HOME

TheInfoList



OR:

"Labyrinthodontia" ( Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
and early Mesozoic eras (about 390 to 150 million years ago). Traditionally considered a subclass of the class Amphibia, modern classification systems recognize that labyrinthodonts are not a formal natural group ( clade) exclusive of other tetrapods. Instead, they consistute an evolutionary grade (a
paraphyletic group In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
), ancestral to living tetrapods such as lissamphibians (modern amphibians) and amniotes ( reptiles, mammals, and kin). "Labyrinthodont"-grade vertebrates evolved from lobe-finned fishes in the Devonian, though a formal boundary between fish and amphibian is difficult to define at this point in time. "Labyrinthodont" generally refers to extinct four-limbed tetrapods with a large body size and a crocodile-like lifestyle. 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
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
ize. They are also distinguished by a broad, strongly-built skull roof composed of many small heavily-textured skull bones. "Labyrinthodonts" generally have complex multi-part vertebrae, and several classification schemes have utilized vertebrae to define subgroups. Because labyrinthodonts do not form a monophyletic group, many modern researchers have abandoned the term. However, some have continued to use the group in their classifications, at least informally, pending more detailed study of their relationships. Many authors prefer to simply use the term tetrapod, while others have re-defined the previously obsolete term
Stegocephalia Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...
("roof heads") as a cladistic alternative to "Labyrinthodontia" or "Tetrapoda".


Labyrinthodont traits

The labyrinthodonts flourished for more than 200 million years. Particularly the early forms exhibited a lot of variation, yet there are still a few basic anatomical traits that make their fossils very distinct and easily recognizable in the field: * Strongly folded tooth surface, involving infolding of the dentin and enamel of the teeth. The cross section resembles a classical
labyrinth In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by ...
(or maze), hence the name of the group. * Massive skull roof, with openings only for the nostrils, eyes and a parietal eye, similar to the structure of the "
anapsid An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolv ...
" reptiles. With the exception of the later more reptile-like forms, the skull was rather flat and strongly ornamented with presumably tough dermal covering, accounting for an older term for the group: "Stegocephalia". *
Otic notch Otic notches are invaginations in the posterior margin of the skull roof, one behind each orbit. Otic notches are one of the features lost in the evolution of amniotes from their tetrapod ancestors. The notches have been interpreted as part of an ...
behind each eye at the back edge of the skull. In earlier fully aquatic forms such as '' Ichthyostega'', it may have formed an open spiracle. In later terrestrial forms such as '' Seymouria'', it may possibly have held a tympanic membrane (eardrum). Laurin M. (1998): The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I-systematics, middle ear evolution, and jaw suspension. ''Annales des Sciences Naturelles, Zoologie, Paris'', 13e Série 19: pp 1–42. * Complex vertebrae made of multiple components: an intercentrum (wedge-shaped front lower piece), two pleurocentra (upper rear piece), and a
vertebral arch The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
/spine (upper projection). The relative development and shape of the elements is highly variable.


The labyrinthodonts in life


General build

Labyrinthodonts were generally amphibian-like in build. They were short-legged and mostly large headed, with moderately short to long tails. Many groups, and all the early forms, were large animals. Primitive members of all labyrinthodont groups were probably true water predators, and various degrees of amphibious, semi-aquatic and semi terrestrial modes of living arose independently in different groups.Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. ''Indiana University Press'', Bloomington, Indiana. 369 pp Some lineages remained waterbound or became secondarily fully aquatic with reduced limbs and elongated, eel-like bodies.


Skull

With the exception of the snake-like aïstopods, the skulls of labyrinthodonts were massive. The broad head and short neck may have been a result of respiratory constraints. Their jaws were lined with small, sharp, conical teeth and the roof of the mouth bore larger tusk-like teeth. The teeth were replaced in waves that traveled from the front of the jaw to the back in such a way that every other tooth was mature, and the ones in between were young. All teeth were labyrinthodont. The sole exception were the chisel-like teeth of some of the advanced herbivorous diadectomorphs. The skull had prominent otic notches behind each eye and a parietal eye.


