P3.148 Friday, Jan. 6 Middle Ear Cavity Morphology Across Testudines WILLIS, Katie L*; CARR, Catherine E; Univ of Maryland; Univ of Maryland firstname.lastname@example.org
We hypothesize that the function of the large middle ear cavity in testudines is to act as a resonator for underwater sound. To determine if middle ear cavity size was affected by environment or phylogeny, we measured the middle ear in different species. Using microMRI and microCT scans of over 20 species, which have a wide range of ecologies and include examples from most of the testudine families, we reconstructed and measured the middle ear cavities using Neurolucida (MBF Bioscience). Because the frequencies in question are low, volume matters more than the shape of the cavity in determining best resonance frequency. In all but one species examined, the middle ear cavity was a curved ellipsoid that scaled with head size. The exception to this pattern was the Chelus fimbriatus, which displayed a more hourglass morphology but scaled in the same manner. Unlike the middle ears of lepidosaurs and archosaurs, testudine ears are uncoupled. Middle ear cavities ranged in volume from 0.03 mL to 2.56 mL; head widths from 19-96 mm. These values scaled approximately linearly. By treating the middle ear cavity as a sphere, we calculated their best resonance frequencies underwater as 385-1737 Hz, corresponding to the largest and smallest heads respectively. After separating species by ecology and phylogeny, we found no significant difference (ANOVA p= 0.987 and p=0.943, respectively) in the variation of middle ear cavity volume with head width, suggesting that there has been little modification from the ancestral condition. Given that the cavity resonates underwater within the testudine hearing range, but resonates in air out of range, the middle ear morphology supports the hypothesis of an aquatic origin for testudines.