Extinct turtles belong to a gigaorder Proganochelydria. The other gigaorder Casichelydia became dominant in the Jurassic and includes the 2 modern megaorders, Cryptodira and Pleurodira (Ernst, 1994).
The first known fossil turtle, Proganochelys, is known from the Triassic Period. Its shell was about a meter long and it was a terrestrial herbivore. Its teeth were lost and it had a horny beak. There was an extra row of plates in the back of its shell, which are not present in modern turtles. Neck retraction was not possible and bony plates in the skin protected neck. The plastron (underside of the shell) was similar to the plastron of modern turtles but there were many more elements.
By the end of the Jurassic Period, early
representatives of both groups of modern turtles are known. Pleurodires (the more
primitive of the two groups of turtles which cannot retract their necks)
|Kayentachelys is the oldest known cryptodire (the group of turtles which can retract their necks which include all North American turtles).|
wood turtle in the following photo can obviously retract its head inside
Some fossil turtles possessed shells which could reach 2 meters in length, such as Archelon.
|The two most significant features of turtle anatomy is their bony shell and the position of the limbs within the rib cage, rather than outside the rib cage. A turtle’s shell is composed of an upper carapace and a lower plastron.|
|The rib cage and vertebral column help form the shell. There are about 50 bones which compose the carapace and 23 in the plastron. These bones are covered by horny scutes whose margins tend to lie over the sutures between shell bones. Leatherbacks and softshell turtles have lost the horny covering over their shells and have reduced their shell bones. Turtles lack teeth and use a horny beak to bite their food (Ernst, 1994). The following images depict the bones which compose the shell and the position of the limbs inside the shell.|
|Turtles are most vulnerable when they are developing in their eggs and when they are young. Hognose snakes, crows, bears, foxes, mink, skunk, raccoons prey on nests. It is estimated that more than half of all turtle nests are destroyed by predators. Herons, egrets, bitterns, bald eagles, bullfrogs, water snakes, bears, otters, coyotes, and adult snapping turtles can prey on young turtles. The following images are of turtle nests whose eggs have been unearthed by a predator.|
Most turtles determine gender by the incubation
temperature of the eggs. In most
turtle nests, eggs which develop at a lower temperature usually hatch
as males. In some species, the
eggs incubated at the warmest and coolest temperatures are female while
the intermediate temperatures produce males.
Spiny softshells and wood turtles have
a genetic mechanism to determine gender although the sex chromosomes are
the same in males and females (homomorphic). The only four species of turtle which do not rely on temperature to determine the gender of embryos known to date are Trionyx spiniferus, Emydura macquarii, Emydura signata, and Clemmys insculpta (of the families Trionychidae, Chelidae, and Emydidae) (Spotila, 1994; Ernst,
Like other reptiles, turtles depend on their environment to control the temperatures of their bodies and can often be seen basking in the sun.
North American turtles range in the length
of their carapace from under 12 cm (Clemmys muhlenbergii) to almost 2 meters (leatherback
sea turtles, which can weigh up to 916 kg). There are 7 families of cryptodires
Charles Darwin once said “Rarity is the
precursor to extinction.” (Klemens,
2000). The main reason for
the reduction of turtle populations is habitat loss. More than half of the wetland that was present