Morphology of the tapir forelimb: anatomy, function and implications for perissodactyl locomotor evolution
25 April 2019
Campus Drie Eiken, O.04 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Organization / co-organization:
Department of Biology
Sandra Nauwelaerts & Peter Aerts
PhD defence Jamie MacLaren - Faculty of Science, Department of Biology
The shape of the mammalian forelimb is inexorably tied to the functional demands of the species in question. Mammals which utilise their forelimbs for walking and running on the ground possess a multitude of forelimb shapes and postures adapted to achieve the basic functions of gravitational support, directional change, shock absorption, and propulsion. The shape of the forelimb has undergone no greater changes in terrestrial mammals than that of the perissodactyl family Equidae (horses, zebras and asses). The earliest equids were small-bodied, four-toed (tetradactyl) forest-dwellers living approximately 56 million years ago. Through time, equids have adapted their limbs though digit reduction (from four to three to one), distal element elongation, and tendonisation of distal muscles, ultimately leading to the evolution of the modern, single-toed (monodactyl) genus Equus. To provide insights into this presumably adaptive locomotor transition, this thesis takes an alternative approach by investigating the forelimb functional morphology of another extant perissodactyl family – the Tapiridae (tapirs) – which have remained both forest-dwelling and tetradactyl in their forelimb digit condition. By quantifying the anatomy of the tapir forelimb bones and muscles, I am able to gain an understanding of the functional similarities between the forelimbs of early tetradactyl perissodactyls and their modern counterparts. Furthermore, important shifts in osteology and muscular arrangement (with known functional outcomes) can be highlighted during the transition between four and three functional forelimb digits in equids and their kin. To quantify morphology, I used a three-dimensional geometric morphometric approach, based on laser surface scans. In addition, I also utilised linear ratios such as speed proxies, lever-arm measurements; body mass estimates; phylogenetic comparative methods; and muscular architecture quantification, in order to achieve a holistic understanding of tapir forelimb functional anatomy.
This thesis has revealed the interspecific variation present in the forelimb of the crown tapir genus Tapirus, across both extant and extinct species, revealing hitherto unrecognised interspecific variation which suggests: a) differential load application across the four toes during movement, and b) the postcranial skeleton of tapirs exhibits some morphological variation independent of both size and phylogenetic relatedness. I also demonstrate that tapirs share a series of morphological features indicating functional shifts in common with Eocene equids and palaeotheres (centrally placed scapular spine; extended volar process of the magnum; reduced flexion / increased adduction and abduction of the phalanges). These features indicate that early three-toed (tridactyl) equids and palaeotheres interacted with their underfoot substrate in a different manner, with equids exhibiting highly reduced limb stability early in their shift from four to three functional digits.
The inclusion of both osteological and muscular quantification of the tapir forelimb has made it possible, and justifiable, to draw inferences on the evolution of locomotion in equids. This thesis represents the first geometric quantification of such data in a tetradactyl perissodactyl, and can act as a springboard for further study of locomotor functional morphology across the Perissodactyla.