Ryan. N. Felice

The Australasian crocodylian fossil record is dominated by the extinct endemic clade Mekosuchinae, with known members spanning the Eocene‐to‐Holocene. This diverse clade exhibited high morphological disparity, from early members superficially similar to Crocodylus , to more terrestrial forms later in the clade's evolutionary history. However, it remains unclear why mekosuchines went extinct whilst Crocodylus continues to thrive in the region, and how the ancestors of this terrestrial clade reached Australasia without long‐distance transoceanic dispersal. One possible explanation is that there was intraclade variation in the growth pattern and life habit of mekosuchine species, and that these, in turn, differ from those of Crocodylus . Here, we present the first osteohistological study of mekosuchines, evaluating the femora of the middle–late Eocene species Kambara taraina and late Oligocene–middle Miocene Baru ( B . wickeni , B . darrowi ). Bone compactness profiles are more characteristic of a terrestrial habit than those of extant crocodylians, with Baru darrowi demonstrating the most terrestrialized profile. Apposition rates are similar in absolute value to those of extant crocodylians, indicating a comparable metabolism; however, high appositional rates throughout mid‐cortical cyclical growth marks suggest that mekosuchines were able (or needed) to maintain a moderate rate of growth for longer than extant crocodylians. Our results indicate that mekosuchines were more terrestrial than Crocodylus and that later‐diverging mekosuchines showed increased terrestrialization; however, they do not indicate that early mekosuchines were well‐adapted to transoceanic dispersal. Heightened terrestrialization and an extended moderate growth period might have made mekosuchines more vulnerable to late Cenozoic aridification in Australia than semi‐aquatic Crocodylus .

Testing hypotheses of phenotypic modularity involves assessing whether groups of traits covary more strongly with each other than with parts outside the group. Structural Equation Modelling (SEM) is a flexible statistical framework for interrogating complex relationships between sets of variables, making it ideally suited to studies of hierarchical modularity and integration. However, quantifying the modular organization of high-dimensional traits using SEM in a phylogenic context has only recently become possible through new methodological advances. Here, we applied SEM to investigate patterns and correlates of phenotypic modularity in the skull and brain of birds. Birds independently evolved relatively large brains multiple times, as well as a wide range of different skull and brain morphologies. While some have proposed the bird skull is composed of several functional or developmental modules, others have suggested the skull is highly integrated, with share allometric scaling structuring trait correlations. The data best supported a model in which brain shape is influenced by changes in shape of the neurocranium as well as a ‘jaw’ module consisting of the rostrum shape and jaw musculature. Rostrum shape itself does not strongly covary with other aspects of the skull and brain, suggesting decoupling of beak morphology from the rest of the avian cranium. All variables, with the exception of rostrum shape, are strongly influenced by size, supporting the idea that allometry is a major influence on craniofacial integration in birds. These results provide new insights into likely drivers shaping the evolution of the skull in birds and highlight the usefulness of phyloSEM testing hypotheses of evolutionary modularity and integration.

Tyrannosaurus is viewed as a model organism in vertebrate paleontology, with numerous studies analyzing its feeding biomechanics. Nonetheless, the evolution of this feeding performance has been under‐addressed in Tyrannosauroidea, especially in basal tyrannosauroids. Here we used muscle‐force reconstruction and finite element analysis (FEA) to quantify the cranial performance of tyrannosauroids and outgroup theropod clades. 2D (planar) cranial models, set to standardized skull lengths and jaw adductor forces, were used to analyze the evolution of feeding behavior in a large sample size of Tyrannosauroidea and other theropods. Sampled stresses matched well between planar and 3D analyses of three disparately shaped crania, suggesting valid interpretations from 2D models along the lateral sides of theropod crania if symmetrical bite loadings are assumed. We traced cranial evolution by sampling stresses at homologous points of theropod crania and input their average stress values into a maximum likelihood ancestral character state estimation. Our results show tyrannosauroids having moderate‐to‐low cranial stresses. We further tested whether the average stress value correlates with head size through phylogenetic generalized least square regressions. We found that 2D FEA provides significant information on the evolution of feeding performance in a major dinosaur clade. Along internal branches of Tyrannosauroidea, hypothetical common ancestors exhibit low cranial stress values owing to a combination of robust skulls and cranial protuberances. These traits may have been passed down to later tyrannosauroids, enabling them to handle high forces. Our results additionally demonstrate a possible correlation between cranial shape (brevirostrine versus longirostrine) and inferred cranial performance in non‐tyrannosauroid clades.

