Turtles, tortoises and terrapins

Peter A. Meylan
Click on an image to view larger version & data in a new window
Click on an image to view larger version & data in a new window
Olive ridley turtle
taxon links extinct icon extinct icon extinct icon extinct icon extinct icon extinct icon Phylogenetic position of group is uncertain[down<--]Amniota Interpreting the tree
close box

This tree diagram shows the relationships between several groups of organisms.

The root of the current tree connects the organisms featured in this tree to their containing group and the rest of the Tree of Life. The basal branching point in the tree represents the ancestor of the other groups in the tree. This ancestor diversified over time into several descendent subgroups, which are represented as internal nodes and terminal taxa to the right.

example of a tree diagram

You can click on the root to travel down the Tree of Life all the way to the root of all Life, and you can click on the names of descendent subgroups to travel up the Tree of Life all the way to individual species.

For more information on ToL tree formatting, please see Interpreting the Tree or Classification. To learn more about phylogenetic trees, please visit our Phylogenetic Biology pages.

close box

Tree from Gaffney and Meylan (1988)

Containing group: Amniota


Several different scientific names are used for turtles including Chelonia, Chelonii, Testudines, and Testudinata. The earliest fossils are from the beginning of the age of dinosaurs, in the late Triassic. The Testudines reached its greatest diversity by the end of the Cretaceous. Today only 260 species representing 13 families survive. Although turtles are abundant in the tropics, they also are quite diverse in temperate regions and have been recorded in Arctic waters.

Records from Olduvai Gorge indicate that men have eaten turtles for at least 2 million years. We have had a severe impact on turtles, causing the extinction of many forms, especially land tortoises. Today the problem is quite serious with many land tortoises, sea turtles and aquatic forms facing extinction. Loss and degradation of habitat and continued killing of reproductive females on the nesting beach and removal of their eggs are the biggest problems. The future of many of these survivors from the age of dinosaurs will depend on a conscious effort on our part.

Turtles (including tortoises and terrapins) are characterized by a shell that completely encloses both of the limb girdles. The shell is composed of a dorsal carapace of dermal bone that incorporates endochondral contributions from the vertebrae and ribs and a ventral plastron of clavicles and interclavicles anteriorly and abdominal ribs posteriorly. No turtles have teeth on their jaws, and all have the external ear supported by a large, semicircular quadrate.

Click on an image to view larger version & data in a new window
Click on an image to view larger version & data in a new window
turtle skeleton

Figure 1- The turtle shell makes all members of this group immediately identifiable. This specimen has been prepared to show that the shoulder girdle and pelvis are enclosed within the shell, a feature that is unique to turtles. Photograph copyright © E. S. Gaffney.


The monophyly of turtles has never been questioned. The following characters are synamorphies for the group:

Proganochelys is the most primitive turtle (for an illustration see the Amniotes page). Essentially, all other turtles are placed in the Casichelydia.


Much of the early evolution of turtles involved a reduction in the number of bones of the skull.

In addition to the reduction in the number of elements there is also a closing up of the skull which results in a more solid structure.


Most of the diversity of living turtles (10 of 13 families) belong to this group of turtles. All living members can pull their neck inside the shell between the shoulder girdles, hence the name Cryptodira, which means "hidden neck".


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. The Pleurosternidae is known from North America, Europe, and possibly Asia. 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.

Fossil Forms

The major groups of turtles already were in existence by the late Triassic, about 210 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.

Another potential candidate for the closest relatives of turtles identified during the 1970's (Gaffney & Meylan 1988, Gauthier et al. 1988) is the "cotylosaur" family, Captorhinidae. 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. However, "parareptile" groups including procolophonids (Laurin & Reisz 1995) and pareiasaurs (Lee 1997) have subsequently been argued to be closer to the origin of turtles than captorhinids. More recently, an old idea, that the origin of turtles actually lies among the diapsids, has received new support from some morphologists (deBraga & Rieppel 1997). Molecular biologists have also provided data that suggest that turtles are diapsids and now consider that the main question is where among the diapsids turtles have their origin (Hedges & Poling 1999, Cao et al. 2000). 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.

