The middle ear--morphological types in amphibians and reptiles.

Abstract

The middle ear regions of reptiles and amphibians frequently are grouped into morphological types on the basis of structural resemblances. The arrays of animals resulting from such a grouping have a fair degree of taxonomic continuity. The types in most instances include a “central” pattern, presumed to be primitive, and modifications that are considered to have been derived from the “central” type by evolutionary processes. Some understanding of phylogeny, thus, is a necessary precursor to the formation of the groups. This raises problems when, as is often the case, the groups based on ear structures are used as a basis for phylogenetic interpretations. Among reptiles the theropsid-sauropsid case is the best known. The principal morphological types are described and discussed. The middle ear, as a sound-transmitting apparatus and in its associations with the masticatory apparatus, is highly susceptible to adaptive modifications. Some morphological types, such as those in archosaurs and labyrinthodonts, are quite stable. Others show marked evolutionary diversity. The middle ear structures, hence, appear quite useful as phylogenetic indicators within morphological types but less so when relationships between types are considered.