Side-necked turtles are distinguished from hidden-necked turtles by the way they fold their neck into their shell. Side-necked turtles fold their neck and head sideways and tuck it under the edge of the shell closer to the shoulder so that the head and neck bend at an angle relative to the axis of the spine. Hidden-necked turtles, in contrast, retract their neck inwards along the axis of the spine, curving it in an S shape along the spinal plane so that their head moves directly into the shell.
Side-necked turtles also differ from hidden-necked turtles in the structure of their carapace and plastron. The carapace and plastron of side-necked turtles are more oval in shape relative to those of the hidden-necked turtles, whose carapace and plastron are rounder in shape. Additionally, the bones and scutes take on different arrangements in the two clades of turtles.
There are 3 families of side-necked turtles, the Austro-American side-necked turtles, the African side-necked turtles and the American side-necked river turtles (a group that also includes the Madagascan big-headed turtle).
The first turtle-like reptiles appeared during the late Triassic, about 220 million years ago. The earliest known turtle that was equipped with a compete shell is Proganochelys, a creature that roamed the earth during the late Triassic. Another ancient turtle was Odontochelys which had a semi-soft carapace. Hidden-necked turtles arose and diversified during the Jurassic, displacing side-necked turtles from many of their aquatic habitats.