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Publication

Morphology of the tapir forelimb

Book - Dissertation

Subtitle:anatomy, function and implications for perissodactyl locomotor evolution
The shape of the mammalian forelimb is inexorably tied to the functional demands of the species in question. The versatility of this morphological unit has enabled mammals to meet the locomotor needs involved in swimming, flying, climbing, digging and running. 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). Widely regarded as the posterchild for terrestrial locomotor macroevolution, the so-called ‘equid transition’ charts the changes in limb morphology – most importantly the adaptation and reduction of digits – through the numerous lineages of equids. 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, distal element elongation, and tendonisation of distal muscles, ultimately leading to the evolution of the modern, single-toed (monodactyl) genus Equus. This transition has been studied for over 150 years, and is broadly believed to be a result of an increasingly cursorial lifestyle necessitating an increase in stride length and reduction in rotational inertia. Despite a diverse range of studies investigating the locomotor transition in equids, the timing, mechanisms and driving forces behind the changes are not yet fully resolved. To provide insights into this presumably adaptive locomotor transition from four to one digit, this thesis takes an alternative taxonomic approach by investigating the forelimb functional morphology of a modern family of tetradactyl perissodactyls – the Tapiridae (tapirs). By quantifying the anatomy of tapir forelimb bones and muscles, I am able to gain an understanding of the functional anatomy of a living tetradactyl perissodactyl, which in turn enables informed comparisons between extinct tetradactyl perissodactyls and their modern counterparts. Furthermore, identification of important shifts in osteology and muscular arrangements (with known functional outcomes) can be highlighted during the transition between four and three functional forelimb digits in equids and their kin. Quantifying the functional morphology of the tapir forelimb forms the integral backbone of this thesis. Historically, tapirs have been considered uniform in their bauplan, with some morphological changes in the cranium due to the development of their characteristic proboscis, and in the post-cranium only as an artefact of increases in body size. To offer insights into the evolution of the equid forelimb, it was therefore important to ascertain the morphological variation in the tapir forelimb before any comparisons could be drawn. The thesis was therefore divided roughly into two sections: 1) quantifying the variation in tapir forelimb functional morphology, both in extant and extinct species; and 2) using information gained in the first section, to draw inferences on the functional morphology of the Equidae and their closest relatives (the Palaeotheriidae). To quantify morphology in both sections, I used a three-dimensional geometric morphometric approach based on laser surface scans of the bones of the forelimb. In addition, I also implemented limb long-bone ratios (speed proxies), lever-arm measurements (proxy for mechanical advantage), area ratios of muscle attachments / joint articular surfaces, body mass estimates, phylogenetic comparative methods, and quantified muscular architecture in order to achieve a holistic understanding of tapir forelimb functional anatomy. Initial results of the thesis revealed the interspecific variation present in the forelimb of the tapir genus Tapirus, across both extant and extinct species. I also revealed hitherto unrecognised interspecific variation in the forelimbs of tapirs which suggests: a) differential load application across the four toes during movement, b) a spectrum of muscular application at the shoulder and beneath the foot-pad, c) potential shifts in resting posture between modern tapir species based on their upper forelimb bone shape, and d) the postcranial skeleton of tapirs has undergone morphological changes independent of both body size and phylogenetic relatedness. Investigating the muscular attachments and their resultant action on the skeleton revealed that tapirs share a series of features in common with tetradactyl Eocene equids and palaeotheres (centrally placed scapular spine; unspecialised humerus; extended volar process of the magnum; reduced flexion / increased adduction and abduction of the phalanges), which are rapidly lost in the equid locomotor transition. The shift in shape of these features indicate that early three-toed (tridactyl) equids and palaeotheres interacted with their underfoot substrate in a different manner, with equids exhibiting reduced upper limb stability and reduced potential for digit adduction/abduction early in their shift from four to three functional digits. This is not observed in palaeotheres or tapirs, leading to the conclusion that both these groups were better adapted for locomotion on compliant surfaces (e.g. rainforest floor) than the tridactyl equids. The inclusion of both osteological and muscular quantification of the tetradactyl forelimb of tapirs in this thesis has made it possible, and justifiable, to draw inferences on the evolution of locomotion in equids. Aspects which warrant further study beyond the work presented here include the quantification of shape change in bones defining origination and insertion of muscle groups which have 1) undergone radical reorganisation through the equid transition, and 2) pertain to established outcomes relevant for locomotion. This thesis represents the first quantification of such data in a tetradactyl perissodactyl, and can act as a springboard for further study of locomotor functional morphology across the Perissodactyla.
Number of pages: 354
Publication year:2019
Keywords:Doctoral thesis
Accessibility:Open