Post-cranial skeleton

The vertebrae were complex and not particularly strong, consisting of numerous, often poorly ossified elements. The long bones of the limbs were short and broad and the ankle had limited mobility and the toes lacked claws, limiting the amount of traction the feet could produce. This would have made most labyrinthodonts slow and clumsy on land. In adulthood, most of the larger species were likely confined to water. Some late Paleozoic groups, particularly microsaurs and seymouriamorphs, were small to medium-sized and appear to have been competent terrestrial animals. The advanced diadectomorphs from the
Late Carboniferous Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effe ...
and Early Permian were fully terrestrial with stout skeletons, and were the heaviest land animals of their time. The Mesozoic labyrinthodonts were primarily aquatic with increasingly cartilaginous skeleton.White, T. & Kazlev, M. A. (2006)
Temnospondyli: Overview
from Palaeos website


Sensory apparatus

The eyes of most labyrinthodonts were situated at the top of the skull, offering good vision upwards, but very little lateral vision. The parietal eye was prominent, although there is uncertainty as to whether it was a true image producing organ or one that could only register light and dark, like that of the modern tuatara. Most labyrinthodonts had special sense organs in the skin, forming a lateral line organ for perception of water flow and pressure, like those found in
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
and a number of modern amphibians. This would enable them to pick up the vibration of their prey and other waterborne sounds while hunting in murky, weed filled waters. Early labyrinthodont groups had massive stapes, likely primarily anchoring the brain case to the skull roof. It is a question of some doubt whether early terrestrial labyrinthodonts had the stapes connected to a tympanum covering their otic notch, and if they had an aerial sense of hearing at all. The tympanum in anurans and amniotes appear to have evolved separately, indicating most, if not all, labyrinthodonts were unable to pick up airborne sound.


Respiration

The early labyrinthodonts possessed well developed internal gills as well as primitive lungs, derived from the swim bladders of their ancestors. They could breathe air, which would have been a great advantage for residents of warm shoals with low oxygen levels in the water. There was no diaphragma and the ribs in many forms were too short or spaced too closely to aid in expanding the lungs. Likely air was inflated into the lungs by contractions of a throat sac against the skull floor like in modern amphibians, which may be the reason for the retention of the very flat skull in later forms. Exhalation with the aid of the ribs probably evolved only in the line leading to amniotes. Many aquatic forms retained their larval gills in adulthood. With the high atmospheric oxygen and carbon dioxide pressure, particularly during the Carboniferous, the primitive throat sac breathing would have been sufficient for obtaining oxygen even for the large forms. Getting rid of carbon dioxide would present a greater problem on land, and the larger labyrinthodonts probably combined a high tolerance for blood carbonic acid with returning to the water to dissipate the carbon dioxide through the skin. The loss of the armour of rhomboid
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...
of their piscine ancestors allowed for this as well as additional respiration through the skin as in modern amphibians.


Hunting and feeding

Like their sarcopterygian ancestors, the labyrinthodonts were carnivorous. The rather broad, flat skulls and hence short jaw muscle would however not allow them to open their mouth to any great extent. Likely the majority of them would employ a sit-and-wait strategy, similar to that of many modern amphibians. When suitable prey swam or walked within reach, the jaw would slam shut, the palatine tusks stabbing the hapless victim. The strain put on the teeth by this mode of feeding may have been the reason for the reinforcing labyrinthodont enamel typifying the group. Swallowing was done by tipping the head back, as seen in many modern amphibians and in crocodiles. Evolution of a deeper skull, better jaw control and a reduction of the palatine tusks is only seen in the more advanced reptile-like forms, possibly in connection with the evolution of more effective breathing, allowing for a more refined hunting style.


