The Sarcopterygii or lobe-finned fish —sometimes considered synonymous with Crossopterygii —constitute a clade of the bony fish, though a strict cladistic view includes the terrestrial vertebrates. The living sarcopterygians include six species of lungfish. Early lobe-finned fishes are bony fish with fleshy, paired fins, which are joined to the body by a single bone; the fins of lobe-finned fishes differ from those of all other fish in that each is borne on a fleshy, scaly stalk extending from the body. The scales of sarcopterygians are true scaloids, consisting of lamellar bone surrounded by layers of vascular bone, dentine-like cosmine, external keratin; the morphology of tetrapodomorphs, fish that are similar-looking to tetrapods, give indications of the transition from water to terrestrial life. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs; these fins evolved into the legs of the first tetrapod land vertebrates, amphibians. They possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians.
The braincase of sarcopterygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcopterygians have a symmetrical tail. All sarcopterygians possess teeth covered with true enamel. Most species of lobe-finned fishes are extinct; the largest known lobe-finned fish was Rhizodus hibberti from the Carboniferous period of Scotland which may have exceeded 7 meters in length. Among the two groups of extant species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching 2 m in length and weighing up 110 kg; the largest lungfish is the African lungfish which can weigh up to 50 kg. Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within this group, which in turn consists of all species of four-limbed vertebrates; the fin-limbs of lobe-finned fishes such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. The lobe-finned fishes followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia and the Actinistia.
The classification below follows Benton 2004, uses a synthesis of rank-based Linnaean taxonomy and reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank. Subclass Sarcopterygii †Order Onychodontida Order Actinistia Infraclass Dipnomorpha †Order Porolepiformes Subclass Dipnoi Order Ceratodontiformes Order Lepidosireniformes Infraclass Tetrapodomorpha †Order Rhizodontida Superorder Osteolepidida †Order Osteolepiformes †Family Tristichopteridae †Order Panderichthyida Superclass Tetrapoda The cladogram presented below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project, Mikko's Phylogeny Archive and Swartz 2012. Sarcopterygii incertae sedis †Guiyu oneiros Zhu et al. 2009 †Diabolepis speratus †Langdenia campylognatha Janvier & Phuong, 1999 †Ligulalepis Schultze, 1968 †Meemannia eos Zhu, Yu, Zhao & Jia, 2006 †Psarolepis romeri Yu 1998 sensu Zhu, Yu, Zhao & Jia, 2006 †Megamastax ambylodus Choo, Zhao, Jia, & Zhu, 2014 †Sparalepis tingi Choo,Zhu,Qu,Yu,Jia & Zhaoh, 2017 Paraphyletic Osteolepida incertae sedis: †Bogdanovia orientalis Obrucheva 1955 †Canningius groenlandicus Säve-Söderbergh, 1937 †Chrysolepis †Geiserolepis †Latvius †L. grewingki †L. porosus Jarvik, 1948 †L. obrutus Vorobyeva, 1977 †Lohsania utahensis Vaughn, 1962 †Megadonichthys kurikae Vorobyeva, 1962 †Platyethmoidia antarctica Young, Long & Ritchie, 1992 †Shirolepis ananjevi Vorobeva, 1977 †Sterropterygion brandei Thomson, 1972 †Thaumatolepis edelsteini Obruchev, 1941 †Thysanolepis micans Vorobyeva, 1977 †Vorobjevaia dolonodon Young, Long & Ritchie, 1992 Paraphyletic Elpistostegalia/Panderichthyida incertae sedis †Parapanderichthys stolbovi Vorobyeva, 1992 †Howittichthys warrenae Long & Holland, 2008 †Livoniana multidentata Ahlberg, Luksevic & Mark-Kurik, 2000 Stegocephalia incertae sedis †Antlerpeton clarkii Thomson, Shubin & Poole, 1998 †Austrobrachyops jenseni Colbert & Cosgriff, 1974 †Broilisaurus raniceps Kuhn, 1938 †Densignathus rowei Daeschler, 2000 †Doragnathus woodi Smithson, 1980 †Jakubsonia livnensis Lebedev, 2004 †Limnerpeton dubium Fritsch, 1901 †Limnosceloides Romer, 1952 †L. dunkardensis Romer, 1952 †L. brahycoles Langston, 1966 †Occidens portlocki Clack & Ahlberg, 2004 †Ossinodus puerorum emend Warren & Turner, 2004 †Romeriscus periallus Baird & Carroll, 1968 †Sigournea multidentata Bolt & Lombard, 2006 †Sinostega pani Zhu et al. 2002 †Ymeria denticulata Clack et al. 2012 Lobe-finned fishes and their relatives the ray-finned fishes comprise the superclass of bony fishes characterized by their bony skeleton rather than cartilage.
There are otherwise vast differences in fin and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmoid layers in the scales of sarcopterygians. The earliest fossils of sarcopterygians, found in the uppermost Silurian resembled the
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
Gnathostomata are the jawed vertebrates. The term derives from Greek: γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes have teeth, paired appendages, a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheathes of neurons. Another is an adaptive immune system that uses VJ recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene, it is now assumed that Gnathostomata evolved from ancestors that possessed a pair of both pectoral and pelvic fins. These ancestors, known as antiarchs, were thought to not possess pectoral or pelvic fins until recently. In addition to this, some placoderms were shown to have a third pair of paired appendages, modified to claspers in males and basal plates in females--a pattern not seen in any other vertebrate group.
