Amniotes (from Greek ἀμνίον amnion, "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός amnos, "lamb"[2]) are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes lay their eggs on land or retain the fertilized egg within the mother, and are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the evolutionary great chain of being.

Amniotes are tetrapods (descendants of four-limbed and backboned animals) that are characterised by having an egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anamniotes (including frogs) typically do. Amniotes include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their ancestors, back to amphibians. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians.[3]

The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago,[4] in the Carboniferous geologic period. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms.

The amniotic egg represents a critical divergence within the vertebrates, one enabling amniotes to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago,[4] while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago).

Temporal range:
(Possible Mississippian record)
A baby tortoise emerges from an amniotic egg.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Haeckel, 1866


Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis, as amphibians do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis.[5] The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the massive size and yolk content of eggs may have evolved to allow the direct development of the embryo to a larger size.

Anatomy of an amiotic egg
Anatomy of an amniotic egg:
  1. Eggshell
  2. Outer membrane
  3. Inner membrane
  4. Chalaza
  5. Exterior albumen (outer thin albumen)
  6. Middle albumen (inner thick albumen)
  7. Vitelline membrane
  8. Nucleus of Pander
  9. Germinal disk (blastoderm)
  10. Yellow yolk
  11. White yolk
  12. Internal albumen
  13. Chalaza
  14. Air cell
  15. Cuticula
Crocodile Egg Diagram
Crocodilian egg diagram:
  1. eggshell
  2. yolk sac
  3. yolk (nutrients)
  4. vessels
  5. amnion
  6. chorion
  7. air space
  8. allantois
  9. albumin (egg white)
  10. amniotic sac
  11. crocodile embryo
  12. amniotic fluid

Adaptions for terrestrial living

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois evolved to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the placenta.

The ancestors of true amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondary aquatic).[6] The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell.[7] Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating.[8] Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion. The amniotic egg formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later.[9] It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4 inch) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.[10]

The egg membranes

Fish and amphibian eggs have only one inner membrane, the embryonic membrane. The amniote egg evolved new internal structures to accommodate gas exchange between the embryo and the atmosphere and to deal with wastes. After these structures developed, further adaption allowed amniotes to lay bigger eggs and to breed in drier habitats. The evolution of larger eggs opened up for larger offspring and consequently larger adults, with those who adapted to a diet of fish and other aquatic animals getting bigger than the terrestrial tetrapods. Further growth for the latter, however, was limited by their position in the terrestrial food-chain, which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators.

Amniote traits

While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof. In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage.[11] There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of temporal fenestrae (openings) behind each eye. In anapsids, the ancestral condition, there are none, in synapsids (mammals and their extinct relatives) there is one, and most diapsids (including birds, crocodilians, squamates, and tuataras), have two. Turtles were traditionally classified as anapsids because they lack fenestrae, but molecular testing firmly places them in the diapsid line of descent - they therefore secondarily lost their fenestrae.

Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs, a sternum in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the ankle.[12]

Definition and classification

Amniota was first formally described by the embryologist Ernst Haeckel in 1866 on the presence of the amnion, hence the name. A problem with this definition is that the trait (apomorphy) in question does not fossilize, and the status of fossil forms has to be inferred from other traits.

Archaeothyris BW
Archaeothyris, one of the most basal synapsids, first appears in the fossil records about 306 million years ago.[13]
Europasaurus holgeri Scene 2
By the Mesozoic, 150 million years ago, sauropsids included the largest animals anywhere. Shown are some late Jurassic dinosaurs and the early bird Archaeopteryx perched on a tree stump.

Traditional classification

Classifications of the amniotes have traditionally recognised three classes based on major traits and physiology:[14][15][16][17]

This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics, as the class Reptilia is paraphyletic—it has given rise to two other classes not included in Reptilia.

Classification into monophyletic taxa

A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton, is presented in simplified form below.[20]

Phylogenetic classification

With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants".[12] As Gauthier makes use of a crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy.[21]


The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995).[22] The cladogram covers the group as defined under Gauthier's definition.