Reproduction

The labyrinthodonts had an amphibious reproduction — they laid eggs in water, where they would hatch to tadpoles. They would remain in water throughout the larval stage until metamorphosis. Only the metamorphosed individuals would eventually venture onto land on occasion. Fossil tadpoles from several species are known, as are
neotenic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...
adults with feathery
external gills External gills are the gills of an animal, most typically an amphibian, that are exposed to the environment, rather than set inside the pharynx and covered by gill slits, as they are in most fishes. Instead, the respiratory organs are set on a fril ...
similar to those found in modern lissamphibian tadpoles and in the fry of
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
and bichirs. The existence of a larval stage as the primitive condition in all groups of labyrinthodonts can be fairly safely assumed, in that tadpoles of ''
Discosauriscus ''Discosauriscus'' was a small seymouriamorph which lived in what is now Central and Western Europe in the Early Permian Period. Its best fossils have been found in the Broumov and Bačov Formations of Boskovice Furrow, in the Czech Republic. ...
'', a close relative of the amniotes, are known. Špinar, Z. V. (1952): Revision of some Morovian Discosauriscidae. ''Rozpravy ustrededniho Uštavu Geologickeho'' no 15, pp 1–160


Groups of labyrinthodonts

The systematic placement of groups within Labyrinthodontia is notoriously fickle. Carroll, R. L. (2001): The origin and early radiation of terrestrial vertebrates. ''Journal of Paleontology'' no 75(6), pp 1202–121
PDF
Several groups are identified, but there is no consensus of their
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
relationship. Many key groups were small with moderately ossified skeletons, and there is a gap in the fossil record in the early Carboniferous (the "
Romer's gap Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist ...
") when most of the groups appear to have evolved. Carroll, R. L. (1995): Problems of the phylogenetic analysis of Paleozoic choanates. ''Bulletin du Muséum national d'Histoire naturelle de Paris'', 4ème série 17: pp 389–445
Summary
Further complicating the picture is the amphibian larval-adult life cycle, with physical changes throughout life complicating phylogenetic analysis. The Labyrinthodontia appear to be composed of several nested clades. The two best understood groups, the Ichthyostegalia and the reptile-like amphibians have from the outset been known to be paraphyletic. Tellingly, labyrinthodont systematics was the subject of the inaugural meeting of International Society for Phylogenetic Nomenclature.


Ichthyostegalia

The early labyrinthodonts are known from the Devonian and possibly extending into the
Romer's Gap Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist ...
of the early Carboniferous. These labyrinthodonts are often grouped together as the
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
Ichthyostegalia, though the group is an evolutionary grade rather than a clade. Ichthyostegalians were predominantly aquatic and most show evidence of functional internal
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s throughout life, and probably only occasionally ventured onto land. Their polydactylous feet had more than the usual five digits for tetrapods and were paddle-like. The tail bore true fin rays like those found in fish. The vertebrae were complex and rather weak. At the close of the Devonian, forms with progressively stronger legs and vertebrae evolved, and the later groups lacked functional gills as adults. All were however predominantly aquatic and some spent all or nearly all their lives in water.


Reptile-like amphibians

An early branch was the terrestrial reptile-like amphibians, variously called Anthracosauria or Reptiliomorpha. ''
Tulerpeton ''Tulerpeton'' is an extinct genus of Devonian four-limbed vertebrate, known from a fossil that was found in the Tula Region of Russia at a site named Andreyevka. This genus and the closely related ''Acanthostega'' and ''Ichthyostega'' represen ...
'' has been suggested as the earliest member of the line, indicating the split may have happened before the Devonian-Carboniferous transition. Their skulls were relatively deep and narrow compared to other labyrinthodonts. Front and hind feet bore five digits on most forms. Several of the early groups are known from brackish or even marine environments, having returned to a more or less fully aquatic mode of living. With the exception of the diadectomorphs, the terrestrial forms were moderately sized creatures that appeared in the early Carboniferous. The vertebrae of the group foreshadowed that of primitive reptiles, with small pleurocentra, which grew and fused to become the true centrum in later vertebrates. The most well known genus is '' Seymouria''. Some members of the most advanced group, the Diadectomorpha, were herbivorous and grew to several meters in length, with great, barrel-shaped bodies. Small relatives of the diadectomorphs gave rise to the first reptiles in the
Late Carboniferous Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effe ...
.


