Jamie Alexander MacLaren

Osteoderms are bony elements in the dermis of a disparate set of vertebrates, including several taxa of reptiles. Why and to what extent osteoderms vary in occurrence, number, volume, and shape, particularly at the intraspecific level, remains poorly understood. Here we explored the extent of intraspecific variation in the cordylid lizard Hemicordylus capensis, which is broadly distributed across the Western Cape of South Africa. We sampled lizards from three geographically distinct populations along a longitudinal gradient and used micro-computed tomography and 3D geometric morphometrics to quantify osteoderm number, volume, and shape. Our results show substantial variation across populations, sexes, and body regions. Specifically, the northernmost population has significantly fewer and smaller osteoderms than the central and southern populations. We also observed sexual dimorphism: males have more and larger osteoderms than females. Interestingly, sexual shape dimorphism was restricted to the northern population, where males exhibit flatter, ‘pancake’-shaped osteoderms and females more elongated, ‘sausage’-like forms. Additionally, osteoderm morphology varied across body regions, with the neck bearing thicker, keel-shaped (often spiked) osteoderms, contrasting with the ‘pancake’- or ‘sausage’-shaped osteoderms on the trunk. We discuss potential functional interpretations for this variation—including roles in protection, thermoregulation, and hydroregulation—that merit further investigation.

Equid evolution is characterized by a high diversity of extinct species and morphologies, whereas extant equids share a superficially similar, monodactyl morphology. This inferred musculoskeletal similarity of modern equid limbs remains unexplored, and it is often assumed that domestic horse limbs are representative for wild equids (e.g., zebras, onagers, etc.). Our aim was to quantitatively describe the muscle architecture and arrangement of all forelimb muscles in extant wild Equus species to test this assumption, and investigate any differences between the species. We hypothesized that there would be subtle variation linked to locomotion on the different substrates that these species encounter. Gross dissections were performed to record muscle attachment sites and to quantify architectural metrics: muscle‐tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and fascicle length; physiological cross‐sectional area (PCSA) of the muscles and tendon cross‐sectional area (TCSA) of the distal ligaments were then calculated. Qualitative results confirm common origin and insertion sites of all muscles across all Equus species. When normalized for size, the forelimb muscles across equids generally exhibit comparable muscular architecture and force‐generating capacities. However, we observed a trend for higher force‐generating potential in the distal limb flexor muscles in two species of zebra naturally found in habitats with inclined or uneven substrates. Although limited by sample size, these results indicate that scaled data for modern wild equids are generally very comparable, which may enable much smoother translation of experimental data from domestic horses into digital simulations of wild equid locomotion, including for extinct equids.

Body length is a crucial ecological predictor in vertebrates, yet total body length proxies have seldom been assessed for ancient marine top predators. Here, we test the strength of phylogenetic imputation and 23 linear measurements, sampling both broad skeletal regions and frequently fossilized elements (such as vertebral centra), in predicting the body length of the main clades of tail-propelled Mesozoic marine reptiles (Ichthyosauria, Mosasauridae and pelagic thalattosuchians). We embed this marine reptile sample within a comparative framework with raptorial cetaceans, and analyse the evolution of body proportions in these clades. We find that trunk length and centrum dimensions are strong predictors of body length, opening up the possibility to build vast datasets of body length estimations for Mesozoic marine reptiles from minimal preserved remains. We provide body length calculation equations for all traits and all clades. Proxies fared much better and often had distinct slopes when applied clade-wide rather than when applied to the global dataset. We also show that body length in Mesozoic marine reptiles is more labile than their skeletal architectures, rendering phylogenetic imputation methods less effective than skeletal proxies for assessing body lengths.

Orthopedic conditions, congenital or acquired, are frequent reasons for presentation of companion animals to veterinarians

The distal limb of many hooved mammals (ungulates) has become highly specialised, with tendonised muscles and elongate bones and ligaments. Several clades of ungulates retain fleshy, muscularised distal forelimbs; these include hippopotamuses, rhinoceroses, and tapirs. Of these species, tapirs (Tapiridae: Tapirus) represent the most plesiomorphic manus anatomy for its higher taxonomic group (Perissodactyla); the tetradactyl manus of tapirs is reminiscent of the earliest members of the lineages leading to modern horses (equids) and rhinocerotids. Within the tapir manus, osteological evidence indicates clear differences in load distribution, digit use during locomotion, and phylogenetic signal in the shape of certain bony elements. To date, no quantitative investigation has ever been performed to explore differences in the muscular anatomy of the tapir distal forelimb (manus). Here, we conducted a comparative muscle architecture quantification of the muscles which are intrinsic to the tapir manus, across all four extant species (Tapirus indicus, T. bairdii, T. pinchaque, T. terrestris). Despite limited sample sizes, we observed notable variation across the different species with regard to the force‐generating potential of the muscles (based on physiological cross‐sectional area, PCSA) and the shortening range of each muscle (based on fascicle length). High force‐generating capacities were recovered for the interosseus muscles (preventing hyperextension) for the third and fourth digits, as may be expected for a mesaxonic manus such as that of Tapirus. Our results also indicate subtle differences in patterns of force‐generating potential in the interosseus muscles between specimens housed in captivity and those from the wild, specifically living in upland rainforest and exhibiting ranging behaviour up and downhill on a regular basis. These data offer tantalising insights into the variation in the force‐excursion relationship in the muscles of the ungulate manus, providing both qualitative and quantitative information for veterinarians, biologists, and palaeontologists investigating perissodactyl locomotor anatomy and evolution.

