|SAUROPSIDA - The Reptiles|
o Amniota (Reptilia grade begins here) |--o SAUROPSIDA = Reptilia when Mesosaurs are placed within Parareptiles (clade) | |?--Mesosauria | |--o Anapsida/Parareptilia | `--o Eureptilia | |--Captorhinidae | `--o=="Protorothyrididae" (grade) | `--Diapsida `--Synapsida
|Linnaean Hierarchy||Local Cladogram|
Reptiliomorpha `--Amniota (Reptile grade begins here) |--Sauropsida (sometimes = "Reptilia") | |--Anapsida/Parareptilia | |?--Testudines | `--Eureptilia | `--Diapsida | `--Sauria | |--Lepidosauria | | |--Sphenodontia | | `--Squamata | `--Archosauria | |--Crocodylia | `--Dinosauria (reptiles) | `--BIRDS `--Synapsida `--Pelycosauria `--Therapsida (reptiles) `--MAMMALS
Introductiontetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane. Today they are represented by four surviving orders:
- Crocodylia (crocodiles, caimans and alligators): 23 species
- Sphenodontia (tuataras from New Zealand): 2 species
- Squamata (lizards, snakes and amphisbaenids ("worm-lizards")): approximately 7,600 species
- Testudines (turtles): approximately 300 species
For more details on the biology, physiology, evolution, and so on of each of these groups, see the specific pages.
The standard 20th Century Classification
Duriong the 20th century, the standard classification of the reptiles was based on the system first proposed by Osborn and popularised in Romer's classic texbook Vertebrate Paleontology. This divides the reptiles into four subclasses according to the positioning of temporal fenestrae, the openings in the sides of the skull behind the eyes for attachment of jaw muscles. Those divisions were:
- Anapsida - no fenestrae - primitive reptiles, and turtles
- Synapsida - one low fenestra (beneath the postorbital and squamosal bones) - the extinct "mammal-like" reptiles
- Euryapsida - one high fenestra (above the postorbital and squamosal) - some extinct marine reptiles (Sauropterygia)
- Paraspida - also high fenestra, but different to the Euryapsida - some extinct marine reptiles (Ichthyosauria)
- Diapsida - two fenestrae - most reptiles
Although the "Parapsida" were later for the most part discarded as a group (the ichthyosaurs being classified as incertae sedis or with the Euryapsida) this schema remained more or less in use until the cladistic revolution.
The Cladistic revolution - reptiles as a paraphyletic gradeclassical standpoint, reptiles included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term.
However, in recent years, many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:
- Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synamorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptila are 'non-avian, non-mammalian amniotes'. (Tudge, p.85)
Some cladists thus redefine Reptilia as a monophyletic group, including both the classic reptiles as well as the birds and perhaps the mammals (depending on ideas about their relationships). Others abandon it as a formal taxon altogether, dividing it into several different classes. However, other biologists believe that the common characters of the standard four orders are more important than the exact relationships, or feel that redefining the Reptilia to include birds and mammals would be a confusing break with tradition. A number of biologists have adopted a compromise system, marking paraphyletic groups with an asterisk, e.g. class Reptilia*. Colin Tudge notes other uses of this compromise system:
- By the same token, the traditional class Amphibia becomes Amphibia*, because some ancient amphibian or other gave rise to all the amniotes; and the phylum Crustacea becomes Crustacea*, because it may have given rise to the insects and myriapods (centipedes and millipedes). If we believe, as some (but not all) zoologists do, that myriapods gave rise to insects, then they should be called Myriapoda*....by this convention Reptilia without an asterisk is synonymous with Amniota, and includes birds and mammals, whereas Reptilia* means non-avian, non-mammalian amniotes. (Tudge, p.85)
Recent college-level references, such as Benton (2004) (taxonomy here), offer another compromise by applying traditional ranks to accepted phylogenetic relationships. In this case, reptiles belong to the class Sauropsida, and mammal-like reptiles to the class Synapsida, with birds and mammals separated into their own traditional classes.
A more cladistic approach uses Reptilia as a monophyletic group and subclade within Sauropsida. However there is still not yet any consensus on phylocode-style definitions of a monophyletic Reptilia (except that it would include the Aves and not include the Synpasida obviously).
The following list includes only tradtional (Linnaean) reptiles, although the indenting is in keeping with the cladistic system:
- Family Captorhinidae (extinct)
- Family Protorothyrididae - Hylonomus (extinct)
- Subclass Anapsida
- Subclass Diapsida
- Superorder Ichthyopterygia - Ichthyosaurs (extinct) (= "Parapsida")
- Infraclass Lepidosauromorpha
- Infraclass Archosauromorpha
- Subclass Synapsida (here, "Mammal-like reptiles")
- Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.
Credits: The main text of this article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Reptile". Copied to Palaeos org and modified with new material MAK061002