Unique arrangement of carapace bones in the South American chelid turtle Hydromedusa maximiliani (Mikan)

The discovery of fossil chelid fragments from the Eocene of South America initiated a study of the shells of Hydromedusa maximiliani (Mikan) 1820, and Hydromedusa tectifera Cope 1869, the only living species of this genus. These species are poorly documented and ill described with no detailed characterization available for consultation. Our investigation revealed that the nuchal bone of H. maximiliani is recessed from the anterior border of the carapace, a condition unique amongst chelonians. A number of previously unrecognized characters important for taxonomy at both the generic and species level are also identified and discussed.