The cranial musculature of turtles

Ingmar Werneburg

ABSTRACT

Up to this date, no clear common reference system for muscle nomenclature in vertebrates exists due to 1. human medical anatomy dominated traditions, 2. typological, 'box-like' approaches, and 3. simplifications based on the taxonomic and topographical focus of the respective authors. Hence, a large terminological and homologisation confusion in the literature is recognisable, hindering evolutionary and developmental analyses. In this paper, a comprehensive study on the cranial musculature is presented, in which more than 100 references on cranium-associated musculature of turtles were critically reviewed. Following a new traceable approach to muscular terminology, a set of 88 adult 'muscular units' – the smallest parts of macroscopic muscular structures – were identified across turtle species, exemplarily demonstrated in a side-necked turtle. For example, the homology of jaw muscle portions and that of epaxial and hypaxial muscular structures are defined by a comprehensive consideration of criteria such as innervation, spatial characteristics, and ontogeny. Adult muscle arrangement variability among specimens, fusions of muscular units, and drop-like apoptosis are recorded. These phenomena are the result of a fluid pattern formation – first driven by neural crest stream patterning in ontogeny. Considering this fact of ontogeny, a new discussion of the evolutionary history of turtles and of particular cranial structures is possible.

Ingmar Werneburg. RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku, 650-0047 Kobe, Japan
Paläontologisches Institut und Museum der Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland