Systematic revision of the Protostegidae, with a redescription of Calcarichelys gemma Zangerl, 1953

Abstract

ABSTRACT A revised classification is proposed for the Protostegidae. This revision is aided significantly by a redescription of Calcarichelys gemma Zangerl, 1953 based on a nearly complete specimen from the Mooreville Chalk (Campanian) of Alabama. The phylogenetic relationships of “Protostega” eaglefordensis are unresolved but it does not belong in the genus Protostega. Protostega dixie and Protostega potens are junior synonyms of Protostega gigas. Archelon copei is placed in a new genus, Microstega, and is included with Archelon ischyros, Calcarichelys gemma, Chelosphargis advena, and Protostega gigas in the family Protostegidae. The protostegids have the following synapomorphies: absence of the foramen palatinum posterius; a basioccipital with knob-like processes fitting into sockets on the posterior end of the pterygoids; the presence of only one suprapygal; and a T-shaped entoplastron that is not sutured to the other plastral elements. Evidence exists for a larger group containing Desmatochelys, Rhinochelys, Cimochelys, “Protostega” eaglefordensis and the protostegids based on the following synapomorphies: medial contact between palatines; foramen posterius canalis carotici interni located between the basisphenoid and pterygoid; coracoid longer than scapula; and radius bends posteriorly.