Temnospondyli

The most diverse group of labyrinthodonts was the Temnospondyli. Temnospondyls appeared in the
early Carboniferous Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Ear ...
and came in all sizes, from small salamander-like Stereospondyli that scurried along the waters edge and undergrowth, to giant, well armoured
Archegosauroidea Archegosauroidea is an extinct superfamily of Permian temnospondyls. The superfamily is assigned to the clade Stereospondylomorpha and is the sister taxon to the suborder Stereospondyli. It includes the families Actinodontidae and Archegosaurid ...
that looked more like crocodiles. '' Prionosuchus'' was an exceptionally large member of the Archegosauridae, estimated to have been up to 9 meters long, it is the largest amphibian ever known to have lived. Temnospondyls typically had large heads and heavy shoulder girdles with moderately long tails. A fossil trackway from Lesotho shows larger forms dragged themselves by the front limbs over slippery surfaces with limited sideways movement of the body, very unlike modern salamanders. A temnospondyl's fore-foot had only four toes, and the hind-foot five, similar to the pattern seen in modern amphibians. Temnospondyls had a conservative vertebral column in which the pleurocentra remained small in primitive forms, vanishing entirely in the more advanced ones. The intercentra bore the weight of the animal, being large and forming a complete ring. All were more or less flat-headed with either strong or secondarily weak vertebrae and limbs. There were also fully aquatic forms, like the
Dvinosauria Dvinosaurs are one of several new clades of Temnospondyl amphibians named in the phylogenetic review of the group by Yates and Warren 2000. They represent a group of primitive semi-aquatic to completely aquatic amphibians, and are known from t ...
, and even marine forms such as the
Trematosauridae Trematosauridae are a family of large marine temnospondyl amphibians with many members. They first appeared during the Induan age of the Early Triassic, and existed until around the Carnian stage of the Late Triassic, although by then they wer ...
. The Temnospondyli may have given rise to the modern
frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...
s and
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s in the late
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
or early
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
.Laurin, M. (1996)
Phylogeny of Stegocephalians
from the Tree of Life Web Project


Lepospondyli

A small group of uncertain origin, the Lepospondyli evolved mostly small species that can be found in European and North American Carboniferous and early Permian strata. They are characterized by simple spool-shaped vertebrae formed from a single element, rather than the complex system found in other labyrinthodont groups. Most were aquatic and external
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s are sometimes found preserved. The Leposondyli were generally salamander-like, but one group, the Aïstopoda, was snakelike with flexible, reduced skulls, though whether the families belong with the other lepospondyls is uncertain. Some
microsaur Microsauria ("small lizards") is an extinct, possibly polyphyletic order of tetrapods from the late Carboniferous and early Permian periods. It is the most diverse and species-rich group of lepospondyls. Recently, Microsauria has been considered ...
lepospondyls were squat and short-tailed and appear to have been well adapted to terrestrial life. The best known genus is '' Diplocaulus'', a nectridean with a boomerang-shaped head. The position of Lepospondyli in relation to other labyrinthodont groups is uncertain, and it is sometimes classified as a separate subclass.Kent, G. C. & Miller, L. (1997): Comparative anatomy of the vertebrates. 8th edition. Wm. C. Brown Publishers. Dubuque. 487 pages. The teeth were not labyrinthodont, and the group has classically been seen as separate from the Labyrinthodontia. There is some doubt as to whether the lepospondyls form a
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
unit at all, or is a wastebin taxon containing the padamorphic forms and tadpoles of other labyrinthodonts, notably the reptile-like amphibians, or even very small primitive amniotes with reduced skulls.


Evolutionary history

The labyrintodonts have their origin in the early middle Devonian (398–392 Mya) or possibly earlier. They evolved from a bony fish group: the fleshy-finned Rhipidistia. The only other living group of Rhipidistans alive today are the
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
, the sister group of the landliving vertebrates. Earliest traces of the land-living forms are fossil trackways from Zachełmie quarry,
Poland Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populou ...
, dated to 395 million years ago, attributed to an animal with feet very similar to Ichthyostega.


