The cervical and caudal vertebrae of the cryptodiran turtle, Meiolania platyceps, from the Pleistocene of Lord Howe Island, Australia. American Museum novitates ; no. 2805

Abstract

"Meiolania platyceps, a cryptodiran turtle from the Pleistocene of Lord Howe Island, Australia, has cervical and caudal vertebrae exhibiting a number of phylogenetically interesting features. The cervicals have fully formed central articulations with a formula of (2( (3( (4) )5) )6) )7) )8), interpreted as a synapomorphy of eucryptodires. Free ribs are present on cervicals two through six, a retained primitive character in Meiolania, lost independently in other eucryptodires, baenids, and Pleurodires. The presence in Meiolania of cervical ribs articulating with paired intercentra shows that the supposed vestigial ribs identified by previous authors in the neck of Recent turtles are probably intercentra and not rib remnants. The neural spines of cervicals seven and eight articulate with the nuchal bone; this is interpreted as an autapomorphy, occurring independently in chelonioids. Complete tails are unknown for Meiolania platyceps, but a conservative reconstruction based on degree of serial variation and comparison with Recent turtles, yields a total of at least ten caudals plus tail club. The caudals are all opisthocoelus and have well-developed haemal spines; characters that are primitive for cryptodires. Meiolania has an ossified tail club consisting of a conical layer of bone surrounding and fused to the terminal caudals. Proganochelys has a similar, but not identical, terminal ossification, and the presence of a tail club is interpreted as a retained primitive feature in Meiolania, being lost independently in pleurodires and other cryptodires. The tail of Meiolania platyceps has a series of ventrally incomplete rings, contrasting with the complete rings found in 'Meiolania' oweni and Niolamia argentina. The vertebral features of Meiolania platyceps are consistent with its hypothesized systematic position as a primitive eucryptodire"--P. [1].