Early development of the mammalian superficial masseter muscle in cynodonts
Date
2004
Authors
Abdala, Fernando
Damiani, Ross
Journal Title
Journal ISSN
Volume Title
Publisher
BERNARD PRICE INSTITUTE FOR PALAEONTOLOGICAL RESEARCH
Abstract
The masseter muscle is a hallmark of the jaw-closing mechanism of modern mammals, acting in concert with other mandibular
adductor muscles to fine-tune oral food processing. The model explaining the origin of this muscle within non-mammalian therapsids
involves the differentiation of a masseter-like muscle from a primitive external adductor, a downward migration of the masseter
insertion on the mandible, and a division and distribution of the muscle to a condition that is similar to that in living mammals. The
presence of a suborbital process of the jugal, which is interpreted as the site of origin for the superficial masseter, has been previously
regarded as the earliest putative morphological evidence used to infer masseteric division. The suborbital process is first recorded in late
Early Triassic (c. 245 Ma) cynognathian cynodonts. Here it is shown that primitive galesaurid cynodonts of earliest Triassic age
(c. 251 Ma) display a distinct angulation of the zygomatic arch below the orbit, indicating the presence of a divided masseter amongst
more basal cynodonts. This alters the timing of masseter muscle evolution by showing that the downward migration and division of the
masseter occurred simultaneously, prior to the evolution of advanced cynodonts (=eucynodonts).