Synapsida (mammals and their extinct relatives)Ruskea rotta


MesosauridaeMesosaurus BW flipped


MillerettidaeMilleretta BW flipped


PareiasauriaScutosaurus BW flipped


ProcolophonoideaRhipaeosaurusDB12 flipped

Testudines (turtles, tortoises, and terrapins)Psammobates geometricus 1872 white background


CaptorhinidaeLabidosaurus flipped


Protorothyrididae Protorothyris

Diapsida (lizards, snakes, crocodiles, birds, etc.)Zoology of Egypt (1898) (Varanus griseus)

The inclusion of Testudines within Parareptilia is unsupported by more recent morphological phylogenetic studies, which placed turtles firmly within diapsids.[23][24] All molecular studies have also strongly upheld the placement of turtles within diapsids. Within diapsids, some place turtles within Archosauria,[25] or, more commonly, as a sister group to extant archosaurs,[26][27][28][29] though an analysis conducted by Lyson et al. (2012) recovered turtles as the sister group of lepidosaurs instead.[30]


  1. ^ a b c d e Jason D. Pardo; Matt Szostakiwskyj; Per E. Ahlberg; Jason S. Anderson (2017). "Hidden morphological diversity among early tetrapods". Nature. 546 (7660): 642–645. doi:10.1038/nature22966. PMID 28636600.
  2. ^ Oxford English Dictionary
  3. ^ Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 105–109. ISBN 978-0-412-73810-4.
  4. ^ a b Benton M.J. and Donoghue P.C.J. 2006. Palaeontological evidence to date the tree of life. Molecular biology and evolution. 24(1): 26–53. [1]
  5. ^ Alexander M. Schreiber; Donald D. Brown (2003). "Tadpole skin dies autonomously in response to thyroid hormone at metamorphosis". Proceedings of the National Academy of Sciences. 100 (4): 1769–1774. doi:10.1073/pnas.252774999. PMC 149908. PMID 12560472.
  6. ^ "the_mid_palaeozoic_biotic_crisis - Ocean and Earth Science, National Oceanography Centre Southampton - University of Southampton".
  7. ^ Stewart J. R. (1997): Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291–326. London: Academic Press.
  8. ^ Cunningham, B.; Huene, E. (Jul–Aug 1938). "Further Studies on Water Absorption by Reptile Eggs". The American Naturalist. 72 (741): 380–385. doi:10.1086/280791. JSTOR 2457547.
  9. ^ Shell Game » American Scientist
  10. ^ Michel Laurin (2004). "The evolution of body size, Cope's rule and the origin of amniotes". Systematic Biology. 53 (4): 594–622. doi:10.1080/10635150490445706. PMID 15371249.
  11. ^ Lombard, R. E. & Bolt, J. R. (1979): Evolution of the tetrapod ear: an analysis and reinterpretation. Biological Journal of the Linnean Society No 11: pp 19–76 Abstract
  12. ^ a b Gauthier, J., Kluge, A.G. and Rowe, T. (1988). "The early evolution of the Amniota." Pp. 103–155 in Benton, M.J. (ed.), The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Oxford: Clarendon Press.
  13. ^ Falcon-Lang H J, Benton M J and Stimson M (2007) "Ecology of early reptiles inferred from Lower Pennsylvanian trackways". Journal of the Geological Society, 164 (6): 1113-1118. doi:10.1016/j.palaeo.2010.06.020
  14. ^ Romer A S and Parsons T S (1985) The Vertebrate Body. (6th ed.) Saunders, Philadelphia.
  15. ^ Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
  16. ^ Hildebrand, M. & G. E. Goslow, Jr. Principal ill. Viola Hildebrand. (2001). Analysis of vertebrate structure. New York: Wiley. p. 429. ISBN 978-0-471-29505-1.CS1 maint: Uses authors parameter (link)
  17. ^ Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York — ISBN 978-0-471-38461-8.
  18. ^ Reeder, Tod W.; Townsend, Ted M.; Mulcahy, Daniel G.; Noonan, Brice P.; Wood, Perry L.; Sites, Jack W.; Wiens, John J. (2015). "Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa". PLOS ONE. 10 (3): e0118199. doi:10.1371/journal.pone.0118199. PMC 4372529. PMID 25803280.
  19. ^ *Hope, S. (2002) The Mesozoic record of Neornithes (modern birds). In: Chiappe, L.M. and Witmer, L.M. (eds.): Mesozoic Birds: Above the Heads of Dinosaurs: 339–388. University of California Press, Berkeley. ISBN 0-520-20094-2
  20. ^ Benton, M.J. (2004). Vertebrate Paleontology. Blackwell Publishers. xii–452. ISBN 978-0-632-05614-9.
  21. ^ Lee, M.S.Y. & Spencer, P.S. (1997): Crown clades, key characters and taxonomic stability: when is an amniote not an amniote? In: Sumida S.S. & Martin K.L.M. (eds.) Amniote Origins: completing the transition to land. Academic Press, pp 61–84. Google books
  22. ^ Laurin, M. and Reisz, R.R. (1995). "A reevaluation of early amniote phylogeny." Zoological Journal of the Linnean Society, 113: 165–223.
  23. ^ Rieppel & DeBraga 1996
  24. ^ Müller 2004
  25. ^ Mannen & Li 1999
  26. ^ Zardoya & Meyer 1998
  27. ^ Iwabe et al. 2004
  28. ^ Roos, Aggarwal & Janke 2007
  29. ^ Katsu et al. 2010
  30. ^ Lyson et al. 2012

