Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this.
Reptiles, as most commonly defined, are the animals in the class Reptilia, a paraphyletic grouping comprising all sauropsid amniotes except Aves (birds). Living reptiles comprise turtles, crocodilians, squamates and rhynchocephalians (tuatara). In the traditional Linnaean classification system, birds are considered a separate class to reptiles. However, crocodilians are more closely related to birds than they are to other living reptiles, and so modern cladistic classification systems include birds within Reptilia, redefining the term as a clade. Other cladistic definitions abandon the term reptile altogether in favor of the clade Sauropsida, which refers to all animals more closely related to modern reptiles than to mammals. The study of the traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.
Vertebrates comprise all species of animals within the subphylum Vertebrata, including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. Vertebrates comprise such groups as the following:
Tetrapods are four-limbed animals constituting the superclass Tetrapoda. It includes extant and extinct amphibians, reptiles, and synapsids. Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient sarcopterygian fish around 390 million years ago in the middle Devonian period; their forms were transitional between lobe-finned fishes and the four-limbed tetrapods. The first crown-tetrapods appeared by the very early Carboniferous, 350 million years ago. The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known. The first tetrapods (stem) or "fishapods" were primarily aquatic. Modern amphibians, which evolved from earlier groups, are generally semiaquatic; the first stage of their lives is as fish-like tadpoles, and later stages are partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes, most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods about 340 million years ago. The key innovation in amniotes over amphibians is the amnion, which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water.
Herpetology is the branch of zoology concerned with the study of amphibians and reptiles. Birds, which are cladistically included within Reptilia, are traditionally excluded here; the scientific study of birds is the subject of ornithology.
Amniotes are a clade of tetrapod vertebrates that comprise sauropsids and synapsids. They are distinguished by a membrane (amnion) protecting the embryo and a lack of a larval stage. Because of this trait, amniotes lay eggs on land or retain them within the mother, unlike anamniotes, which typically lay eggs in water. Older sources, particularly before the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the antiquated idea of the evolutionary great chain of being. The term amniote comes from the Greek ἀμνίον amnion, "membrane surrounding the fetus", and earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός amnos, "lamb".
Caecilians are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics of South and Central America, Africa, and southern Asia. Their diet consists of small subterranean creatures such as earthworms.
Caeciliidae is the family of common caecilians. They are found in Central and South America. Like other caecilians, they superficially resemble worms or snakes.
Labyrinthodontia is an extinct amphibian subclass, which constituted some of the dominant animals of late Paleozoic and early Mesozoic eras. The group evolved from lobe-finned fishes in the Devonian and is ancestral to all extant landliving vertebrates. As such it constitutes an evolutionary grade rather than a natural group (clade). The name describes the pattern of infolding of the dentin and enamel of the teeth, which are often the only part of the creatures that fossilize. They are also distinguished by a heavily armoured skull roof, and complex vertebrae, the structure of which were used in older classifications of the group.
Lepospondyli is a diverse taxon of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco, lepospondyls lived from the Early Carboniferous (Mississippian) to the Early Permian and were geographically restricted to what is now Europe and North America. Five major groups of lepospondyls are known: Adelospondyli; Aïstopoda; Lysorophia; Microsauria; and Nectridea. Lepospondyls have a diverse range of body forms and include species with newt-like, eel- or snake-like, and lizard-like forms. Various species were aquatic, semiaquatic, or terrestrial. None were large, and they are assumed to have lived in specialized ecological niches not taken by the more numerous temnospondyl amphibians that coexisted with them in the Paleozoic. Lepospondyli was named in 1888 by Karl Alfred von Zittel, who coined the name to include some tetrapods from the Paleozoic, that shared some specific characteristics in the notochord and teeth. Lepospondyls have sometimes been considered to be either related or ancestral to modern amphibians or to Amniota. It has been suggested that the grouping is polyphyletic, with aïstopods being primitive stem-tetrapods, while microsaurs are primitive reptiles.
