IMPLICATIONS OF THE MORPHOLOGY OF THE BONY LABYRINTH FOR THE PALEOECOLOGY AND SYSTEMATICS OF TURTLES

I compare the anatomy of the bony labyrinth of extant turtles in an attempt to link inner ear morphology to locomotion/ecology. The anatomy of the inner ear, particularly the semicircular canals (SSCs), often correlates with the locomotor mode of tetrapods. Understanding the correlation between inner ear morphology and locomotion in extant taxa can then be used to help infer the paleoecology of extinct taxa. Little is known about the evolution and anatomical diversity of the bony labyrinth in turtles, a clade that includes fully terrestrial, semi-aquatic, and fully marine clades. The paleoecology of fossil turtles typically is inferred from appendicular and shell morphology, as well as the depositional setting in which the fossils are discovered. I hypothesize that, as in other tetrapod clades, the anatomy of the SSCs is correlated with ecology and locomotion.

I used computed tomography to examine the morphology of the bony labyrinth in 18 species, including examples of most major clades of extant turtles. Trionychids, a group of highly-aquatic freshwater turtles, exhibit dorsoventrally depressed SSCs, with a long anterior SSC. Semi-aquatic mud turtles possess taller SSCs, with similar overall labyrinth morphology to that of snapping turtles. The fully-terrestrial tortoise Agrionemys, a member of Testudinoidea, possesses dorsoventrally tall and roughly symmetrical anterior and posterior SSCs that are more robust than those of aquatic taxa. Other testudinoid turtles exhibit a mosaic of characteristics that are known from fully terrestrial, semi-aquatic, and aquatic taxa from other turtle clades. For example, the semi-aquatic taxon Glyptemys possesses tall, robust SSCs, but also possesses a relatively long anterior SSC, similar to the aquatic trionychids. However, another semi-aquatic taxon, Emys orbicularis, possesses inner ear morphology more similar to that of Agrionemys.

Disparity within inner ear morphology of semi-aquatic taxa may be indicative of a phylogenetic rather than an ecological signal. Relationships within Testudinoidea are contentious; morphology-based studies are often incongruent with molecular analyses. With an increased taxonomic sample of both extinct and extant taxa, the inner ear may yield useful data on the interrelationships of problematic turtle taxa.