Early Tetrapods: Experimenting with Form and Function

Abstract

This chapter describes the ear regions of tetrapods from the Paleozoic. In the past couple of decades, understanding of their morphology has been increased by discoveries of fossils from the Late Devonian and early Carboniferous (Mississippian) periods. The primitive condition, as found in several only distantly related taxa, consisted of a bulky stapes with a large footplate and a stout wing-shaped distal portion. In some, the stapes seems to have been a strut supporting the braincase. The hearing capabilities of these taxa were probably poor, confined to low-frequency detection primarily in water but, possibly, also in air. Ichthyostega had a highly modified version probably specialized for aquatic audition. Taxa from the later Carboniferous (Pennsylvanian) and Permian show a great deal of variation, especially among more terrestrially adapted forms. These include the larger stem amniotes, such as seymouriamorphs and diadectomorphs. All these taxa have some kind of a notch or embayment at the back of the skull, which in early forms was probably part of a spiracular mechanism but, in later ones, might have housed a tympanic membrane that closed off an air-filled middle ear cavity. The small recumbirostran microsaurs and the earliest amniotes had no such notch. Microsaur stapes had large footplates and short, stubby shafts. These and the earliest amniotes probably had no middle ear cavity. The earliest amniote stapes were robust, long, and laterally or downwardly projecting. In some cases they contacted the cheek bones or the jaw joint, likely precluding good aerial sound reception.