The baubellum (os clitoridis) is a bone found in the clitoris of many female eutherian mammals and is homologous to the baculum in males. In contrast to the baculum, the baubellum has received very little attention regarding its morphological or interspecific diversity, or on hypotheses for its function. The presence of the baubellum in bears (Ursidae) has only been established and mentioned in the literature for the Ursus genus, and not for the other genera of bears. Moreover, no scaled photographs are available for baubella of this clade, and the sizes reported vary between sources. We hereby present and describe the baubellum of a spectacled bear (Tremarctos ornatus), providing a detailed account of baubella in a basal ursid species. The baubellum of Tremarctos is slightly bowed dorsally, with two small prominences at the distal apex. The length of the Tremarctos baubellum in this study is comparable to that of Ursus americanus (American black bear). We infer the specific shape, with longitudinal ridges, of the baubellum in Tremarctos could indicate a discrete function during copulation or sexual arousal. However, future studies, especially regarding the associated soft tissues, will be required to confirm whether this is indeed the case. Our study expands the understanding of baubella within Ursidae, providing new data (including a three‐dimensional model) that can be used to further explore the morphological diversity and function of this enigmatic extra‐skeletal bone.

Accurate specimen digitization through glass achieved and validated using 3D surface scanning

The musculoskeletal anatomy of the shoulder of many ungulates has been inferred from veterinary model taxa, with uniformity in muscle arrangements and attachment sites often assumed. In this study, I investigated the muscular and osteological anatomy of tapirs and their relatives (Perissodactyla: Tapiroidea), using a combination of gross dissection and digital imaging (photography and laser surface scanning). Dissections of three modern tapir species revealed that the m. infraspinatus originates from both supraspinous and infraspinous fossae for all species, lying on both sides of the distal scapular spine. The epimysial border between the m. supraspinatus and m. infraspinatus origin sites are marked in all species by an ossified ridge, sometimes extending the length of the scapular spine. This “supraspinous ridge” is clearly visible on the scapular surface of both modern and extinct Tapirus scapulae; however, the ridge does not appear present in any non‐Tapirus tapiroids examined (e.g., Helaletes, Nexuotapirus), nor in other perissodactyls or artiodactyls. Moreover, the ridge exhibits a clearly distinct morphology in Tapirus indicus compared to all other Tapirus species examined. Combined, these findings indicate that the presence and position of the “supraspinous ridge” may represent a robust phylogenetic character for reconstructing relationships within tapiroids. Unfortunately, any functional locomotor outcomes or benefits of the m. infraspinatus straddling the scapular spine remains elusive. This study represents a firm reminder for anatomists, veterinarians, and paleontologists to (where possible) look beyond veterinary model systems when inferring musculoskeletal form or function in non‐model organisms.

Finite element analysis (FEA) is a computational method used to predict the behaviour (stresses, strains and deformation) of a structure under predefined loading conditions. It can be applied to different biological structures, such as bone, to study defined muscle‐driven scenarios. However, as muscle is an extremely complex structure to model, evolutionary biologists usually model muscle forces indirectly. In 2007, the BONELOAD MATLAB routine was developed to distribute muscle forces on a surface defined by the user. This routine then had to be coupled with a pre‐existing FEA software (e.g. Strand7) to perform the analyses and has been widely used ever since. In this manuscript, we present a new method to run muscle‐driven finite element simulations on a bone by distributing muscle forces on their insertions area, all within a single environment. We apply this protocol in three different situations: two biting simulations (unilateral and bilateral) and a shoulder flexion simulation. We demonstrate how to prepare the mesh, delineate the muscle origins and insertions, define the constraints, adjust material properties, choose a loading scenario (uniform, tangential or tangential‐plus‐normal), and extract the results. Our automated script meshes the 3D model, defines the constraints and distributes muscle forces within a single simulation software: ‘Metafor’ (nonlinear solver, owned and distributed by Gesval S.A) or ‘Fossils’ (a new open‐source linear static solver developed in the frame of this work). ‘Metafor’ and ‘Fossils’ can perform the entire protocol (from the meshing to the muscle‐induced simulations) on high‐resolution volumetric meshes (millions of tetrahedra) and rapidly, exceeding the processing time of other widely used software protocols by up to four times. We demonstrate that the results obtained from our protocol are highly congruent with brands such as Strand7. Thus, our protocol opens up the possibility to routinely and rapidly simulate the behaviour of high‐precision muscle‐driven FE models containing millions of tetrahedra.