An anapsid is an amniote whose skull does not have openings (fenestra) near the temples. Traditionally, the Anapsida are the most primitive subclass of reptiles, the ancestral stock from which Synapsida and Diapsida evolved, making anapsids paraphyletic. It is however doubtful that all anapsids lack temporal fenestra as a primitive trait, and that all the groups traditionally seen as anapsids truly lacked fenestra.


Anolis is a genus of anoles (US: (listen)), lizards in the family Dactyloidae, native to the Americas. With more than 425 species, it represents the world's most species-rich amniote tetrapod genus, although it has been proposed that many of these should be moved to other genera, in which case only about 45 Anolis species remain. Previously, it was classified under the family Polychrotidae that contained all the anoles as well as Polychrus, but recent studies place it under Dactyloidae.


Casea is an extinct genus of pelycosaur synapsids which was about 1.2 metres (4 ft) long from Texas, United States and Aveyron, France. It was slightly smaller than the otherwise very similar Caseoides. Casea was one of the first amniote herbivores, sharing its world with animals such as Dimetrodon and Eryops. It was possibly also aquatic.


Casineria is an extinct genus of tetrapod which lived about 334 million years ago in the Mississippian epoch of the Carboniferous period. Its generic name, Casineria, is a latinization of Cheese Bay, the site near Edinburgh, Scotland where the holotype fossil was found. When originally described in 1999, it was identified as a transitional fossil noted for its mix of basal (amphibian-like) and advanced (reptile-like) characteristics, putting it at or very near the origin of the amniotes, the group containing all mammals, birds, modern reptiles, and other descendants of their reptile-like common ancestor. However, the sole known fossil is lacking key elements such as a skull, making exact analysis difficult. As a result, the classification of Casineria has been more controversial in analyses conducted since 1999. Other proposed affinities include a placement among the lepospondyls, seymouriamorphs, or as a synonym of Caerorhachis, another controversial tetrapod which may have been an early temnospondyl.


Colobomycter is an extinct genus of small parareptile known from the Early Permian of Oklahoma. The genus was first described from fossil remains in 1958, at which time it was believed to represent a synapsid, specifically, a pelycosaur. However, the discovery of new material and reexamination of the holotype led to its reclassification as a member of the Eureptilia. More recent studies indicate that Colobomycter is properly placed within the amniote clade Parareptilia, closely related to the taxon Acleistorhinus. Together, the two taxa form the Family Acleistorhinidae.

No postcranial material is known for Colobomycter, and the skull material referred to the genus (representing four individuals) has all been recovered from a single locality, the Dolese Brothers Quarry, near Richard's Spur, 11 kilometers north of Fort Sill, Comanche County, Oklahoma, dating to the early Kungurian stage. This site comprises a fissure-fill deposit yielding a unique upland fauna. Other taxa recovered from these strata preserves a wide array of tetrapods, including lepospondyl and temnospondyl amphibians, the anthracosaur Seymouria, microsaurs, captorhinomorphs, protorothyridids, and synapsids.

