Cladistics is a modern branch of taxonomy which classifies organisms through evolutionary relationships, as reconstructed by phylogenetic analyses. To resolve this potential concern, the apomorphy-based definition is often supported by an equivalent cladistic definition. ĭefining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through convergent evolution. The first vertebrates with limbs and digits evolved in the Devonian, including the Late Devonian-age Ichthyostega and Acanthostega, as well as the trackmakers of the Middle Devonian-age Zachelmie trackways. Limbs and digits are major apomorphies (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to the group. Apomorphy-based definitions Ī majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct digits (fingers and toes), as well as legless vertebrates with limbed ancestors. The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with the earliest members of the group. Amniotes include the tetrapods that further evolved for flight-such as birds from among the dinosaurs, pterosaurs from the archosaurs, and bats from among the mammals. One group of amniotes diverged into the reptiles, which includes lepidosaurs, dinosaurs (which includes birds), crocodilians, turtles, and extinct relatives while another group of amniotes diverged into the mammals and their extinct relatives. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during the Carboniferous period, others as recently as the Cenozoic. Some tetrapods, such as snakes and caecilians, have lost some or all of their limbs through further speciation and evolution some have only concealed vestigial bones as a remnant of the limbs of their distant ancestors. (Some amniotes later evolved internal fertilization, although many aquatic species outside the tetrapod tree had evolved such before the tetrapods appeared, e.g. 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. However, most tetrapod species today are amniotes, most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods early in the Late Carboniferous. 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. The first tetrapods ( stem) or "fishapods" were primarily aquatic. These include the structure of the head for feeding and breathing, limb girdles and digits for locomotion, eyes for seeing, ears for hearing, and the heart and lungs for gas circulation and exchange outside water. Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic ancestors. 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 specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The first crown-tetrapods (last common ancestors of extant tetrapods capable of terrestrial locomotion) appeared by the very early Carboniferous, 350 million years ago. Limbed vertebrates (tetrapods in the broad sense of the word) are first known from Middle Devonian trackways, and body fossils became common near the end of the Late Devonian but these were all aquatic. Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient lobe-finned ( 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. It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mammals). Tetrapods ( / ˈ t ɛ t r ə ˌ p ɒ d z/ from Ancient Greek τετρα- (tetra-) 'four', and πούς (poús) 'foot') are four-limbed vertebrate animals constituting the superclass Tetrapoda ( / t ɛ ˈ t r ɒ p ə d ə/). Japanese pentatonic scale guitar.Four-limbed vertebrates (tetrapods sensu lato) originated in the Eifelian stage of the Middle Devonian Ĭlockwise from top left: Mercurana myristicapaulstris, a shrub frog Dermophis mexicanus, a legless amphibian Equus quagga, a plains zebra Sterna maxima, a tern (seabird) Pseudotrapelus sinaitus, a Sinai agama Tachyglossus aculeatus, a short-beaked echidna
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