Cranial Geometric Morphometric Analysis of the Genus Tapirus (Mammalia, Perissodactyla)

New Insights into the Molecular Evolution of Tapirus pinchaque (Tapiridae, Perissodactyla) and the Rise and Fall of Tapirus kabomani as a Full Species

Large wild mammals are extremely important in their respective ecological communities and are frequently considered to be emblematic. This is the case of the different tapir species, the largest terrestrial mammals from the Neotropics. Despite their large size and being objects of interest for many naturalists, the field still lacks critical genetics and systematics information about tapir species. In the current work, we analyzed four molecular datasets (mitogenomes, and three nuclear genes, RAG 1-2, IRBP, and BRCA1) of two South American tapirs: the Andean tapir (Tapirus pinchaque) and the alleged new species of tapir, Tapirus kabomani. We derived four main findings. (1) Our molecular phylogenetic analyses showed T. pinchaque as the youngest tapir branch in Neotropics and a sister species of Tapirus terrestris. This contradicts the traditional morphological observations of renowned zoologists and paleontologists, who considered T. pinchaque as the oldest Neotropical tapir. (2) Our data does not support that the alleged T. kabomani is a full species. Rather, it is a specific group within T. terrestris. (3) T. pinchaque is the Neotropical tapir species which yielded the lowest levels of genetic diversity (both for mitochondrial and nuclear data). (4) The spatial genetic structure for T. pinchaque shows differences depending on the type of molecular marker used. With mitogenomes, the spatial structure is relatively weak, whereas with two nuclear genes (RAG 1-2 and IRBP), the spatial structure is highly significant. Curiously, for the other nuclear gene (BRCA1), the spatial structure is practically nonexistent. In any case, the northernmost population of T. pinchaque we studied (Los Nevados National Park in Colombia) was in a peripatric situation and was the most genetically differentiated. This is important for the adequate conservation of this population. (5) T. pinchaque showed clear evidence of population expansion during the last part of the Pleistocene, a period during which the dryness and glacial cold extinguished many large mammals in the Americas. However, T. pinchaque survived and spread throughout the Northern Andes.

Sagittal crest morphology decoupled from relative bite performance in Pleistocene tapirs (Perissodactyla: Tapiridae)

Bite force is often associated with specific morphological features, such as sagittal crests. The presence of a pronounced sagittal crest in some tapirs (Perissodactyla: Tapiridae) was recently shown to be negatively correlated with hard-object feeding, in contrast with similar cranial structures in carnivorans. The aim of this study was to investigate bite forces and sagittal crest heights across a wide range of modern and extinct tapirs and apply a comparative investigation to establish whether these features are correlated across a broad phylogenetic scope. We examined a sample of 71 specimens representing 15 tapir species (5 extant, 10 extinct) using the dry-skull method, linear measurements of cranial features, phylogenetic reconstruction, and comparative analyses. Tapirs were found to exhibit variation in bite force and sagittal crest height across their phylogeny and between different biogeographical realms, with high-crested morphologies occurring mostly in Neotropical species. The highest bite forces within tapirs appear to be driven by estimates for the masseter-pterygoid muscle complex, rather than predicted forces for the temporalis muscle. Our results demonstrate that relative sagittal crest height is poorly correlated with relative cranial bite force, suggesting high force application is not a driver for pronounced sagittal crests in this sample. The divergent biomechanical capabilities of different contemporaneous tapirids may have allowed multiple species to occupy overlapping territories and partition resources to avoid excess competition. Bite forces in tapirs peak in Pleistocene species, independent of body size, suggesting possible dietary shifts as a potential result of climatic changes during this epoch.

