The Function of the Reflected Lamina in Therapsids and the Origin of the Mammalian Middle Ear

Abstract

The selective pressures that drove the remarkable transition of the postdentary and jaw hinge bones of non-mammalian synapsids into the mammalian middle ear are uncertain because the hearing capability of early synapsids is poorly understood. This work investigates the hypothesis that non-mammalian therapsids used their reflected lamina, a plate of bone on the mandible, for sound reception. A systematic survey of reflected lamina anatomy is conducted to assess evolutionary patterns across Therapsida, and a consistent arrangement of ridges and fossae on the angular surface is observed, suggesting a shared ancestral pattern for therapsids. The pterygoid of chameleons is chosen as a modern analogue for a sound-receptive therapsid reflected lamina, and the distribution of pterygoid-based hearing across Chamaeleonidae is assessed via dissections. Finally, the allometry of the pterygoid in chameleons is examined, as the scaling of a sound receptor is likely important for its function. Chameleons with a pterygoid ear show signs of stabilizing selection around a specific allometric coefficient compared to chameleons without this ear. Similarly constricted allometry is also found in the reflected lamina of therocephalian and basal anomodont therapsids, suggesting that selective pressures consistent with sound reception were present long before the origin of the mammalian ear.