Evolution of Otolithic Membrane. Structure of Otolithic Membrane in Amphibians and Reptilians

Otolithic membrane of utricles, saccules, and lagena of amphibians (Bufo bufo, Xenopus laevis, Rana temporaria) and reptiles (Teratoscincus scincus, Agama sanguinolenta, Ophisaurus apodus, Caiman crocodilus) were studied using light and scanning electron microscopy. Otolithic membrane in various otolithic organs in all studied animals was found to differ by shape, size, structure, and composition of otoconia. Otolithic membrane of utricle of amphibians and reptiles represents a thin plate of non-uniform structure. Otolithic apparatus in saccule represents a large cobble-stone-like conglomerate of otoconia. Otolithic membrane of lagena looks like a bent plate and is poorly differentiated in amphibians, but well differentiated in reptiles. Thus, transition of vertebrates to the earth surface was accompanied by a fundamental reorganization of otolithic membrane structure. Otolithic membrane containing constantly growing large otolith (in fish) was replaced by a thin structurally differentiated otolithic membrane that ceases its growth at early stages of ontogenesis. However, this replacement did not occur simultaneously in all otolithic organs. The changes initially involved otolithic membrane of utricle. Saccule of amphibians and reptiles has a typical compositional otolith. In the course of further phylogenetic development of tetrapods the process of structural differentiation of otolithic membrane was enhanced and otoliths were completely lost. In parallel, there proceeded a process of replacement of prismatic and spindle-shaped aragonitic otoconia by calcitic barrel-shaped otoconia. The data obtained confirm our hypothesis put forward earlier about two directions of evolution of otolithic membrane.