A systematic compendium of turtle mandibular anatomy using digital dissections of soft tissue and osteology

Abstract

Turtles are a charismatic reptile group with a peculiar body plan, which most notably includes the shell. Anatomists have often focused descriptive efforts on the shell and other strongly derived body parts, such as the akinetic skull, or the cervical vertebrae. Other parts of turtle osteology, like the girdles, limbs, and mandibles, are documented with less rigor and detail. The mandible is the primary skeletal element involved in food acquisition and initial food processing of turtles, and its features are thus likely linked to feeding ecology. In addition, the mandible of turtles is composed of up to seven bones (sometimes fused to as little as three) and has thus anatomical complexity that may be insightful for systematic purposes and phylogenetic research. Despite apparent complexity and diversity to the mandible of turtles, this anatomical system has not been systematically studied, not even in search of characters that might improve phylogenetic resolution. Here, we describe the mandibular osteology for all major subclades of extant turtles with the help of digitally dissected 3D models derived from high‐resolution computed tomography (μCT) scans of 70 extant species. We provide 31 fully segmented mandibles, as well as 3D models of the mandibular musculature, innervation, and arterial circulation of the cryptodire Dermatemys mawii. We synthesize observed variation into 51 morphological characters, which we optimize onto a molecular phylogeny. This analysis shows some mandibular characters to have high systematic value, whereas others are highly homoplastic and may underlie ecological influences or other factors invoking variation.

The first report of Chelonioidea cf. Ctenochelys from the Late Cretaceous of the Maastrichtian type area

“…First, the absence of an anterior process of the surangular supports a position outside of crown-Chelonioidea. The reduced foramen dentofaciale majus in MAB13689 is more characteristic of Dermochelys, yet the hook development as described in other Ctenochelys specimens (Zangerl, 1953;Matzke, 2007;Karl and Nyhuis, 2012;Gentry, 2017) is modest compared to the prominent spike-like symphyseal hook in Dermochelys (Evers et al, 2022).…”

“…Evers et al, 2019). The recent compendium of extant turtle mandibular anatomy by Evers et al (2022) highlighted several synapomorphies for both Cheloniidae and Dermochelyidae, as well as Chelonioidea as a whole, which may prove useful for distinguishing members of respective stem lineages. However, the mandibular characters present in Ctenochelys do not unambiguously favour any placement.…”

“…In our description, we follow the updated anatomical nomenclature as defined by Evers et al (2022). The ventral side of the dentary remains mostly embedded in the sediment, so that only the dorsal, medial and part of the lateral sides are clearly exposed.…”

“…The width of the intact half of the dentary is 19.7 mm (measured between the apex of the symphyseal ridge and posterolateral extremity), and it has a length of 34.5 mm (measured between the most anterior and posterior parts). No remnant of the foramen dentofaciale majus is visible on either the lateral or mediolateral surface of the dentary, suggesting that it is completely reduced, as in Dermochelys (Evers et al, 2022). The dentaries bear a sharp symphyseal ridge, which ends posteriorly in a triangular elevation.…”

The topography of diet: Orientation patch count predicts diet in turtles

“…A blue overlay indicates the accessory triturating ridge. Scale bar indicates 1 cm been observed in both extant and fossil turtles, including how specific modifications of the triturating surface correlate with dietary specialization (e.g., Ernst & Barbour, 1989;Claude et al, 2004;Joyce & Bell, 2004;Sterli, 2015;Foth et al, 2017;Evers et al, 2022). Some of the studies on this subject have also incorporated geometric morphometric data as part of a broader study on the relationship between cranial shape and ecology in turtles (Ferreira et al, 2015).…”