The skull of Colobomycter is considered one of the most enigmatic found in any of the parareptiles primarily due to the presence of greatly enlarged caniniform teeth possessing serrated edges in the premaxilla and, to a lesser extent, the maxilla. The length of the premaxillary fang is greater than half the height of the skull. Modesto et Reisz (2008) note that "The large size of the first premaxillary tooth is [otherwise] unheard of among early reptiles." The taxon also possesses unusual "folding" of the dentine at the bases of its larger marginal teeth, a state known as polyplycodont (a condition also seen to have evolved independently in diadectomorphs, ichthyosaurs, and mosasaurs). Modesto et Reisz (2008, p. 682) speculate that hard-shelled insects and other arthropods may have formed the bulk of its diet, but that Colobomycter could also have fed on vertebrates, including small amphibians and eureptiles. It is notable as the smallest predatory amniote from the Richard's Spur deposits, with a skull measuring a mere 70-80 millimeters in length.


Deltaherpeton is an extinct genus of colosteid from middle Mississippian (late Viséan age) deposits of Delta, Iowa, United States. It was first named by John R. Bolt and R. Eric Lombard in 2010 and the type species is Deltaherpeton hiemstrae.Deltaherpeton can be differentiated from other colosteids due to possessing several unique bones along the midline of the skull, separating paired skull bones which typically contact each other along the midline. These include an internasal, an oval-shaped bone which lies at the intersection of the paired premaxillae and nasal bones at the top of the snout. Internasals are known from several of the earliest four-limbed vertebrates, such as Acanthostega, Ichthyostega, and baphetids. Further back, what seems to be a pair of lozenge-shape bones lie at the intersection of the nasal bones and frontal bones. These bones may be interfrontonasals, which have been found in some eryopoids and microsaurs.In addition, Deltaherpeton has a single postparietal (rather than a pair), which separates the left and right supratemporal and tabular bones at the rear edge of the skull. Lone postparietals are rare among non-amniote tetrapods and tetrapod relatives; only Ichthyostega and diadectomorphs are known to possess them. The discovery of Deltaherpeton prompted a review and re-augmentation of the defining characteristics for the family Colosteidae, though it did not help to clarify the relationship between colosteids and other early tetrapods.Though it is believed that Deltaherpeton is more derived, the species is thought to have shared its environment with other early four-limbed vertebrates such as Whatcheeria and Siguornea, approximately 339.4 to 336 million years ago.


Diadectomorpha are a clade of large reptile-like amphibians that lived in Euramerica during the Carboniferous and Early Permian periods and in Asia during Late Permian (Wuchiapingian), and are very close to the ancestry of the Amniota. They include both large (up to 2 meters long) carnivorous and even larger (to 3 meters) herbivorous forms, some semi-aquatic and others fully terrestrial. The Diadectomorpha seem to have evolved during late Mississippian times, although they only became common after the Carboniferous rainforest collapse and flourished during the Late Pennsylvanian and Early Permian periods.


Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all crocodiles, lizards, snakes, tuatara, turtles, and birds. Although some diapsids have lost either one hole (lizards), or both holes (snakes and turtles), or have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. At least 7,925 species of diapsid reptiles exist in environments around the world today (nearly 18,000 when birds are included).


The egg is the organic vessel containing the zygote in which an embryo develops until it can survive on its own; at which point the animal hatches. An egg results from fertilization of an egg cell. Most arthropods, vertebrates (excluding mammals), and mollusks lay eggs, although some, such as scorpions do not.

Reptile eggs, bird eggs, and monotreme eggs are laid out of water, and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches, i.e. breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering.

The largest recorded egg is from a whale shark, and was 30 cm × 14 cm × 9 cm (11.8 in × 5.5 in × 3.5 in) in size. Whale shark eggs typically hatch within the mother. At 1.5 kg (3.3 lb) and up to 17.8 cm × 14 cm (7.0 in × 5.5 in), the ostrich egg is the largest egg of any living bird, though the extinct elephant bird and some dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg, which weighs half of a gram (around 0.02 oz). Some eggs laid by reptiles and most fish, amphibians, insects and other invertebrates can be even smaller.

Reproductive structures similar to the egg in other kingdoms are termed "spores," or in spermatophytes "seeds," or in gametophytes "egg cells".


In amniote animal embryology, the epiblast (also known as the primitive ectoderm) is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amnionic ectoderm and extraembryonic mesoderm also originate from the epiblast.


