A three‐dimensional morphometric analysis of upper forelimb morphology in the enigmatic tapir (Perissodactyla: Tapirus) hints at subtle variations in locomotor ecology

Abstract

Forelimb morphology is an indicator for terrestrial locomotor ecology. The limb morphology of the enigmatic tapir (Perissodactyla: Tapirus) has often been compared to that of basal perissodactyls, despite the lack of quantitative studies comparing forelimb variation in modern tapirs. Here, we present a quantitative assessment of tapir upper forelimb osteology using three-dimensional geometric morphometrics to test whether the four modern tapir species are monomorphic in their forelimb skeleton. The shape of the upper forelimb bones across four species (T. indicus; T. bairdii; T. terrestris; T. pinchaque) was investigated. Bones were laser scanned to capture surface morphology and 3D landmark analysis was used to quantify shape. Discriminant function analyses were performed to reveal features which could be used for interspecific discrimination. Overall our results show that the appendicular skeleton contains notable interspecific differences. We demonstrate that upper forelimb bones can be used to discriminate between species (>91% accuracy), with the scapula proving the most diagnostic bone (100% accuracy). Features that most successfully discriminate between the four species include the placement of the cranial angle of the scapula, depth of the humeral condyle, and the caudal deflection of the olecranon. Previous studies comparing the limbs of T. indicus and T. terrestris are corroborated by our quantitative findings. Moreover, the mountain tapir T. pinchaque consistently exhibited the greatest divergence in morphology from the other three species. Despite previous studies describing tapirs as functionally mediportal in their locomotor style, we find osteological evidence suggesting a spectrum of locomotor adaptations in the tapirs. We conclude that modern tapir forelimbs are neither monomorphic nor are tapirs as conserved in their locomotor habits as previously described. J. Morphol. 277:1469-1485, 2016. © 2016 Wiley Periodicals, Inc.

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

“…The supraspinatus and infraspinatus have been shown to be of particular interest for tapir locomotion (MacLaren & Nauwelaerts, ), with the mountain tapir Tapirus pinchaque exhibiting a very large supraspinous fossa, interpreted as an adaptation for shock absorption at the shoulder (MacLaren & Nauwelaerts, ). In this study, the infraspinatus of T. indicus was observed not only to occupy the infraspinous fossa but also to pass over the scapular spine with an accessory head originating from the supraspinous fossa as well.…”

“…). The cranial and postcranial elements have for many years been known to differ from those of neotropical taxa (Earle, ), with recent quantitative analyses demonstrating clear divergences between Malayan and neotropical tapir osteology (MacLaren & Nauwelaerts, , ; Dumbá et al. ).…”

“…tapirs) may be considered more appropriate musculoskeletal analogues than modern, monodactyl horses. Within the four modern tapir species, the Malayan tapir ( Tapirus indicus ) represents the most functionally tetradactyl taxon (Earle, ; Bressou, ; Steiner & Ryder, ; MacLaren & Nauwelaerts, , ). This study draws on the existing tapir literature describing muscular arrangements (Murie, ; Campbell, ; Bressou, ; Pereira, ) to inform a systematic dissection and muscular analysis of a juvenile Malayan tapir.…”

Interspecific variation in the tetradactyl manus of modern tapirs (Perissodactyla:Tapirus) exposed using geometric morphometrics

“…Increased loading and necessity for stability in this comparatively small tapir may be due to a number of factors. Reduced reliance on the digital pad in favour of the toes, as is seen through equid evolution (MacFadden, ; Thomason, 1985), would cause a shift in loading forces to the toes and may account for increased sesamoid facets and necessity for toe stability in T. pinchaque . No quantitative comparisons of toepad size has been reported in T. pinchaque , and so this interpretation remains speculative until further investigation has been undertaken.…”

“…In addition to equids (horses, asses, and zebras) and rhinoceroses, tapirs represent the last members of a formerly highly speciose order of small to very large herbivores (Janis, ; Norman & Ashley, ). The tetradactyl (four‐toed) manus of the modern tapir is a unique feature in extant perissodactyls, with equids and rhinoceroses having reduced their functional digit number to one and three, respectively (MacFadden, ); the earliest ancestors of rhinoceroses, tapirs, and equids also displayed a tetradacyl manus (Holbrook, ). The small, basal members of the Perissodactyla (e.g., Propalaeotherium , Hyracotherium , Heptodon ) are interpreted as forest‐dwelling browsers with a ‘ancestral’ digital condition, bearing three toes on the hind foot (tridactyly), and four on the forefoot (Hellmund, ; Holbrook, ; Radinsky, ; Wood et al, ).…”