On the Occurrence of Calcified Tympanic Membranes in the Mosasaur Platecarpus

has been assumed by authors (Dollo, 1925, for example) that a calcified tympanic membrane is found, among known genera of mosasaurs, only in the European Plioplatecarpus, in which this structure was described by Dollo (1905). Dollo (1925) considered the calcified tympanic membrane to be one of the diagnostic characters of his monogeneric family Plioplatecarpidae. is generally agreed (Dollo, 1905; Abel, 1912; Camp, 1942) that the calcified membrane in Plioplatecarpus was an adaptation to deep water, habits. There are statements in the literature which point out the occurrence of calcified tympanic membranes in other mosasaurian genera. Williston (1897, p. 96) wrote: It may not be amiss to state here that this cavity [the external auditory cavity of the quadrate bone] in the Kansas specimens of the order is frequently filled with thick plates of cartilage, which extend through the auditory slit or foramen and surround the stapedial pit to a greater or less degree. In 1914 (p. 154), Williston noted that The ears, . . . in most if not all mosasaurs, had a thick cartilaginous ear-drum in place of a simple membrane, evidently, as Dollo has shown, for better protection under undue pressure of the water in deep diving. We may safely assume that Williston's statements were based on observation of calcified cartilaginous eardrums. In his Osteology of the Reptiles (1925, p. 272), Williston recognized three adaptive types of mosasaurs: a surface-swimming a deeper-sea type and a diving type. As a characteristic of the type, he cited heavy cartilaginous protections for the ears; he did not cite this structural modification as a characteristic of either of the