AN X-RADIOGRAPHIC AND SEM STUDY OF THE OSSEOUS INNER-EAR OF MULTITUBERCULATES AND MONOTREMES (MAMMALIA) - IMPLICATIONS FOR MAMMALIAN PHYLOGENY AND EVOLUTION OF HEARING

Multituberculate petrosals with well-presented, three-dimensional inte rnal anatomy from the Late Cretaceous/early Paleocene Bug Creek Anthil ls, Montana, U.S.A., are described from X-radiographic and SEM images, as well as from conventional visual observations, and are compared wi th the anatomy of the osseous inner ear in monotremes and in primitive nontherian and therian mammals. Results of this study indicate that: (1) the cochlea of at least some multituberculates retained a lagena, previously known only in monotremes among mammals; (2) an enlarged ves tibule evolved in several lineages of multituberculates independently, and hence is not a synapomorphy of the order; (3) the cochlear canal lacks osseous laminae in support of the short, wide basilar membrane, which was probably inefficient in responding to high-frequency airborn e vibrations; and (4) consequently, bone-conducted bearing in some mul tituberculate species may have been important in interpretation of the ir surroundings. Comparisons with the inner ear of monotremes and prim itive therians indicate that curvature of the cochlea and cribriform p lates for passage of vestibulocochlear nerve branches through the petr osal are unlikely homologues between monotremes and therians. From non -therian to therian mammals, there is a distinct morphological gap in the inner ear transition, characterized by acquisition of a number of neomorphs in the therian inner ear; an intermediate stage has yet to b e discovered. (C) 1997 The Linnean Society of London.