Jaw musculature during the dawn of turtle evolution

Abstract

Using a new approach to study muscle anatomy in vertebrates, the fully differentiated jaw musculature of 42 turtle species was studied and character mappings were performed. Soft tissue arrangements were correlated to the temporal openings (emarginations) of the skull and the trochlearis system of the jaw apparatus among turtle taxa. When compared to the cranial anatomy of stem Testudines, most characters detected as diagnostic of particular extant groups have to be considered as being evolved first within Testudines. Hence, jaw muscle anatomy of extant turtles is difficult to compare to that of other reptilian taxa. Moreover, the high number of apomorphic character changes speaks for a divergating turtle and saurian morphotype of jaw musculature, which could indicate either a position of turtles outside of Sauria or a highly derived, undetectable origin within that group. In general, a low direct correlation of most soft and hard tissue characters was detected. This finding could imply that both character complexes are more integrated to each other driven by functional morphology; i.e., the composition of muscle fibre types. That condition highlights the difficulty in using gross anatomy of jaw muscle characters to interpret temporal bone arrangements among amniotes in general.

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…Also, the position of the exits of the cranial nerves change only slightly, even with profound changes in the arrangement of related structures such as the eyes and muscles. For example, in P. quenstedti the external jaw adductor musculature innervated by the trigeminal nerve (CN V 3 ) is vertically oriented and entirely positioned anteriorly to the quadrate (Ferreira and Werneburg, in press), while in crown-turtles it extends far posteriorly, following the enlargement of the supraoccipital and squamosal crests (Poglayen-Neuwall, 1953;Werneburg, 2011Werneburg, , 2013. However, the relative position of the exit of CN V remains roughly the same through turtle evolution FIGURE 5 | Two dimensional morphospace plots of brain endocast outlines based on the first three PC axes using a priori defined phylogenetic groups.…”

Comparative analysis of the shape and size of the middle ear cavity of turtles reveals no correlation with habitat ecology

“…The quadrate furthermore forms the medial portions of the middle ear, helps brace the chondrocranium against the dermatocranium, and articulates with the mandible (Gaffney, 1979) and the squamosal serves as the attachment site for numerous muscles pertaining to jaw closure and neck movement (Werneburg, 2011(Werneburg, , 2013(Werneburg, , 2015Jones et al 2012;Ferreira & Werneburg, 2019). It is therefore not surprising that these bones, in particular the squamosal, show an enormous amount of shape diversity across the phylogeny of turtles (Werneburg, 2011(Werneburg, , 2013(Werneburg, , 2015Foth & Joyce, 2016). As the function of the air-filled portion of the middle ear is mostly determined by its size, not its shape (Mason, 2016), we hypothesize that its shape may at least in part be controlled by factors fully unrelated to hearing, such as jaw closure, neck retraction or shape of the temporal region.…”

The Tendinous Framework in the Temporal Skull Region of Turtles and Considerations About Its Morphological Implications in Amniotes: A Review

“…If a muscular structure is reduced, tendons and aponeuroses can turn into ligaments. Also, tendons can flatten and evolutionarily develop into aponeuroses in spite of this plasticity (see examples in Werneburg, 2013).…”

“…All other topologies, with a count of 14, have higher tree lengths. These numbers should not be used to decide for one preferred amniote phylogeny as the characters were only plotted and not used to build the tree (Assis and Rieppel, 2010;Werneburg, 2013). However, the comparison of tree lengths shows that the characters defined herein best fit into a phylogenetic framework with a turtle position outside of Sauria.…”