Swamp predators

By the late Devonian, land
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
s had stabilized freshwater habitats, allowing the first
wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in t ...
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s to develop, with increasingly complex food webs that afforded new opportunities. The early labyrinthodonts were wholly aquatic, hunting in shallow water along tidal shores or weed filled tidal channels. From their piscine ancestors, they had inherited swim bladders that opened to the
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
and were capable of functioning as
lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either si ...
(a condition still found in lungfish and some physostome ray-finned fishes), allowing them to hunt in stagnant water or in waterways where rotting vegetation would have lowered oxygen content. The earliest forms, such as '' Acanthostega'', had vertebrae and limbs quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land—either in search of prey like modern mudskippers, or to find water when the pond they lived in dried out. Early fossil tetrapods have been found in marine sediments, suggesting marine and brackish areas were their primary habitat. This is further corroborated by fossils of early labyrinthodonts being found scattered all around the world, indicating they must have spread by following the coastal lines rather than through freshwater only. The first labyrinthodonts were all large to moderately large animals, and would have suffered considerable problems on land despite their members ending in toes rather than fin-rays. While they retained gills and fish-like skulls and tails with fin rays, the early forms can readily be separated from Rhipidistan fish by the cleithrum/ scapula complex being separate from the skull to form a pectoral girdle able to carry the weight of the front end of the animals. They were all carnivorous, initially eating fish and possibly going ashore to feed off washed up carrion of sea animals caught in tidal ponds, only later turning into predators of the large
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
s of the Devonian at the waters edge. The various early forms are for convenience grouped together as Ichthyostegalia. While the body shape and proportions of the ichthyostegalians went largely unchanged throughout their evolutionary history, the limbs underwent a rapid evolution. The proto-tetrapods like from '' Elginerpeton'' and '' Tiktaalik'' had extremities ending in fin-rays with no clear fingers, primarily suited for movement in open water, but also capable of propelling the animal across sandbanks and through vegetation filled waterways. '' Ichthyostega'' and '' Acanthostega'' had paddle-like polydactyl feet with stout bony toes that also would have enabled them to drag themselves across land. The aquatic ichthyostegalians flourished in tidal channels and swampland through the remainder of the Devonian, only to disappear from the
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
record at the transition to the Carboniferous.


Onto land

The end of the Devonian saw the
late Devonian extinction The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...
event, followed by a gap in the
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
record of some 15 million years at the start of the Carboniferous, called "
Romer's gap Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist ...
". The gap marks the disappearance of the ichthyostegalian forms as well as the origin of the higher labyrinthodonts. Finds from this period found in
East Kirkton Quarry East Kirkton Quarry is a former limestone quarry in West Lothian, Scotland ( East Kirkton Limestone), now better known as a fossil site known for terrestrial fossils from the fossil-poor Romer's gap, a 15 million year period at the beginning of ...
includes the peculiar, probably secondarily aquatic ''
Crassigyrinus ''Crassigyrinus'' (from la, crassus, 'thick' and el, γυρίνος el, gyrínos, 'tadpole') is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous Limestone Coal Group of Scotland and possibly Greer, West Virginia. Th ...
'', which may represent the sister group to later labyrinthodont groups. Early Carboniferous saw the radiation of the family Loxommatidae, a distinct if mysterious group that may have been the ancestors or sister taxon of the higher groups, characterized by keyhole-shaped eye openings. By the Visean age of mid-Carboniferous times the labyrinthodonts had radiated into at least three main branches. Recognizable groups are representative of the temnospondyls, lepospondyls and reptile-like amphibians, the latter which were the relatives and ancestors of the Amniota. While most labyrinthodonts remained aquatic or semi-aquatic, some of the reptile-like amphibians adapted to explore the terrestrial ecological niches as small or medium-sized predators. They evolved increasingly terrestrial adaptions during the Carboniferous, including stronger vertebrae and slender limbs, and a deeper skull with laterally placed eyes. They probably had watertight skin, possibly covered with a horny epidermis overlaying small bony nodules, forming scutes, similar to those found in modern caecilians. To the modern eye, these animals would appear like heavyset, lizards betraying their amphibious nature only by their lack of claws and by spawning aquatic eggs. In the middle or late Carboniferous, smaller forms gave rise to the first reptiles.R. L. Paton, R. L., Smithson, T. R. & Clack, J. A. (1999): An amniote-like skeleton from the Early Carboniferous of Scotlan
(abstract)
''
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
'' 398, 508–513 (8 April 1999)
In the late Carboniferous, a global
rainforest collapse Ecological collapse refers to a situation where an ecosystem suffers a drastic, possibly permanent, reduction in carrying capacity for all organisms, often resulting in mass extinction. Usually, an ecological collapse is precipitated by a disastro ...
favoured the more terrestrially adapted reptiles, while the many of their amphibian relatives failed to reestablish. Some reptile-like amphibians did flourish in the new seasonal environment. The reptiliomorph family Diadectidae evolved herbivory, becoming the largest terrestrial animals of the day with barrel-shaped, heavy bodies. There were also a family of correspondingly large carnivores, the Limnoscelidae, that flourished briefly in the late Carboniferous.


