Brandon P. Hedrick

Veterinary education often relies on cadaveric specimens, but there is increasing demand for alternatives due to limited resources and ethical considerations. To address this, we developed a 3D printed 'explodable' model of a dog cranium with detachable, magnetized cranial components for teaching anatomy to students. This model was generated from a computed tomographic scan of a juvenile dog cranium for which cranial sutures were still partially open and segmented such that major cranial bones were isolated. All bones are printed at actual size and retain openings for cranial nerves and major vessels. This interactive model enhances anatomical education by supplying a hands-on tool that can be used either in the classroom setting or for independent learning and can be incorporated at the high school, college, or veterinary school level. It is currently being integrated into the first-year anatomy foundation course at Cornell University's College of Veterinary Medicine. The model can be printed using any hobbyist or specialist 3D printer and we outline assembly instructions on how to attach magnets at prefabricated attachment points. Using both digital and 3D printed resources, we hope to help to address current shortages of anatomical resources and also inspire future generations of practicing veterinarians by making anatomy more accessible and engaging.

Birds have repeatedly evolved diverse developmental strategies, including multiple origins of sophisticated parental care, making them a model system to explore the consequences of developmental strategy upon phenotypic evolution. Here, we assess evolutionary covariance between limb proportions and ecological diversity of different bird lineages with altricial (high parental care) or precocial (lower parental care) developmental strategies. In addition, we model overall rates of evolutionary divergence between wing and leg skeletal proportions, allowing us to investigate the influence of developmental strategy upon adaptive traits. We show that while wing and leg proportions evolve independently of one another in altricial lineages, conforming to a modular pattern of evolution attested in birds more generally, there are strong correlations between wing and leg trait evolution in precocial lineages. Unlike precocial groups, divergent wing and leg evolution in altricial lineages is associated with access to novel flight-style combinations and is strongly associated with body mass. This suggests an adaptive association with mechanisms of growth and development. Inspection of internal wing proportions within major clades demonstrates that lineages with more altricial developmental strategies explore a wider range of mechanically relevant wing proportions, such as Brachial Index.

Body mass plays a fundamental role in the macroevolutionary dynamics of morphological, ecological, and phylogenetic diversification. Given biomechanical principles, large body masses in terrestrial vertebrates may impose important constraints on the adaptative potential of skeletal morphology. This is especially true for the limbs, which are involved in both supporting and propelling the body during locomotion. We present a novel framework for evaluating how body mass structures patterns of morphological, ecological, and phylogenetic diversification using a dataset of forelimb traits for more than 600 terrestrial mammal species. We found that forelimb shape disparity increases with body mass for mammals generally as well as within mammalian subclades, suggesting that this trend is robust to phylogenetic scale. However, both phylogenetic and locomotor diversity (a proxy for ecological diversity) were high for all except the largest mammals and were not strongly associated with body mass. This suggests that small mammals are capable of speciating widely and evolving novel locomotor modes without requiring drastic changes to forelimb shape. However, as body mass increases, biomechanical constraints require substantial morphological changes to the forelimb to adapt to similar levels of locomotor mode disparity. We also show that different limb bone elements do not respond in the same way to increases in body mass when analyzed individually, perhaps due to differing developmental constraints. We provide new insights on how body mass structures macroevolutionary processes in mammals, and our approach can be generalized to examine this question for a variety of traits, ecological modes, and phylogenetic groups.

Despite the remarkable morphological diversity found in vertebrate genitalia, it has historically been difficult to quantify shape variation of soft tissue structures due to limitations of 3D landmarking methods. New techniques such as automatic landmarking now allow us to examine such structures in detail, and with these methods we quantify the intraspecific variation in the genitalia of Burmese pythons ( Python bivittatus ). Despite previous assertions that a vaginal pouch is not present in pythons, we find that P. bivittatus have well developed vaginal pouches, that are morphologically diverse, and change shape over ontogeny. Vaginal pouches and hemipenes are isometric. Hemipenes also vary in shape ontogenetically, but we find no evidence of directional asymmetry in shape or size between adult right and left hemipenes suggesting a lack of laterality. We identify a potentially intersex neonate with hemipenes, testes, and a vaginal pouch. We discuss our results in the context of snake genital evolution and suggest other mechanisms for selection beyond the standard “lock and key” hypothesis. Future work examining genital shape variation of other snake families will provide more insight into the coevolutionary patterns shaping the genitalia diversity across snakes and vertebrates more broadly.

The avian respiratory system has been an area of biological interest for centuries, with zebra finches ( Taeniopygia castanotis ) emerging in recent decades as a primary avian model organism popularized across numerous disciplines. The pulmonary system of birds is unique in that air moves unidirectionally through the gas-exchanging lung, and previous works have suggested anatomical constraints within the bronchial network that may be coupled to the inspiratory valving mechanism in Aves. We used µCT-based segmented models to visualize and describe the morphology of the zebra finch lower respiratory system and to examine intra- and interspecific differences of the bronchial tree with the phylogenetically and ecologically different African grey parrot ( Psittacus erithacus ). Here, we show that zebra finches have highly variable lung and air sac morphology within individuals but generally do not diverge from the anatomical bauplan previously described for passerines. Additionally the parabronchi in the zebra finch lung are arranged into isolated segments between secondary bronchi, which has not been described and may be coupled with airflow patterns in this species. Both zebra finches and African grey parrots show constrained interostial distances and robust, caudally directed third ventrobronchi that may play an unexplored role in the unidirectional airflow patterns of birds. This article is part of the theme issue ‘Biology of the avian respiratory system: development, evolutionary morphology, function and clinical considerations’.

