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Home / Equine Research Bank / J Anat / Comparative forelimb myology and muscular architecture of a ...
Journal of anatomy2019; 236(1); 85-97; doi: 10.1111/joa.13087

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

Abstract: 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.
© 2019 Anatomical Society.
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Publication Date: 2019-09-13 PubMed ID: 31515803PubMed Central: PMC6904598DOI: 10.1111/joa.13087Google Scholar: Lookup
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  • Comparative Study
  • Journal Article
  • Research Support
  • Non-U.S. Gov't

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research focuses on studying the forelimb structures of the Malayan tapir (a tetradactyl perissodactyl) and comparing it to that of a modern horse (a monodactyl perissodactyl) to understand their musculoskeletal changes and functions that happened through evolution, thereby providing insight into the locomotor transition of equids (horse family).

Study Design and Objectives

  • The study aims to investigate the morphological differences and locomotion features in the musculoskeletal structure of the Malayan tapir and a modern horse.
  • It is intended to provide a quantitative analysis of the muscular architecture of a four-toed tapir, a feature lost in the course of evolution in horses.
  • Such a comparison of musculature will help the researchers understand changes in hard tissues and movements during evolutionary transitions, using modern species as an interpretive guide.

Methodology

  • The researchers dissected each muscle of the tapir forelimb and measured architectural properties like muscle-tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and resting fibre length.
  • They then used these measurements to calculate comparative parameters like physiological cross-sectional area (PCSA), muscle volume, and percentage muscle mass.
  • These results from the Malayan tapir were compared with similar measurements from a modern horse.

Observations and Findings

  • The study found that in the shoulder area, tapirs have dual origination sites on either side of the deflected scapular spine – a feature only recorded in suids among ungulates.
  • There were differences in the relative contribution to limb muscle mass between the tapir and the horse, highlighting forelimb muscles that affect mobility in the lateral and medial digits, which were likely reduced in equids during evolution.
  • Patterns of PCSA across the forelimb were similar between the tapir and the horse, except for the biceps brachii and flexor carpi ulnaris muscles, which were much larger in the horse.

Conclusion and Implications

  • The research provides the first quantitative data of muscle architecture in a functionally tetradactyl perissodactyl.
  • The study’s results highlight muscles essential for maintaining stability in the monodactyl limb (horse) while moving at high speeds, potentially explaining some aspects of equid’s evolutionary transition to monodactyly.
  • The study provides pivotal initial steps towards understanding the locomotor capabilities of modern perissodactyls’ extinct four-toed ancestors and the locomotor transition of equids.

Cite This Article

APA
MacLaren JA, McHorse BK. (2019). Comparative forelimb myology and muscular architecture of a juvenile Malayan tapir (Tapirus indicus). J Anat, 236(1), 85-97. https://doi.org/10.1111/joa.13087

Publication

Journal of anatomy
ISSN: 1469-7580
NlmUniqueID: 0137162
Country: England
Language: English
Volume: 236
Issue: 1
Pages: 85-97

Researcher Affiliations

MacLaren, Jamie A
  • Functional Morphology Lab, Department of Biology, Universiteit Antwerpen, Antwerp, Belgium.
  • Evolution and Diversity Dynamics Lab, Department of Geology, Université Liège, Liège, Belgium.
McHorse, Brianna K
  • Museum of Comparative Zoology, Cambridge, MA, USA.
  • Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
  • Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, Bedford, MA, USA.

MeSH Terms

  • Animals
  • Forelimb / anatomy & histology
  • Horses / anatomy & histology
  • Muscle, Skeletal / anatomy & histology
  • Perissodactyla / anatomy & histology
  • Phylogeny

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Citations

This article has been cited 2 times.
  1. Charles J, Kissane R, Hoehfurtner T, Bates KT. From fibre to function: are we accurately representing muscle architecture and performance?. Biol Rev Camb Philos Soc 2022 Aug;97(4):1640-1676.
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