Variation and development of the turtle chondrocranium, with a description of the common musk turtle (Sternotherus odoratus, Kinosternidae, Cryptodira, Testudines)

Abstract

Based on histological cross‐sections, the chondrocranium of the common musk turtle (Sternotherus odoratus) was reconstructed, described, and compared with other turtles. It differs from that of other turtle chondrocrania by possessing elongated, slightly dorsally orientated nasal capsules with three dorsolateral foramina, which might be homologous to the foramen epiphaniale, and by having an enlarged crista parotica. Additionally, the posterior part of the palatoquadrate is more elongated and more slender than in other turtles, while its ascending process is connected to the otic capsule by appositional bone. The proportions of the chondrocranium were also compared with those of “mature” chondrocrania of other turtle species in a Principal Component Analysis (PCA). Other than expected, the S. odoratus chondrocranium is not similar in proportions to those of chelydrids, the closest related species in the sample. The results indicate to differences in the proportions among larger turtle clades (e.g., Durocryptodira, Pleurodira, and Trionychia). S. odoratus is an exception to this pattern since it shows elongated nasal capsules similar to the trionychid Pelodiscus sinensis. A second PCA comparing the chondrocranial proportions of multiple developmental “stages” mostly shows differences between trionychids and all other turtles. S. odoratus is again similar to trionychids along PC1, but its proportions are the most similar along PC2 and PC3 to older “stages” of americhelydians, including the chelydrid Chelydra serpentina, which is related to chondrocranium height and quadrate width. We discuss potential ecological correlations of our findings mirrored in late embryonic stages.

Chondrocranial anatomy of Testudo hermanni (Testudinidae, Testudines) with a comparison to other turtles

“…Both specimens were compared to the pleurodire turtles (Leicht et al, 2023), Emydura subglobosa (Werneburg & Yaryhin, 2018) and Phrynops hilarii (Bona & Alcalde, 2009), and to the following cryptodire turtles: Trionychidae: Apalone spinifera (Sheil, 2003) and Pelodiscus sinensis (Sánchez-Villagra et al, 2009;Tokita et al, 2021); Geoemydidae:…”

“…Mauremys reevesii (Tokita et al, 2021); Kinosternidae: Sternotherus odoratus (Leicht et al, 2023); Chelydridae: Macrochelys temminckii (Sheil, 2005) and Chelydra serpentina (Rieppel, 1976(Rieppel, , 1977Sheil & Greenbaum, 2005); Emydidae: Graptemys pseudogeographica (Danielson & Sheil, 2017), Trachemys scripta (Tulenko & Sheil, 2007), and Emys orbicularis (Kunkel, 1912); Cheloniidae: Chelonia mydas (Tokita et al, 2021), Caretta caretta (Kuratani, 1987(Kuratani, , 1989(Kuratani, , 1999, and Eretmochelys imbricata (Fuchs, 1915;Sheil, 2013); and Testudinidae: A. gigantea (Gerlach, 2012).…”

“…This difference in length could be a result of the progressed developmental stage of the older specimen since the older specimen surpassed the "tempus optimum" (Werneburg et al, 2021). A correlation between the developmental stage and the length of the anterior process was also suspected to be present in C. serpentina and S. odoratus (Leicht et al, 2023;Werneburg et al, 2021).…”

“…Anterior of the optic fenestra is a small gap between the interorbital septum and the wings of the planum supraseptale. The gap is filled with a cellular membrane (cm) similar to that described in Sternotherus odorathus (Leicht et al, 2023). The pila metoptica is connected to the trabeculae through a process on the right side, forming a foramen for the ophthalmic artery.…”

“…pseudogeographica (Danielson & Sheil, 2017;Leicht et al, 2023), which makes an exception for the emydids. A complete taenia marginalis is present in most descriptions of marine turtles (Fuchs, 1915;Kuratani, 1987Kuratani, , 1989Kuratani, , 1999Tokita et al) The presence of a taenia marginalis is unknown in A. gigantea (Gerlach, 2012).…”