Changes in the structure and relative size of the brain are thought to be key transformations in the evolution of birds, reflecting innovations and diversity of neurosensory and cognitive capabilities. These changes do not occur in isolation, being accompanied by many other derived morphological characteristics. In the avian head alone, these include the evolution of a toothless beak, an increase in relative eye size, and a reduction and restructuring of jaw muscles. Several developmental trade-offs have been proposed to explain the interrelationships among these traits, but how these developmental patterns translate into evolutionary correlations among cranial traits is poorly understood. Here, we use two-block partial least squares analyses and Ornstein–Uhlenbeck models of adaptive trait evolution to explore the phenotypic evolution of hard and soft cranial tissues and test hypotheses of correlated trait evolution. In pairwise analyses, we found that all traits are significantly correlated, and found support for a form of adaptive trait evolution across the whole head in which traits interact reciprocally via the neurocranium. Together, these results highlight the integrated nature of the avian head and reveal that the evolution of diverse phenotypes is a result of complex multiple interactions among hard and soft tissue traits.

The origin and evolution of snakes has been marked by the acquisition of many morphological and functional novelties, one of which is the possession of a highly kinetic skull allowing for the consumption of prey that are often larger than their head diameter. One feature of the iconic wide gape of macrostomate (large‐mouthed) snakes is due to changes in the rostral midline where the left and right hemi‐mandible come together. Across vertebrates, the two sides of the lower jaw are held together by the mandibular symphysis. In snakes, the two halves of the lower jaw do not fuse and the symphysis remains free, facilitating gape expansion. The symphysis has previously been explored in lizards and crocodiles, where ligamentous fibres and cartilages span the joint. Here, we compared the anatomy of the forming ‘free’ mandibular symphysis in the corn snake ( Pantherophis guttatus ) to symphysis development in two lizards, the veiled chameleon ( Chamaeleo calyptratus ) and the ocelot gecko ( Paroedura picta ), and an outgroup sauropsid, the chicken ( Gallus gallus domesticus ). Microcomputed tomography imaging, whole‐mount skeletal staining and histology staining confirmed the absence of bone and cartilage fusion at the mandibular symphysis in the corn snake during development, in contrast to the complete fusion of cartilage, but not bone, in both lizards and the fusion of the bone in the chick. Trichrome staining under circular polarised light and whole fast green staining highlighted that, while the symphyseal region was populated by a dense network of collagen fibres, the snake hemi‐mandibles were not connected across the rostral region by this fibrous network. Instead, collagen fibres extended backwards and around the snake mental groove to an intermandibular nodule. This nodule attached to the midline dorsally, allowing integration of the movement of the soft and hard tissues. Our analysis highlights the adaptations required to allow extreme lower jaw mobility and independence of the two sides of the jaw as found in macrostomate snakes.

Low reproductive success rates can be a common problem for threatened species, impacting population viability and limiting recovery potential. Reproductive success can also be particularly low in populations that are managed ex‐situ. This can pose a challenge for threatened species recovery programmes because high reproductive success rates are often required to provide offspring for wild releases while ensuring population viability. Understanding the underlying causes of low reproductive success rates in ex‐situ populations of threatened species is therefore essential so that management programmes can optimise species' recovery potential. Here, we quantify rates of egg viability (i.e., indicating fertility and/or early embryo mortality) and total hatching success in the Extinct in the Wild sihek (Guam kingfisher, Todiramphus cinnamominus ). Using Bayesian generalised linear mixed models, we investigate effects of parental age, parental inbreeding coefficient ( f ), egg f and incubation method on egg viability and hatching success of viable eggs. We find that the sihek population has extremely low egg viability rates (~48% ± 2.15%, N = 304/635) and total egg‐hatching success rates (~30% ± 1.82% SE 190/635) compared to other threatened and non‐threatened bird species, both ex‐situ and in the wild. We find that increased paternal age and f are key drivers of decreased egg viability. In contrast, increased maternal age and use of artificial incubation are important contributors to decreased egg‐hatching success. Our results are particularly pertinent given current active recovery planning for sihek, which may require increased offspring production for wild releases. Furthermore, our results suggest that, in closed ex‐situ populations where f inevitably increases across generations such that at a given time point older aged individuals may have lower f , there is a need for breeding recommendations to quantitively and systematically balance genetic considerations with species' biological limitations such as reproductive senescence to meet recovery programme goals.