The Eupelycosauria originally referred to a suborder of 'pelycosaurs' (Reisz 1987), but has been redefined (Laurin and Reisz 1997) to designate a clade of synapsids that includes most pelycosaurs, as well as all therapsids and mammals. They first appear during the Early Pennsylvanian epoch (i.e.: Archaeothyris, and perhaps an even earlier genus, Protoclepsydrops), and represent just one of the many stages in the acquiring of mammal-like characteristics (Kemp 1982), in contrast to their earlier amniote ancestors. The defining characteristics which separate these animals from the Caseasauria (also pelycosaurs) are based on details of proportion of certain bones of the skull. These include a long, narrow supratemporal bone (in contrast to caseasaurs where this bone is almost as wide as it is long), and a frontal bone with a wider connection to the upper margin of the orbit (Laurin and Reisz 1997).


Gephyrostegus is an extinct genus of gephyrostegid reptiliomorph amphibian. It was a small animal, 22 cm in total length, of generally lizard-like build and presumably habit. It had large eyes and a large number of small, pointed teeth, indicating it was an active insectivorous hunter. The remains have been found in Nýřany, Czech Republic, dating from around 310 million years ago (upper Carboniferous).Originally thought to have been a seymouriamorph, the phylogenetic position is uncertain, and now it belongs to the family Gephyrostegidae, together with the genus Bruktererpeton. Several phylogenetic studies indicate that Gephyrostegus is only distantly related to amniotes, more distantly than diadectomorphs, lepospondyls and seymouriamorphs were.At 22 cm snout-vent length, Gephyrostegus is one of the smallest (if not the smallest) advanced reptiliomorphs found. The type species is Gephyrostegus bohemicus, the type of which is the specimen with its skull and scattered elements of the anterior postcranial skeleton preserved; another known specimen is an articulated postcranial skeleton, lacking only the tail and a few phalanges. Brough and Brough (1967) erected the species Gephyrostegus watsoni for a smaller, possibly juvenile individual, previously considered to be an individual of Diplovertebron punctatum. Carroll (1970, 1972) considered the holotype of Gephyrostegus watsoni to be a skeleton of an immature individual of G. bohemicus. The skeleton of the smaller individual has few of the features characterizing the skeletons of larvae of discosauriscid seymouriamorphs, and its morphology is overall very similar to that of adult specimens. Carroll (1970, 1972) states that while the skeleton of Gephyrostegus shows some adaptations for terrestrial life (e.g. in the specialization of the tarsus), it also retained some traits characteristic to aquatic non-amniote tetrapods, such the large size of the skull and the loose attachment of the vertebral elements, making it overall not as well adapted to the terrestrial environment as amniotes are. According to Carroll, Gephyrostegus probably spent a large proportion of its adult life on land, but it may be assumed that it retained aquatic reproductive habits.Brough and Brough (1967) considered Solenodonsaurus to be a junior synonym of Gephyrostegus, but other authors consider them to be a separate genera. Some specimens classified by Brough and Brough (1967) as individuals of Gephyrostegus bohemicus were subsequently recognized as basal eureptilians; their "specimen I" became a holotype of Brouffia orientalis, while "specimen II" became a holotype of Coelostegus prothales.

Jacques Gauthier

Jacques Armand Gauthier (born June 7, 1948 in New York City) is an American vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology.


Lucianosaurus is an extinct genus of amniote of unknown affinities, known only from teeth. Initially described as a basal ornithischian dinosaur, subsequently reclassified as a member of the clade Archosauriformes of uncertain phylogenetic placement and later, taking into account the similarity of its teeth to the teeth of traversodontid cynodonts such as Dadadon (shared presence of teeth with sub-triangular crowns, enlarged denticles, and thecodont tooth implantation), as an amniote of uncertain affinities (though based on dissimilarities in gross morphology and geographic separation it is still more likely that the taxon is indeed an archosauriform rather than a traversodontid).Fossil remains of Lucianosaurus were first found in Late Triassic strata in Eastern New Mexico, United States. The generic name refers to Luciano Mesa (34.980121°N 104.150048°W / 34.980121; -104.150048) in Guadalupe and Quay counties of New Mexico where the teeth of Lucianosaurus were first uncovered and identified.


Mesosaurus (meaning "middle lizard") is an extinct genus of reptile from the Early Permian of southern Africa and South America. Along with it, the genera Brazilosaurus and Stereosternum, it is a member of the family Mesosauridae and the order Mesosauria. Mesosaurus was long thought to have been one of the first marine reptiles, although new data suggests that at least those of Uruguay inhabited a hypersaline water body, rather than a typical marine environment. In any case, it had many adaptations to a fully aquatic lifestyle. It is usually considered to have been anapsid, although Friedrich von Huene considered it to be a synapsid, and this hypothesis has been revived recently.


