The periotic of Moeritherium (Mammalia, Proboscidea): homology or homoplasy in the ear region of Tethytheria McKenna, 1975?



The periotic of Moeritherium is described for the first time. A number of potentially autapomorphic features are identified; however, in common with living elephants and seacows the moerithere periotic is shown to lack the fenestra cochleae and aqueductus cochleae typical of therian mammals. Instead, it possesses a secondarily undivided perilymphatic foramen, a feature considered to be a synapomorphy uniting Proboscidea and Sirenia within the Tethytheria. The supposedly more derived fossil proboscidean, Mimidotherium, along with the primitive fossil sirenian Prorastomus, possess the typical therian pattern of openings to the pars cochlearis of the periotic. It is therefore unclear whether the condition in Recent tethytheres is homologous or independently derived. It is argued that the presence in Moeritherium of a suite of periotic characters more derived than those in Mimidotherium undermines the primitive status of moeritheres relative to numidotheres and all other Proboscidea, and therefore weakens the hypothesis of a secondary reversal to a more primitive auditory region in Numidotheriurn. The biological role of auditory specializations in Recent tethytheres is shown to have been very different, and it is argued that this functional disparity extends to the most primitive members of each order. Demonstrable homoplasy in structure and lack of functional congruence in basal members of both Sirenia and Proboscidea are deemed to be indicative of an independent acquisition of similar structures in the ear region of Recent tethytheres.