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. Laurin and Reisz (2020) defined Pan-Amniota as the largest total clade containing Homo sapiens, but not Pipa pipa, Caecilia tentaculata, and Siren lacertina.
Anthracosauria is an order of extinct reptile-like amphibians that flourished during the Carboniferous and early Permian periods, although precisely which species are included depends on one's definition of the taxon. "Anthracosauria" is sometimes used to refer to all tetrapods more closely related to amniotes such as reptiles, mammals, and birds, than to lissamphibians such as frogs and salamanders. An equivalent term to this definition would be Reptiliomorpha. Anthracosauria has also been used to refer to a smaller group of large, crocodilian-like aquatic tetrapods also known as embolomeres.
Diadectomorpha are a clade of large tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods and in Asia during Late Permian (Wuchiapingian), They have typically been classified as advanced reptiliomorphs close to the ancestry of the Amniota, though some recent research has recovered them as the sister group to Synapsida within Amniota, based on inner ear anatomy. They include both large carnivorous and even larger 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.
Temnospondyli is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few species continued into the Jurassic and Cretaceous periods. Fossils have been found on every continent. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including freshwater, terrestrial, and even coastal marine environments. Their life history is well understood, with fossils known from the larval stage, metamorphosis, and maturity. Most temnospondyls were semiaquatic, although some were almost fully terrestrial, returning to the water only to breed. These temnospondyls were some of the first vertebrates fully adapted to life on land. Although temnospondyls are considered amphibians, many had characteristics, such as scales and armour-like bony plates, that distinguish them from modern amphibians (lissamphibians).
Seymouria is an extinct genus of seymouriamorph from the Early Permian of North America and Europe. Although they were amphibians, Seymouria were well-adapted to life on land, with many reptilian features—so many, in fact, that Seymouria was first thought to be a primitive reptile. It is primarily known from two species, Seymouria baylorensis and Seymouria sanjuanensis. The type species, S. baylorensis, is more robust and specialized, though its fossils have only been found in Texas. On the other hand, Seymouria sanjuanensis is more abundant and widespread. This smaller species is known from multiple well-preserved fossils, including a block of six skeletons found in the Cutler Formation of New Mexico, and a pair of fully grown skeletons from the Tambach Formation of Germany, which were fossilized lying next to each other.
Atretochoana eiselti is a species of caecilian originally known only from two preserved specimens discovered by Sir Graham Hales in the Brazilian rainforest, while on an expedition with Sir Brian Doll in the late 1800s, but rediscovered in 2011 by engineers working on a hydroelectric dam project in Brazil. Until 1998, it was known only from the type specimen in the Naturhistorisches Museum, Vienna. Originally placed in the genus Typhlonectes in 1968, it was reclassified into its own monotypic genus, Atretochoana, in 1996. It was also found to be more closely related to the genus Potamotyphlus than Typholonectes. The species is the largest of the few known lungless tetrapods, and the only known lungless caecilian.
Marine vertebrates are vertebrates that live in marine environments. These are the marine fish and the marine tetrapods. Vertebrates are a subphylum of chordates that have a vertebral column (backbone). The vertebral column provides the central support structure for an internal skeleton. The internal skeleton gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect the spinal cord that lies within the column.
Solenodonsaurus is an extinct genus of reptiliomorphs that lived in what is now Czech Republic, during the Westphalian stage.
Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes. Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms, 6.7 metres (22 ft) for the African giant earthworm, Microchaetus rappi, and 58 metres (190 ft) for the marine nemertean worm, Lineus longissimus. Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land, but instead, live in marine or freshwater environments, or underground by burrowing. In biology, "worm" refers to an obsolete taxon, vermes, used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be paraphyletic. The name stems from the Old English word wyrm. Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slowworm Anguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids, nematodes (roundworms), platyhelminthes (flatworms), marine nemertean worms, marine Chaetognatha, priapulid worms, and insect larvae such as grubs and maggots.