Bats and birds are defined by their convergent evolution of flight, hypothesized to require the modular decoupling of wing and leg evolution. Although a wealth of evidence supports this interpretation in birds, there has been no systematic attempt to identify modular organization in the bat limb skeleton. Here we present a phylogenetically representative and ecologically diverse collection of limb skeletal measurements from 111 extant bat species. We compare this dataset with a compendium of 149 bird species, known to exhibit modular evolution and anatomically regionalized skeletal adaptation. We demonstrate that, in contrast to birds, morphological diversification across crown bats is associated with strong trait integration both within and between the forelimb and hindlimb. Different regions of the bat limb skeleton adapt to accommodate variation in distinct ecological activities, with flight-style variety accommodated by adaptation of the distal wing, while the thumb and hindlimb play an important role facilitating adaptive responses to variation in roosting habits. We suggest that the wing membrane enforces evolutionary integration across the bat skeleton, highlighting that the evolution of the bat thumb is less correlated with the evolution of other limb bone proportions. We propose that strong limb integration inhibits bat adaptive responses, explaining their lower rates of phenotypic evolution and relatively homogeneous evolutionary dynamics in contrast to birds. Powered flight, enabled by the membranous wing, is therefore not only a key bat innovation but their defining inhibition.

The anatomy of the avian lower respiratory system includes a complex interaction between air‐filled pulmonary tissues, pulmonary air sacs, and much of the postcranial skeleton. Hypotheses related to the function and phylogenetic provenance of these respiratory structures have been posed based on extensive interspecific descriptions for an array of taxa. By contrast, intraspecific descriptions of anatomical variation for these features are much more limited, particularly for skeletal pneumatization, and are essential to establish a baseline for evaluating interspecific variation. To address this issue, we collected micro‐computed tomography (μCT) scans of live and deceased African grey parrots (Psittacus erithacus) to assess variation in the arrangement of the lungs, the air sacs, and their respective invasion of the postcranial skeleton via pneumatic foramina. Analysis reveals that the two pairs of caudalmost air sacs vary in size and arrangement, often exhibiting an asymmetric morphology. Further, locations of the pneumatic foramina are more variable for midline, non‐costal skeletal elements when compared to other pneumatized bones. These findings indicate a need to better understand contributing factors to variation in avian postcranial respiratory anatomy that can inform future intraspecific and interspecific comparisons.

The early diversification of tetrapods into terrestrial environments involved adaptations of their locomotor apparatus that allowed for weight support and propulsion on heterogeneous surfaces. Many lineages subsequently returned to the water, while others conquered the aerial environment, further diversifying under the physical constraints of locomoting through continuous fluid media. While many studies have explored the relationship between locomotion in continuous fluids and body mass, none have focused on how continuous fluid media have impacted the macroevolutionary patterns of limb shape diversity. We investigated whether mammals that left terrestrial environments to use air and water as their main locomotor environment experienced constraints on the morphological evolution of their forelimb, assessing their degree of morphological disparity and convergence. We gathered a comprehensive sample of more than 800 species that cover the extant family‐level diversity of mammals, using linear measurements of the forelimb skeleton to determine its shape and size. Among mammals, fully aquatic groups have the most disparate forelimb shapes, possibly due to the many different functional roles performed by flippers or the relaxation of constraints on within‐flipper bone proportions. Air‐based locomotion, in contrast, is linked to restricted forelimb shape diversity. Bats and gliding mammals exhibit similar morphological patterns that have resulted in partial phenotypic convergence, mostly involving the elongation of the proximal forelimb segments. Thus, whereas aquatic locomotion drives forelimb shape diversification, aerial locomotion constrains forelimb diversity. These results demonstrate that locomotion in continuous fluid media can either facilitate or limit morphological diversity and more broadly that locomotor environments have fostered the morphological and functional evolution of mammalian forelimbs. Read the free Plain Language Summary for this article on the Journal blog.

The subpectoral diverticulum (SPD) is an extension of the respiratory system in birds that is located between the primary muscles responsible for flapping the wing^1,2. Here we survey the pulmonary apparatus in 68 avian species, and show that the SPD was present in virtually all of the soaring taxa investigated but absent in non-soarers. We find that this structure evolved independently with soaring flight at least seven times, which indicates that the diverticulum might have a functional and adaptive relationship with this flight style. Using the soaring hawks Buteo jamaicensis and Buteo swainsoni as models, we show that the SPD is not integral for ventilation, that an inflated SPD can increase the moment arm of cranial parts of the pectoralis, and that pectoralis muscle fascicles are significantly shorter in soaring hawks than in non-soaring birds. This coupling of an SPD-mediated increase in pectoralis leverage with force-specialized muscle architecture produces a pneumatic system that is adapted for the isometric contractile conditions expected in soaring flight. The discovery of a mechanical role for the respiratory system in avian locomotion underscores the functional complexity and heterogeneity of this organ system, and suggests that pulmonary diverticula are likely to have other undiscovered secondary functions. These data provide a mechanistic explanation for the repeated appearance of the SPD in soaring lineages and show that the respiratory system can be co-opted to provide biomechanical solutions to the challenges of flight and thereby influence the evolution of avian volancy.