Allometry of sound reception structures and evidence for a mandibular middle ear in non-mammalian synapsids

The origin of sensory structures provides an excellent framework for studying how constraints and selective pressures affect the evolution of complex features. The evolution of the mammalian middle ear from the jaw hinge of non-mammalian synapsids offers a deep-time perspective on sensory evolution but is limited by a poor understanding of early synapsid hearing. This work tests the hypothesis that the size of the reflected lamina of the angular bone in non-mammalian synapsids followed a strict, negative allometric trend that may be expected for a sound receiver. Allometry is first investigated in the pterygoid bone of chameleons, which was co-opted for hearing in some species and represents a possible analog for the synapsid reflected lamina. Results indicate that chameleons with a pterygoid ear exhibit a similar allometric slope, while species without a pterygoid ear have variable slopes, suggesting an optimum allometric pattern in sound receivers. In the reflected lamina, we find reduced variation around the allometric trend in therocephalians and non-bidentalian anomodonts, and evolutionary modeling suggests constraint in these groups. These results are consistent with a mandibular middle ear in non-mammalian synapsids, adding valuable new insights to the hypothesis that selective pressures for hearing ability were present long before the evolution of the mammalian middle ear.