54 - Anatomical Adaptations to the Carapacial Hinge in Kinixys Tortoises

Poster Board Number: 54
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Co-authors:

M. Scott Echols - Parrish Creek Veterinary Hospital and Diagnostic Center; Madison Berg - Department of Anatomy - Midwestern University; Emily Napoles - College of Veterinary Medicine - Midwestern University; Mia Felix - Midwestern University; Jeremy Thompson - The Kinixys Cooperation

Abstract Body : Throughout turtle evolution, shell hinges have evolved numerous times to provide protection via partial or complete closure of the cranial and/or caudal shell openings. Most shell hinges in fossil and modern turtles have evolved within the plastron (ventral part of shell). The extant tortoise genus Kinixys is the only documented example of a hinge evolving within the carapace (dorsal part of shell). This unique carapacial hinge is understudied, and its impacts on the internal anatomy of Kinixys are poorly understood.

We studied the anatomy of three Kinixys species: an adult male forest hinge-back (Kinixys erosa), an adult male Home’s hinge-back (Kinixys homeana), and an adult female Western hinge-back (Kinixys nogueyi). The K. erosa specimen was perfused with BriteVu® contrast agent. All specimens were CT-scanned, and the resulting scans were analyzed and digitally segmented in Aviso 3D 2022.2. Gross dissection, including comparative anatomical descriptions, morphometric data collection, and photodocumentation, was performed for each specimen.

In Kinixys, lung tissue is amorphous and fills all available space in the dorsal carapace. However, unlike most turtles in which the lung tissue extends caudally into the pelvis, the caudal extent of the lung tissue in Kinixys is constrained posteriorly by the carapacial hinge. Thus, the lungs of Kinixys are craniocaudally shortened compared to other tortoises. In contrast, the trachea and primary bronchi are extremely elongated, taking a tortuous path through the lung tissue, including wrapping around the stomach. The trachea and bronchi are thin and translucent, suggesting a permeability to gas exchange. We interpret the sinuous tracheobronchial tree as an adaptation to the reduced lung volume in Kinixys. The winding course allows the presumably permeable bronchi to maximize contact with the limited lung tissue.

The intestines in K. erosa are reduced compared to other, more herbivorous tortoise species, which likely reflects its more carnivorous diet. Its liver is enlarged, which may serve many functions, including adaptation to carnivory and/or filtering out toxins from food items such as millipedes, which may secrete cyanide when they feel threatened.

Dimorphism between the forelimbs and hind limbs also reflects the position of the carapacial hinge. Posteriorly, the carapace closes to allow complete retraction of the hind limbs safely within the shell. In contrast, the forelimbs are exposed externally and function to close the cranial opening of the shell. As a result, the hind limbs of Kinixys are gracile with thin skin and minimal osteoderms, which will facilitate retraction. Conversely, the forelimbs are robust with thick, prominent osteoderms, providing a protective barrier into the cranial opening of the shell.