Tetrapod
A tetrapod 'four' and πούς is any vertebrate animal of the clade Tetrapoda. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids and synapsids. Hox gene mutations have resulted in some tetrapods becoming limbless or two-limbed. Nevertheless, they still qualify as tetrapods through their ancestry, and some retain a pair of vestigial spurs that are remnants of the hindlimbs.
Tetrapods evolved from a group of semiaquatic animals within the tetrapodomorphs which, in turn, evolved from ancient lobe-finned fish around in the Middle Devonian period. Early tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit the body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates are trackways from the Middle Devonian, and body fossils became common near the end of the Late Devonian, around 370–360 million years ago. These Devonian species all belonged to the tetrapod stem group, meaning that they did not belong to any modern tetrapod group.
Limbs evolved prior to terrestrial locomotion, but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods were experimenting with a semiaquatic lifestyle to exploit food and shelter on land. The first crown-tetrapods appeared by the Tournaisian age of the Early Carboniferous.
The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The transition from a body plan for gill-based aquatic respiration and tail-propelled aquatic locomotion to one that enables the animal to survive out of water and move around on land is one of the most profound evolutionary changes known. Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic fish ancestors. These include distinct head and neck structures for feeding and movements, appendicular skeletons for weight bearing and locomotion, more versatile eyes for seeing, middle ears for hearing, and more efficient heart and lungs for oxygen circulation and exchange outside water.
Stem-tetrapods and "fish-a-pods" were primarily aquatic. Modern amphibians are generally semiaquatic; the first stages of their lives are as waterborne eggs and fish-like larvae with gills known as tadpoles, and later undergo metamorphosis to grow limbs and lungs and become partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes, a group of mostly terrestrial tetrapods that evolved early in the Late Carboniferous. The key amniote innovation is the amnion, which enables the eggs to retain their aqueous contents on land. Some tetrapods, such as snakes and caecilians, have lost some or all of their limbs through further speciation and evolution; some only have concealed vestigial bones as a remnant of the limbs of their distant ancestors. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during the Carboniferous period, while others, such as whales, as recently as the Cenozoic.
Amniotes diverged into two branches: one, sauropsids, includes the reptiles: lepidosaurs, archosaurs, turtles, and various other extinct forms. The other branch, synapsids, include mammals and their extinct relatives.
Amniotes include the only animals apart from insects that further evolved powered flight: the extinct pterosaurs and extant birds and bats.
Definitions
The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with the earliest members of the group.Apomorphy-based definitions
A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct digits, as well as legless vertebrates with limbed ancestors. Limbs and digits are major apomorphies which define tetrapods, though they are far from the only skeletal or biological innovations inherent to the group. 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.Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through convergent evolution. To resolve this potential concern, the apomorphy-based definition is often supported by an equivalent cladistic definition. Cladistics is a modern branch of taxonomy which classifies organisms through evolutionary relationships, as reconstructed by phylogenetic analyses. A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered a monophyletic clade, a group with all of its component taxa sharing a single common ancestor. In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates.
Crown group tetrapods
A portion of tetrapod workers, led by French paleontologist Michel Laurin, prefer to restrict the definition of tetrapod to the crown group. A crown group is a subset of a category of animal defined by the most recent common ancestor of living representatives. This cladistic approach defines "tetrapods" as the nearest common ancestor of all living amphibians and all living amniotes, along with all of the descendants of that ancestor. In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a node-based clade, a group with a common ancestry descended from a single "node".Defining tetrapods based on the crown group would exclude many four-limbed vertebrates which would otherwise be defined as tetrapods. Devonian "tetrapods", such as Ichthyostega and Acanthostega, certainly evolved prior to the split between lissamphibians and amniotes, and thus lie outside the crown group. They would instead lie along the stem group, a subset of animals related to, but not within, the crown group. The stem and crown group together are combined into the total group, given the name Tetrapodomorpha, which refers to all animals closer to living tetrapods than to Dipnoi, the next closest group of living animals. Many early tetrapodomorphs are clearly fish in ecology and anatomy, but later tetrapodomorphs are much more similar to tetrapods in many regards, such as the presence of limbs and digits.
Laurin's approach to the definition of tetrapods is rooted in the belief that the term has more relevance for neontologists than paleontologists. In 1998, he re-established the defunct historical term Stegocephali to replace the apomorphy-based definition of tetrapod used by many authors. Other paleontologists use the term stem-tetrapod to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes. The term "fishapod" was popularized after the discovery and 2006 publication of Tiktaalik, an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits.
The two subclades of crown tetrapods are Batrachomorpha and Reptiliomorpha. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes. Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians. Gaffney provided the name Neotetrapoda to the crown group of tetrapods, though few subsequent authors followed this proposal.
The earliest fossils attributed to crown-group tetrapods are footprints from the earliest Carboniferous of Australia, which appear to belong to early amniotes or potentially even sauropsids. Prior to the discovery of these prints, the earliest evidence of crown-group tetrapods were temnospondyl footprints from slightly later in the Tournaisian, with the earliest body fossils being of the temnospondyl Balanerpeton from the Viséan.
Biodiversity
Tetrapoda includes the four traditional living classes: amphibians, reptiles, birds and mammals. Overall, the biodiversity of lissamphibians, as well as of tetrapods generally, has grown exponentially over time; the more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian. However, that diversification process was interrupted at least a few times by major biological crises, such as the Permian–Triassic extinction event, which at least affected amniotes. The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of species and the number of niches that tetrapods have occupied. The first tetrapods were aquatic and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets. The following table shows summary estimates for each tetrapod class from the IUCN Red List of Threatened Species, 2014.3, for the number of extant species that have been described in the literature, as well as the number of threatened species.Classification
The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to caecilians and aquatic mammals. Newer taxonomy is frequently based on cladistics instead, giving a variable number of major "branches" of the tetrapod family tree.As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fails to recognize evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example, the birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. In phylogenetic nomenclature, in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-type of dinosaurs.