Ontogeny of the Alligator Cartilago Transiliens and Its Significance for Sauropsid Jaw Muscle Evolution

Daphne Soares,Casey M Holliday,Henry P Tsai

doi:10.1371/journal.pone.0024935

Abstract

The cartilago transiliens is a fibrocartilaginous structure within the jaw muscles of crocodylians. The cartilago transiliens slides between the pterygoid buttress and coronoid region of the lower jaw and connects two muscles historically identified as m. pseudotemporalis superficialis and m. intramandibularis. However, the position of cartilago transiliens, and its anatomical similarities to tendon organs suggest the structure may be a sesamoid linking a single muscle. Incompressible sesamoids often form inside tendons that wrap around bone. However, such structures rarely ossify in reptiles and have thus far received scant attention. We tested the hypothesis that the cartilago transiliens is a sesamoid developed within in one muscle by investigating its structure in an ontogenetic series of Alligator mississippiensis using dissection, 3D imaging, and polarizing and standard light microscopy. In all animals studied, the cartilago transiliens receives collagen fibers and tendon insertions from its two main muscular attachments. However, whereas collagen fibers were continuous within the cartilaginous nodule of younger animals, such continuity decreased in older animals, where the fibrocartilaginous core grew to displace the fibrous region. Whereas several neighboring muscles attached to the fibrous capsule in older individuals, only two muscles had significant contributions to the structure in young animals. Our results indicate that the cartilago transiliens is likely a sesamoid formed within a single muscle (i.e., m. pseudotemporalis superficialis) as it wraps around the pterygoid buttress. This tendon organ is ubiquitous among fossil crocodyliforms indicating it is a relatively ancient, conserved structure associated with the development of the large pterygoid flanges in this clade. Finally, these findings indicate that similar tendon organs exist among potentially homologous muscle groups in birds and turtles, thus impacting inferences of jaw muscle homology and evolution in sauropsids in general.

Highlights

Sesamoids are organized, incompressible structures that often form inside portions of tendons that wrap around bony protuberances
Sesamoid anatomy In young alligators, the cartilago transiliens is situated at the caudolateral edge of the pterygoid buttress (Fig. 2)
Gross anatomical and histological analysis show that the crocodylian cartilago transiliens fits this definition and is not a novel, solitary soft-tissue structure as typically described

Summary

Introduction

Incompressible structures that often form inside portions of tendons that wrap around bony protuberances. Some sesamoids form as plastic responses to externally applied compression (e.g. the mammalian fabella), whereas others, such as the human patella and retrocalcaneal sesamoid fibrocartilage, form during embryonic development, well before the onset of rigorous, externally applied loads [2,3]. These studies indicate that formation of sesamoid structures, whether genetic or epigenetic, is intimately associated with mechanical demand for muscle tendons to withstand compression during life

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