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The Selmacryptodira includes all crytpodires other than Kayentchelys aprix from the Jurassic of Arizona. The Paracryptodira includes two, largely Cretaceous families that are now extinct, the Pleurosternidae and Baenidae.

"turtle shell"

The Baenidae is only known from North America. The Paracryptodira includes those cryptodires with a more advanced pattern of blood flow to the head and a simplified plastron. The Centrocryptodira includes those cryptodires with a more advanced condition of the cervical vertebrae.

The major groups of turtles already were in existence by the late Triassic, about million years ago. Thus, the origin of turtles must have occurred before this time. The Permian reptile Eunotosaurus in the past was proposed as the ancestor to turtles. This idea has been based on the shell-like structure covering its body which was made up of broadly expanded ribs.

However, it has been pointed out recently that the shell of turtles is made up of narrow ribs covered by dermal bone, not broadly expanded ribs. The make-up of the shell and the presence of an ectopterygoid bone in the skull suggest that this genus is not close to the origin of turtles. Like all turtles, members of this group lack the ectopterygoid and temporal bones and have a large medial process of the jugal. These lizard-like anapsids show no sign of a shell, and there are still no fossils that show a partly developed turtle shell.


Go to the Discussion of Phylogenetic Relationships on the Amniota page for more information. There are many interesting and important turtles that are only known as fossils. The most important of these is the most primitive turtle, Proganochelys. This turtle shows primitive features absent from modern turtles that make it useful as a benchmark for turtle evolution. Equally as old as Proganochelys is the oldest known sideneck, Proterochersis. It has several of the same features of other sidenecks such as the pelvis fused into the shell.

Its presence in the late Triassic indicates that the Pleurodire-Cryptodire dichotomy see phylogeny and classification below had taken place by this time. The earliest known cryptodire is Kayentachelys from the middle Jurassic of North America. By the late Jurassic marine cryptodires were common in many areas of Europe and Asia. Many of these belong to the extinct family Plesiochelyidae. Two closely related families, the Pleurosternidae and Baenidae, were important groups at the close of the Cretaceous, but both were extinct soon after.

The starting point for modern discussions of turtle phylogeny is Williams Since the mid 's, Gaffney and coworkers Gaffney, , , ; Gaffney and Meylan, ; Gaffney et al. Because it is the most complete analysis to date, we follow their arrangement for the turtle pages of the Tree of Life. In addition to the morphological studies, a few papers have examined higher relationships of turtles using nonmorphological data: Chen et al. Shaffer, Meylan, and McKnight published the first study using genetic sequence data to determine the higher relationships among turtles.

They studied two genes in 23 living genera of turtles and combined their findings with a new morphological data set. This study supports much of the phylogenetic hypothesis of Gaffney and coworkers. The major differences are in the relationships among the living families of cryptodires Polycryptodira and among members of the Chelidae Pleurodira. See the molecular phylogeny page for more details.

The morphological evidence of Gaffney and coworkers especially Gaffney and Meylan, ; Gaffney et al. Genetic sequence data Shaffer et al. Within the Cryptodira the Jurassic turtle, Kayentachelys aprix, is considered to be the sister group of all other crytodires Gaffney et al The Pleurosternidae Pleurosternon, Glyptops, Mesochelys is a group of Cretaceous cryptodires that is most likely the sister group of the Baenidae Gaffney, , The Baenidae is a distinctive family of Cretaceous to Eocene North American turtles in which the dorsal lappet of the prefrontal is small or absent.

The Pleurosternidae and Baenidae form the Paracryptodira which is the sister group of advanced cryptodires. The latter group has been given the name Eucryptodira, which means true cryptodires. All of the "true" cryptodires have the carotid artery hidden within the pterygoid bone. The Plesiochelyidae is an extinct late Jurassic to early Cretaceous radiation of marine turtles.

This is a separate marine radiation from the one to which living sea turtles belong. It constitutes the sister group of all eucryptodires that have formed not amphicoelus cervical vertebrae, a group called the Centrocryptodira.

Life in a Shell

The sister group of all other centrocryptodires is the Meiolaniidae. This is an extinct family Cretaceous- Pleistocene of horned turtles found only in South America, and Australia and adjacent islands. All turtles lay eggs. Most bury their eggs in soil, sand or rotting vegetation, but some lay them on the ground in the open. Turtles do not incubate their eggs or attend them in any way, nor do they exhibit any care of the young.

The eggs are incubated by environmental heat. The young break free of the egg using an egg tooth or caruncle after some 45 to 90 days of development and fend for themselves from hatching. The primitive condition for turtles appears to be to lay large clutches of round eggs. Snapping turtles, sea turtles, and soft-shells lay dozens to hundreds of round eggs in a single clutch. Certain side-necks, mud and musk turtles, land tortoises and many pond turtles lay fewer eggs per clutch, sometimes only one or two, and many of these turtles lay oblong rather than round eggs.

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  8. Many turtles are capable of producing more than one clutch of eggs per year, and sea turtles have been known to produce as many as ten clutches in a single year. It has been determined recently that the sex of most species of turtle is determined by environmental factors, that is to say, sex is not determined genetically but rather by such factors as the temperature of incubation.

    Turtles are herbivorous, carnivorous and omnivorous. The majority of turtles are omnivorous, but many have highly specialized diets. Certain land tortoises and sea turtles are strict herbivores, and one marine species has the capacity to digest cellulose. Other marine species are specialists on jellyfish the leatherback and sponges the hawksbill.

    How the Turtle Got Its Shell

    Turtles of several families specialize on mollusks and have broadly expanded jaws for crushing their prey. Others that specialize on swimming prey have developed a vacuum cleaner approach to feeding snappers, softshells, and some side-necks , using a strong hyoid apparatus to suck prey into their mouths. Turtles belong to the reptilian grade of physiological organization. They are ectothermic and have relatively low metabolic rates. Being ectotherms, their body temperature remains close to the temperature of their environment, and they are entirely reliant on external sources of heat.

    Many turtles bask in the sun to raise their body temperature to a point where bodily functions can operate optimally.


    One species, the leatherback, can maintain a body temperature above that of its environment, but how this is achieved is yet to be determined. Most turtles cannot be active during very hot or very cold periods. Therefore, hibernation in winter and aestivation in summer is common for members of this group.

    Turtles breathe with lungs located inside of a rigid ribcage. They therefore must use a different mechanism for breathing than most vertebrates. Muscles in the region of the leg pockets act to inflate the lungs, muscles on the surface of the lungs dorsally and ventrally deflate them.

    Many turtles augment gas exchange at the lungs with gas exchange in the throat or in the cloaca. In addition to providing protection for the turtle, the shell of at least some species has an important physiological function.