permian using quantitative analysis of a dataset incorporating 344 species. We find that lissamphibian precursors show stronger constraints on body size than stem amniotes and increases in relative skull height were facilitated by relaxed constraints on the amniote stem lineage. These differences can be explained by respiratory innovations. Dependence on cutaneous gas exchange constrained lissamphibians and their close relatives to small body sizes, whereas rib-based lung ventilation relaxed constraints on skull shape and maximum body size in terrestrial amniotes.

Because the inner ear is involved in gaze stabilization, balance, and hearing, fossil inner ear endocast morphology has been used to infer the palaeoecology of extinct species. These results have been used to inform major evolutionary transitions, including the ecological origin of snakes. However, prior studies found only modest correlations between inner ear shape and ecological traits, and did not apply machine learning approaches, which could potentially reveal greater predictive relationships between inner ear morphology and ecology. Here, we combine three-dimensional geometric morphometrics with machine learning to evaluate the performance of inner ear morphology as a predictor of habitat use and phylogenetic affinities across a broad sample of toxicoferans (snakes, anguimorphs, and iguanians) representing 73 extant species and 4 fossil species. We find a weak correlation between habitat and inner ear morphology, but machine learning models cannot accurately predict habitat preference in extant species (44% accuracy). In contrast, we find a strongly predictive relationship (95% accuracy) between inner ear shape and higher-order classification. Our results demonstrate that the inner ear shape data we measured strongly predicts evolutionary classifications rather than habitat use in crown toxicoferan squamates. We conclude that machine learning provides a versatile analytical approach to reconstruction of palaeobiology.

In birds, the quadrate bone serves as a hinge articulating with the lower jaw and the skull, playing an important mechanical role in the feeding apparatus. Avian cranial kinesis is dependent on the streptostylic quadrate transferring force from the adductor muscles at the back of the skull toward the beak, as part of a four-bar mechanical linkage to elevate and depress the bill. The complex morphology of the bird quadrate has led to authors adopting a range of alternative terminologies to describe the same anatomical structures and character states, impeding clarity of communication and presenting a barrier to progress in our understanding of the evolution of this important component of the avian feeding apparatus. Here, we reconcile terminological discord among previous studies on avian quadrate morphology and propose a stable nomenclature for future work. To characterise the considerable variation in quadrate form across crown bird diversity, we present an extensive anatomical atlas of the avian quadrate and summarise major patterns of quadrate morphological variation across extant avian phylogeny. In addition, we investigate macroevolutionary patterns in avian quadrate morphology, incorporating comparisons of crown birds and Late Cretaceous near-crown stem birds. We demonstrate that quadrate characters are useful for diagnosing a range of major avian subclades, and suggest that numerous distinctive features are likely to be associated with important biomechanical consequences. This investigation has implications for resolving the unsettled phylogenetic relationships of extinct bird clades such as Pelagornithidae and Gastornithiformes, as well as controversial relationships within several extant groups.

Living reptiles including turtles, crocodilians, birds and squamates are descended from a common ancestor among the Neodiapsida that lived in the late Permian c . 257 million years ago. Their origin was preceded by key evolutionary changes to cranial architecture that are poorly understood due to the rarity of early neodiapsids in the fossil record. Here, we describe a monospecific aggregation of a new non‐saurian neodiapsid from the late Permian of South Africa. Synchrotron microtomography of four complete skulls reveals a mosaic of classic ‘saurian’ features such as a tympanic fossa and cephalic condyle of the quadrate and an open lower temporal bar, alongside surprising plesiomorphies including a rectangular denticle field on the braincase and a comparatively robust stapes. Phylogenetic analysis finds the new taxon within the Younginidae, sister to Akkedops bremneri and Youngina capensis as the earliest‐diverging neodiapsid lineage. Our results demonstrate that the mobile (strepostylic) quadrate evolved only shortly after the origin of the tympanic fossa and the loss of the lower temporal bar, among crownward stem reptiles. This suggests a functional evolutionary linkage between these important traits related to hearing and feeding during the rise of crown reptiles.