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) lived in North America in the Cretaceous but the modern survivors of this group are limited to the Southern Hemisphere.  The following images are of a South American side-necked turtle which cannot retract its neck into its shell.

Hydromedusa tectifera

Kayentachelys is the oldest known cryptodire (the group of turtles which can retract their necks which include all North American turtles).  KAYENTACHELYS
The wood turtle in the following photo can obviously retract its head inside its shell.


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.  TURTLES
     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, 1994). 

     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 in North America and the surrounding oceans.  Turtles are the longest lived vertebrates: Box turtles can live more than 100 years and giant tortoises have survived 120-150 years (Ernst, 1994).

      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 in the United States in the 1700s has been lost due to human development.   Many turtles depend on wetlands smaller than those currently protected by law.  Some turtle populations are decreasing because of introduced species such as rats, cattle, cats, and dogs.  In the northeast United States, the foreign invasive species purple loosestrife is degrading the habitat of bog turtles (Klemens, 2000).   Many turtles are taken from the wild for the pet trade.  From 1986 to 1989, 43,000 turtles were imported to England (one quarter of which were turtles from the U.S.).  In the 1970s, the pet trade caused a Moroccan tortoise population to drop 90% (Klemens, 2000).  It is estimated that there are from 2.5 to 15 million pet turtles in the U.S., more than 1/3 of which are taken from the wild.  The U.S. imports more than 30,000 turtles a year.  In 1992, the bog turtle was listed as CITES I and the wood turtle as CITES II (Klemens, 2000).