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

Using histological cross-sections, the chondrocranium anatomy was reconstructed for two developmental stages of Hermann's tortoise (Testudo hermanni). The morphology differs from the chondrocrania of most other turtles by a process above the ectochoanal cartilage with Pelodiscus sinensis being the only other known species with such a structure. The anterior and posterior processes of the tectum synoticum are better developed than in most other turtles and an ascending process of the palatoquadrate is missing, which is otherwise only the case in pleurodiran turtles. The nasal region gets proportionally larger during development. We interpret the enlargement of the nasal capsules as an adaption to increase the surface area of the olfactory epithelium for better perception of volant odors. Elongation of the nasal capsules in trionychids, in contrast, is unlikely to be related to olfaction, while it is ambiguous in the case of Sternotherus odoratus. However, we have to conclude that research on chondrocranium anatomy is still at its beginning and more comprehensive detailed descriptions in relation to other parts of the anatomy are needed before providing broad-scale ecological and phylogenetic interpretations.

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

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.