Geometric morphometric analysis of the plateau zokor (Eospalax baileyi) revealed significant effects of environmental factors on skull variations

The plateau zokor (Eospalax baileyi) is employed as an ideal model for examining the relationships between phenotypic and ecological adaptations to the underground conditions in which the skull morphology evolves to adapt to tunnel environment. We evaluated the influence of environmental factors (altitude, temperature, and precipitation) and geographical distance on the variations in skull morphology of a native subterranean rodent plateau zokor population. Thin-plate spline showed that the trend of morphological changes along the CV1 axis was as follows: the two zygomatic arch and the two postorbital processes moved down, the two mastoid processes and the tooth row moved upward, and the tympanic bulla grew longer. The changes along the CV2 axis were as follows: the nasal bone and the tooth row became longer, the distance between the two anterior tips of zygomatic arch lengthened, the infraorbital foramen became smaller, the whole posterior part of the skull became shorter, the zygomatic bone and the two posterior tips of zygomatic arch moved down, and the foramen magnum became bigger. Thus we found significant differences in the skull shape among the seven populations studied. Along with the reduction in the altitude and increase in the mean annual temperature and mean annual precipitation, the nasal bone became shorter, the distance between the two anterior tips of the zygomatic arch became shorter, the whole posterior part of the skull lengthened, the infraorbital foramen became smaller, the two mastoid processes moved upward, and the occipital bone moved down on the dorsal surface of the skull. On the ventral surface of the skull, with an increase in the altitude, mean annual temperature, and mean annual precipitation, the tympanic bulla became shorter, the tooth row moved down, and the foramen magnum became smaller. The morphological changes in the skull were significantly positively correlated with environmental factors. Finally, there was a significant positive correlation between the Procrustes distance matrix of the skull and the geographic distance matrix, which indicates that the evolution of the plateau zokor follows the distance isolation model, but it needs to be further explored from genetic perspectives.

Comparative forelimb myology and muscular architecture of a juvenile Malayan tapir (Tapirus indicus)

The absence of preserved soft tissues in the fossil record is frequently a hindrance for palaeontologists wishing to investigate morphological shifts in key skeletal systems, such as the limbs. Understanding the soft tissue composition of modern species can aid in understanding changes in musculoskeletal features through evolution, including those pertaining to locomotion. Establishing anatomical differences in soft tissues utilising an extant phylogenetic bracket can, in turn, assist in interpreting morphological changes in hard tissues and modelling musculoskeletal movements during evolutionary transitions (e.g. digit reduction in perissodactyls). Perissodactyls (horses, rhinoceroses, tapirs and their relatives) are known to have originated with a four-toed (tetradactyl) forelimb condition. Equids proceeded to reduce all but their central digit, resulting in monodactyly, whereas tapirs retained the ancestral tetradactyl state. The modern Malayan tapir (Tapirus indicus) has been shown to exhibit fully functional tetradactyly in its forelimb, more so than any other tapir, and represents an ideal case-study for muscular arrangement and architectural comparison with the highly derived monodactyl Equus. Here, we present the first quantification of muscular architecture of a tetradactyl perissodactyl (T. indicus), and compare it to measurements from modern monodactyl caballine horse (Equus ferus caballus). Each muscle of the tapir forelimb was dissected out from a cadaver and measured for architectural properties: muscle-tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and resting fibre length. Comparative parameters [physiological cross-sectional area (PCSA), muscle volume, and % muscle mass] were then calculated from the raw measurements. In the shoulder region, the infraspinatus of T. indicus exhibits dual origination sites on either side of the deflected scapular spine. Within ungulates, this condition has only been previously reported in suids. Differences in relative contribution to limb muscle mass between T. indicus and Equus highlight forelimb muscles that affect mobility in the lateral and medial digits (e.g. extensor digitorum lateralis). These muscles were likely reduced in equids during their evolutionary transition from tetradactyl forest-dwellers to monodactyl, open-habitat specialists. Patterns of PCSA across the forelimb were similar between T. indicus and Equus, with the notable exceptions of the biceps brachii and flexor carpi ulnaris, which were much larger in Equus. The differences observed in PCSA between the tapir and horse forelimb muscles highlight muscles that are essential for maintaining stability in the monodactyl limb while moving at high speeds. This quantitative dataset of muscle architecture in a functionally tetradactyl perissodactyl is a pivotal first step towards reconstructing the locomotor capabilities of extinct, four-toed ancestors of modern perissodactyls, and providing further insights into the equid locomotor transition.