Changes in the structure and relative size of the brain are thought to be key transformations in the origins and continued evolution of birds, reflecting innovations and diversity of neurosensory and cognitive capabilities. However, these neuro-anatomical and functional changes do not occur in isolation, being accompanied by a host of other derived morphological characteristics associated with the evolution of flight. In the avian head alone, these include the evolution of a toothless beak, increase in relative eye size, and reduction and restructuring of jaw muscles. Several hypothesized developmental trade-offs have been proposed to explain the interrelationships among the hard and soft tissues of the head. How these developmental patterns translate into evolutionary trade-offs in other cranial traits is poorly understood, despite brain shape evolution being well documented in birds. Here, we use two-block partial least squares analyses and Ornstein-Uhlenbeck models of adaptive trait evolution to explore the phenotypic evolution of hard and soft cranial tissues and test hypotheses of correlated trait evolution. In pairwise analyses we found that all traits (endocast shape, neurocranium shape, rostrum shape, jaw muscle shape and residual orbit diameter) are significantly correlated. We found strongest support for a modular hypothesis of trait evolution across the whole head, with the rostrum and jaw muscles forming one module and brain, neurocranium, and eye forming the other. Within modules, traits are tightly integrated, but integration is relaxed between modules, allowing them to develop and evolve with a degree of independence. Together, these results highlight the integrated nature of the avian head and reveal that rather than being driven overwhelmingly by selection on a single trait, the shape of the avian head is a result of multiple interactions among hard and soft tissue traits.

Evolution of the mandibular structure has played a key role in the taxonomic diversification and niche exploration of tetrapods, from the initial water-to-land transition to the origin and radiation of mammals. While jaw evolution through these key transition points have long attracted attention, no studies of jaw evolution to date capture the breadth of tetrapod diversity and evolutionary history. Here, we reconstruct the compositional evolution of the tetrapod mandible with a comprehensive dataset of 36 traits scored in 3047 species. This sampling spans the earliest eotetrapodiforms to extant amphibians, mammals, and reptiles and birds, capturing immense ecological, developmental, and taxonomic diversity among a near-even split of extant and fossil tetrapods. We find that lower jaw morphology rapidly canalises at the base of each major clade, including, Eotetrapodiformes, Amniota, Synapsida, Sauropsida, and Amphibia, as well as subclades within those groups, and that, as a result, jaw compositional variation is highly phylogenetically structured. We estimate an average of over 100 shifts in three key traits: number of elements (dermal and endochondral bones), number of tooth-bearing elements, and total teeth per hemimandible, with symmetrical rates of gains and losses estimated for the number of elements, and correlated evolution between these three characters. Although novel elements arise occasionally, disparity in jaw composition overwhelmingly decreases as clades canalise rapidly, successively, and pervasively through tetrapod evolutionary history.

Both brain and body size vary greatly across birds, with both traits hypothesized to play a key role in diversification patterns across the group. Larger brains facilitate the evolution of greater behavioural plasticity, which could facilitate diversification in two ways, either by driving increased speciation (behavioural drive) or by making lineages more resilient to extinction (cognitive buffer). Here, we model brain size dependent diversification dynamics from published endocranial volume data for a sample of 2291 extant and fossil bird taxa, jointly modelling speciation rate, extinction rate and fossilisation rate separately for perching birds and other groups. We find that increasing relative brain size primarily increases diversification rate through decreasing extinction rate, supporting the cognitive buffer hypothesis and suggesting that larger brains in birds promote diversification by making species more resilient to extinction. Although speciation and extinction are dependent on relative brain size in both perching birds and other groups, this dependence is non-linear. In perching birds, the highest diversification rates are found over a narrow range of relative brain sizes, in contrast to other bird groups which show lower diversification rates over a broader range of relative brain sizes. Significance The potential for speciation and extinction are thought to be shaped by key phenotypic traits. In birds, brain size is one important trait hypothesized to have an impact on diversification. Testing this hypothesis requires the evolution of brain size, speciation rate, and extinction rate to be jointly modelled across the bird phylogeny. The inclusion of information on past diversity has been shown to be important in inferring diversification patterns, and so we include not only data from a broad sample of living birds but also information from the fossil record. Our study reveals patterns of brain size dependent diversification in birds which are consistent with the cognitive buffer hypothesis, with higher relative brain sizes being associated with lower extinction rates.