Reptiliomorpha is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defined by Michel Laurin (2001) and Vallin and Laurin (2004) as the largest clade that includes Homo sapiens, but not Ascaphus truei (tailed frog).The informal variant of the name, "reptiliomorphs", is also occasionally used to refer to stem-amniotes, i.e. a grade of reptile-like tetrapods that are more closely related to amniotes than they are to lissamphibians, but are not amniotes themselves; the name is used in this meaning e.g. by Ruta, Coates and Quicke (2003). An alternative name, "Anthracosauria", is also commonly used for the group, but is confusingly also used for a more primitive grade of reptiliomorphs (Embolomeri) by Benton.As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word. The two major hypotheses for lissamphibian origins are that they are either descendants of dissorophoid temnospondyls or microsaurian "lepospondyls". If the former (the "temnospondyl hypothesis") is true, then Reptiliomorpha includes all tetrapod groups that are closer to amniotes than to temnospondyls. These include the diadectomorphs, seymouriamorphs, most or all "lepospondyls", gephyrostegids, and possibly the embolomeres and chroniosuchians. In addition, several "anthracosaur" genera of uncertain taxonomic placement would also probably qualify as reptiliomorphs, including Solenodonsaurus, Eldeceeon, Silvanerpeton, and Casineria. However, if lissamphibians originated among the lepospondyls according to the "lepospondyl hypothesis", then Reptiliomorpha refers to groups that are closer to amniotes than to lepospondyls. Few non-amniote groups would count as reptiliomorphs under this definition, although the diadectomorphs are among those that qualify.


Seymouria was a reptile-like tetrapod from the early Permian of North America and Europe (approximately 280 to 270 million years ago). It was small, only 2 ft (60 cm) long. Seymouria was well adapted to life on land, with many reptilian features—so many, in fact, that it was first thought to be a primitive reptile.


Tecovasaurus (te-KOH-va-SAWR-us) is an extinct Late Triassic amniote genus of unknown affinities, known only from teeth. It was initially described as a basal ornithischian dinosaur, subsequently reclassified as a member of the clade Archosauriformes of uncertain phylogenetic placement (Irmis et al. (2007), and later, taking into account the similarity of its teeth to the teeth of traversodontid cynodonts such as Dadadon (shared presence of teeth with sub-triangular crowns, enlarged denticles, and thecodont tooth implantation), as an amniote of uncertain affinities (Kammerer et al., 2012; though "based on dissimilarities in gross morphology and geographic separation" the authors considered it more likely that the taxon is indeed an archosauriform rather than a traversodontid). It is named after the Tecovas Formation, in Texas and Arizona, which yielded the holotype remains.


Westlothiana ("animal from West Lothian") is a genus of reptile-like tetrapod that lived about 338 million years ago during the latest part of the Visean age of the Carboniferous. Members of the genus bore a superficial resemblance to modern-day lizards. The genus is known from a single species, Westlothiana lizziae. The type specimen was discovered in the East Kirkton Limestone at the East Kirkton Quarry, West Lothian, Scotland in 1984. This specimen was nicknamed "Lizzie the lizard" by fossil hunter Stan Wood, and this name was quickly adopted by other paleontologists and the press. When the specimen was formally named in 1990, it was given the specific name "lizziae" in homage to this nickname. However, despite its similar body shape, Westlothiana is not considered a true lizard. Westlothiana's anatomy contained a mixture of both "labyrinthodont" and reptilian features, and was originally regarded as the oldest known reptile or amniote. However, updated studies have shown that this identification is not entirely accurate. Instead of being one of the first amniotes (tetrapods laying hard-shelled eggs, including synapsids, reptiles, and their descendants), Westlothiana was rather a close relative of Amniota. As a result, most paleontologists since the original description place the genus within the group Reptiliomorpha, among other amniote relatives such as diadectomorphs and seymouriamorphs. Later analyses usually place the genus as the earliest diverging member of Lepospondyli, a collection of unusual tetrapods which may be close to amniotes or lissamphibians (modern amphibians like frogs and salamanders), or potentially both at the same time.

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