 Osteological characters, such as the construction of the mandible and articulations of the cervical vertebrae distinguish the two families.The enigmatic big-headed turtle ("Platysternon megacephalum") was for some time considered a specialized but still very primitive early offshoot of the Emydidae.With the Geoemydidae being split off, though, it is better reinstated as its own family Platysternidae, though it seems very close to the emydid-geoemydid group.