Discussion of Phylogenetic Relationships

Gaffney (1984) provides a complete historical overview of theories of interrelationships among different groups of turtles, from Linnaeus` concept of the single genus Testudo to the more recent views of the last several decades. The starting point for modern discussions of turtle phylogeny is Williams (1950). Although the emphasis of Williams` work was an analysis of variation in the structure of the cervical vertebrae, he provided a complete classification of fossil and living turtles. Since the mid 1970's, Gaffney and coworkers (Gaffney, 1975, 1984, 1996; Gaffney and Meylan, 1988; Gaffney et al., 1991) have challenged much of the phylogenetic arrangement of the major groups proposed by Williams using an extensive morphological data set. 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. (1980) used an immunological distance approach, and Bickham and Carr (1983) analyzed variation in chromosome number and morphology among the families of the Cryptodira.

Shaffer, Meylan, and McKnight (1997) 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, 1988; Gaffney et al., 1991) supports the long-held view that two major living groups (often recognized as suborders), the Pleurodira and Cryptodira, are each monophyletic (see Gaffney (1984) for history). Genetic sequence data (Shaffer et al., 1997) strongly supports this dichotomy.

Within the Cryptodira the Jurassic turtle, Kayentachelys aprix, is considered to be the sister group of all other crytodires (Gaffney et al 1989). The Pleurosternidae (Pleurosternon, Glyptops, Mesochelys) is a group of Cretaceous cryptodires that is most likely the sister group of the Baenidae (Gaffney, 1975, 1996). 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.

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. 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). 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. It acts as a "calcium bank". Calcium and other cations are taken from the carapace and plastron to buffer the blood during hibernation when metabolic acids are likely to build up. In other species, it appears that in reproductively active females, calcium is removed from the shell and incorporated into eggshells forming around follicles in the oviducts.

Other Names for Testudines


Bickham, J.W. and J.L. Carr. 1983. Taxonomy and phylogeny of the higher categories of cryptodiran turtles based on a cladistic analysis of chromosomal data. Copeia 4: 918-932.

Cao, Y., M.D. Sorenson, Y. Kumazawa, D.P. Mindell, and M. Hasegawa. 2000. Phylogenetic position of turtles among amniotes: evidence from mitochondrial and nuclear genes. Gene 259: 139-148.

Chen et al. 1980. Evolutionary relationships of turtles suggested by immunological cross-reactivity of albumins. Comp. Biochem. Physiol. 66B: 421-425.

deBraga M. and O. Rieppel. 1997. Reptile phylogeny and the interrelationships of turtles. Zool. J. Linn. Soc. 120: 281-354.

Ernst, C.H. and R.W. Barbour. 1989. Turtles of the world. Smithsonian Institution Press, Washington D.C.

Ernst, C.H., J.E Lovich, and R.W. Barbour. 1994. Turtles of the United States and Canada. Smithsonian Institution Press, Washington D.C.

Gaffney, E.S. 1972. The systematics of the North American family Baenidae (Reptilia Crytodira). Bull. Amer. Nat. Hist. 147(5):241-320.

Gaffney, E.S. 1975. A phylogeny and classification of the higher categories of turtles. Bull. Amer. Mus. Nat. Hist. 155: 387-436.

Gaffney, E.S. 1976. Cranial morphology of the European Jurassic turtles Portlandemys and Plesiochelys. Bull. Amer. Mus. Nat. Hist. 157(6): 65-376.

Gaffney, E.S. 1977. The side-necked turtle family Chelidae: a theory of relationships using shared derived characters. American Museum Novitates 2620:1-28.

Gaffney, E.S. 1979. Comparative cranial morphology of recent and fossil turtles. Bull. Amer. Mus. Nat. Hist. 164 (2): 65-376.

Gaffney, E.S. 1984. Historical analysis of theories of chelonian relationship. Syst. Zool. 33: 283-301.

Gaffney, E.S. 1990. The comparative osteology of the Triassic turtle Proganochelys. Bull. Amer. Mus. Nat. Hist. 194: 1-263.

Gaffney et al. 1987. Modern turtle origins: The oldest known cryptodire. Science 237: 289-291.

Gaffney et al. 1991. A computer assisted analysis of the relationships of the higher categories of turtles. Cladistics: 313-335.

Gaffney, E.S. 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bull. Am. Mus. Nat. Hist. 229:1-166.

