Evolution, Homology, and Development of Tetrapod Limb Muscles

Rui Diogo,Julia L. Molnar

doi:10.3390/d13080393

Abstract

Since the early 1900s, researchers have attempted to unravel the origin and evolution of tetrapod limb muscles using a combination of comparative anatomy, phylogeny, and development. The methods for reconstructing soft tissues in extinct animals have been refined over time as our ability to determine muscle homology and phylogenetic relationships between tetrapods has improved. Since many muscles do not leave osteological correlates, muscle reconstruction in extinct animals is largely based on anatomy and development in extant animals. While muscle anatomy in extant tetrapods is quite conservative, the homologies of certain muscles between taxonomic groups are still uncertain. Comparative developmental studies can help to resolve these controversies, as well as revealing general patterns of muscle morphogenesis across tetrapod groups. We review the methods, results, and controversies in the muscle reconstructions of early members of the amniote, mammalian, and lissamphibian lineages, including recent attempts to reconstruct limb muscles in members of the tetrapod stem group. We also review the contribution of recent comparative developmental studies toward understanding the evolution of tetrapod limb muscles, including morphogenic gradients, the origin of paired fins, and the evolution of morphological complexity. Finally, we discuss the role of broad, comparative myological studies as part of an integrative research program on vertebrate evolutionary biology.

Highlights

The study of muscles has too often been neglected in recent decades, especially in a broad phylogenetic context that spans all major tetrapod groups
One research area that integrates comparative anatomy, evolution, and development is the reconstruction of soft tissues in extinct animals
Limb Muscle Reconstructions in Extinct Tetrapods and Persistent Controversies. This first section reviews the literature on limb muscle reconstructions in fossil tetrapods and their implications for the evolution of tetrapod limb function

Summary

Introduction

The study of muscles has too often been neglected in recent decades, especially in a broad phylogenetic context that spans all major tetrapod groups. One research area that integrates comparative anatomy, evolution, and development is the reconstruction of soft tissues in extinct animals. Recent decades have seen a renewed interest in muscles and in developing new tools and methods to reconstruct them more accurately in fossil taxa [7,8]. These include recent studies of animals close to the fin-limb and water-land transitions (e.g., [9,10,11,12]), which employed a modern phylogenetic framework and understanding of the developmental relationship between muscles and bones. Developed embryological approaches and techniques hold great promise for advancing our understanding of the evolution of tetrapod muscle anatomy and musculoskeletal function

Limb Muscle Reconstructions in Extinct Tetrapods and Persistent Controversies

Evolution of Reconstruction Methodology

Stem Amniota: “Bridging the Gap”

Stem Lissamphibia

Stem Reptilia

Stem Tetrapoda

Controversies in Tetrapod Muscle Reconstruction

Evolution of the Deltoid Group

Evolution

A Was subcoracoscapularis consistentlyPresent is present in “lizards”

Was a Teres Major Present Ancestrally in Amniotes?

How many Radial Extensors Were Present Ancestrally in Tetrapods?