Hidden-necked turtles (Cryptodira) are the more diverse of the two modern groups of
turtles, with more than 200 species. The other main group of turtles, the
side-necked turtles, consists of about 76 species.
Hidden-necked turtles are distinguished from side-necked turtles by the way they fold their neck into their shell. Hidden-necked turtles 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, in contrast, 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 also differ from side-necked turtles in the structure of their carapace and plastron. The carapace and plastron of hidden-necked turtles are rounder relative to those of the side-necked turtles, whose carapace and plastron are more oval in shape. Additionally, the bones and scutes take on different arrangements in the two clades of turtles.
Classification:
The hidden-necked turtles are one of the two main groups of
turtles, the other being the side-necked turtles. In general, hidden-necked turtles are larger than side-necked turtles. There are 11 families of hidden-necked turtles some of which include seaturtles, pond turtles, American mud and musk turtles, tortoises and softshell turtles. In total, there are more than 200 species of living hidden-necked turtles.
Evolution:
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.