The Osteostraci are considered the sister taxon of Gnathostomata. It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more – the buccal pump mechanism; the mouth could grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would, with time, become stronger and tougher, being transformed into real jaws. Newer research suggests that a branch of Placoderms was most the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named Entelognathus had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait; the fossil findings of primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived contemporaneously with Entelognathus and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes.
It indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes. Fossils findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making it impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws. Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians or "shark-like fishes", may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils of primitive acanthodians, begin appearing by the early Silurian, become abundant by the start of the Devonian; the group is traditionally a superclass, broken into three top-level groupings: Chondrichthyes, or the cartilaginous fish. Some classification systems have used the term Amphirhina, it is a sister group of the jawless craniates Agnatha.
Tree of Life discussion of Gnathostomata The Gill Arches: Meckel's Cartilage
Amniotes are a clade of tetrapod vertebrates comprising the reptiles and mammals. Amniotes lay their eggs on land or retain the fertilized egg within the mother, are distinguished from the anamniotes, which lay their eggs in water. Older sources 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 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 do. Amniotes include sauropsids, 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, these membranes include the amniotic sac; these embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians. The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago, in the Carboniferous geologic period.
Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could "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 to become the dominant land vertebrates. 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, while the oldest known sauropsid is Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch. Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion and allantois.
Amniotes develop directly into a terrestrial form with a thick stratified epithelium. 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; the unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments. 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 well-suited to water retention. Most mammals do not lay eggs; 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, all known tetrapods were aquatic and fish-like; because the reptiliomorphs were established 20 million years 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 terrestrial.
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. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating. 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 and a calcified shell, were not essential and evolved later.
It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm 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 affect how much the juveniles could grow before they reached adulthood. Fish and amphibian eggs have the embryonic membrane; the amniote egg ev
Embolomeri is an order of tetrapods or stem-tetrapods members of Reptiliomorpha. Embolomeres first evolved from reptile-like amphibians in the Early Carboniferous, they were specialized semiaquatic predators with long bodies for eel-like undulating swimming. Embolomeres are characterized by their vertebral centra, which are formed by two cylindrical segments, the pleurocentrum at the rear and intercentrum at the front; these segments are equal in size. Most other tetrapods have pleurocentra and intercentra which are drastically different in size and shape. Embolomeres were among the earliest large carnivorous tetrapods, with members such as the crocodilian-like Proterogyrinus appearing in the mid-Carboniferous period. However, they declined in diversity during the Permian period, although the most common embolomere known, did live during this time. Embolomeres went extinct shortly after the time of Archeria, but their possible descendant taxa the chroniosuchians survived the Permian-Triassic extinction event that wiped out around 90% of all life on Earth.
The order Embolomeri was first named by Edward Drinker Cope in 1884 during his revision of "batrachian" evolution. Embolomeri was differentiated from several other newly named amphibian orders, such as "Rachitomi", by the presence of intercentra and pleurocentra of the same size and shape, that being large cylinders. At the time, embolomere fossils were uncommon, so Cope could only identify "cricotids" such as Cricotus as possessing embolomerous vertebrae; the genus name "Cricotus" is dubious, as it has been used by Cope to refer to embolomere fossils spanning anywhere between mid-Pennsylvanian deposits of Illinois and the Permian red beds of Texas. Most paleontologists now refer the red bed "Cricotus" specimens to the genus Archeria. Michel Laurin formally defined Embolomeri as "the last common ancestor of Proterogyrinus and Archeria and all of its descendants." This definition excludes Eoherpeton, always considered a close ally of the group. Some authors describe an "embolomere clade" containing not only the taxa included by Laurin, but Eoherpeton and even chroniosuchians and Silvanerpeton.
Like most early tetrapod or stem-tetrapod groups, the phylogenetic position of embolomeres is controversial. For much of the 20th century, they were placed in the group Anthracosauria, a collection of tetrapods which resembled the amphibians biologically but were the ancestors of amniotes rather than modern lissamphibians. Lissamphibians are considered to be descendants of the temnospondyls, an different group of crocodile-like amphibious tetrapods. Anthracosauria is sometimes considered synonymous with Embolomeri, the group's namesake, Anthracosaurus, is an embolomere. However, other authors use the term "Anthracosauria" in reference to a broader group including embolomeres in combination with various other reptile-like amphibians. Many studies conducted since the 1990s have placed the group Lepospondyli as closer to amniotes than embolomeres were. Lepospondyls are a unusual group of tetrapods, with some members similar to lissamphibians and others similar to amniotes. If lepospondyls are both close relatives of amniotes and the ancestors of modern amphibians that means that crown-Tetrapoda is a much more restricted group than assumed.
In this situation, various traditional orders of Tetrapoda such as Embolomeri and Temnospondyli would qualify as stem-tetrapods due to having evolved prior to the split between modern amphibians and amniotes. On the other hand, if temnospondyls are the ancestors of modern amphibians embolomeres would be reptiliomorphs, part of Tetrapoda; however this classification is not stable, as some analyses have found embolomeres to be more basal than temnospondyls. Below is a cladogram from Ruta et al