Gaffney, E.S. and P.A. Meylan. 1988. A phylogeny of turtles. pp. 157-219 in M.J. Benton (ed.) The Phylogeny and Classification of the Tetrapods, Vol.1, Clarendon Press, Oxford, 1988.

Gauthier, J., A.G. Kluge, and T. Rowe. 1988. Amniote phylogeny and the importance of fossils. Cladistics 4: 105-209.

Gibbons, J.H. 1990. Life history and ecology of the slider turtle. Smithsonian Institution Press, Washington D.C.

Harless, M. and H. Morelock, 1979. Turtles: Perspective and Research. Wiley, New York.

Heaton, M.J. 1979. Cranial anatomy of primitive captorhinid reptiles from the late Pennsylvanian and Early Permian Oklahoma and Texas. Oklahoma Geol. Surv. Bull. 127: 1-84.

Hedges, S.B. and L.L. Poling. 1999. A molecular phylogeny of reptiles. Science 283: 998-1001.

Iverson, J.B. 1992. A revised checklist with distribution maps of the turtles of the world. Earlham College (privately printed), Richmond IN.

Kunkel, B.W. 1912. The development of the skull of Emys lutaria. J. Morph. 23: 693-780.

Laurin, M. and R.R. Reisz. 1995. A reevaluation of early amniote phylogeny. Zool. J. Linn. Soc. 113: 165-223.

Lee, M.S.Y. 1997. Pareiasaur phylogeny and the origin of turtles. Zool. J. Linn. Soc. 120:197-280.

Meylan, P.A and E.S. Gaffney. 1989. The skeletal morphology of the Cretaceous Crytodiran turtle, Adocus, and the relationships of the trionychoidea. Amer. Mus. Nov. 2941: 1-60.

Pritchard, P.C.H. 1979. Taxonomy, evolution, and zoogeography. In: M. Hareless and H. Morelock (eds) Turtles, perspectives and research. Wiley, New York, p.1-42.

Pritchard, P.C.H. 1979. Encyclopedia of Turtles. T. F H. Publications, Neptune, N.J.

Romer, A.S. 1956. Osteology of the Reptiles. Univ. of Chicago Press.

Ruckes, H. 1929. Studies in chelonian osteology. Part II. The morpoholgical relationships between the girdles, ribs, and carapace. Ann. N.Y. Acad. Sci. 31, 81-120.

Schumacher, G.H. 1973. The head muscles and hyolaryngeal skeleton of turtles and crocodilians. In: C.Gans and T.S.Parsons (eds) Biology of the Reptilia, Vol.4. Academic Press, London, New York, p 101-199.

Shaffer, H.B., P. Meylan, and M.L. McKnight. 1997. Tests of turtle phylogeny: Molecular, morphological, and paleontological approaches. Systematic Biology 46:235-268.

Williams, E.E. 1950. Variation and selection in the cervical central articulations of living turtles. Bull. Amer. Mus. Nat. Hist. 94: 505-562.

Information on the Internet

Title Illustrations
Click on an image to view larger version & data in a new window
Click on an image to view larger version & data in a new window
Olive ridley turtle
Scientific Name Lepidochelys olivacea
Location Nancite Beach, Costa Rica
Comments Olive ridley turtle
Specimen Condition Live Specimen
Copyright © 1996 Greg and Marybeth Dimijian
About This Page

I have relied heavily on the work of my esteemed colleage, Gene Gaffney, in the preparation of these pages. Eric Ganko was instrumental in preparing a preliminary draft.

Peter A. Meylan
Eckerd College NAS, St. Petersburg, Florida, USA

Correspondence regarding this page should be directed to Peter A. Meylan at

All Rights Reserved.

Citing this page:

Meylan, Peter A. 2001. Testudines. Turtles, tortoises and terrapins. Version 31 May 2001. in The Tree of Life Web Project,

edit this page
close box

This page is a Tree of Life Branch Page.

Each ToL branch page provides a synopsis of the characteristics of a group of organisms representing a branch of the Tree of Life. The major distinction between a branch and a leaf of the Tree of Life is that each branch can be further subdivided into descendent branches, that is, subgroups representing distinct genetic lineages.

For a more detailed explanation of the different ToL page types, have a look at the Structure of the Tree of Life page.

close box


Page Content

articles & notes




Explore Other Groups

random page

  go to the Tree of Life home page