Heyday of the labyrinthodonts

The herbivorous Diadectidae reached their maximum diversity in the late Carboniferous/early
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
, and then quickly declined, their role taken over by early reptilian herbivores like Pareiasaurs and Edaphosaurs. Colbert, E. H. & Morales, M. (1990): Evolution of the Vertebrates: A history of the Backboned Animals Through Time, John Wiley & Sons Inc (4th ed., 470 pp) Unlike the reptile-like amphibians, the Temnospondyli remained mostly denizens of rivers and swampland, feeding on fish and perhaps other labyrinthodonts. They underwent a major diversification in the wake of the Carboniferous rainforest collapse and they too subsequently reached their greatest diversity in the late Carboniferous and early Permian, thriving in the rivers and brackish coal forests in continental shallow basins around equatorial Pangaea and around the Paleo-Tethys Ocean. Several adaptations to
piscivory A piscivore () is a carnivorous animal that eats primarily fish. The name ''piscivore'' is derived . Piscivore is equivalent to the Greek-derived word ichthyophage, both of which mean "fish eater". Fish were the diet of early tetrapod evolut ...
evolved with some groups having crocodile-like skulls with slender snouts, and presumably had a similar life-style ( Archegosauridae, Melosauridae,
Cochleosauridae Cochleosauridae is a family of edopoid temnospondyl amphibians, among the most basal of temnospondyls. Most members of this family are known from the late Carboniferous ( Pennsylvanian) and early Permian (Cisuralian) of Europe and North America, ...
and Eryopidae, and the reptile-like suborder Embolomeri). Others evolved as aquatic ambush predators, with short, broad skulls that allowed for opening the mouth by tipping the skull back rather than dropping the jaw (
Plagiosauridae Plagiosauridae is a clade of temnospondyl amphibians of the Middle to Late Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the begi ...
and the
Dvinosauria Dvinosaurs are one of several new clades of Temnospondyl amphibians named in the phylogenetic review of the group by Yates and Warren 2000. They represent a group of primitive semi-aquatic to completely aquatic amphibians, and are known from t ...
). In life they would have hunted rather like the modern day monkfish, and several groups are known to have retained the larval gills into adulthood, being fully aquatic. The
Metoposauridae Metoposauridae is an extinct family of trematosaurian temnospondyls. The family is known from the Triassic period. Most members are large, approximately long and could reach 3 m long.Brusatte, S. L., Butler R. J., Mateus O., & Steyer S. J. (201 ...
adapted to hunting in shallows and murky swamps, with ∩-shaped skull, much like their Devonian ancestors. In Euramerica, the Lepospondyli, a host of small, mostly aquatic amphibians of uncertain phylogeny, appeared in the Carboniferous. They lived in the tropical forest undergrowth and in small ponds, in ecological niches similar to those of modern amphibians. In the Permian, the peculiar Nectridea found their way from Euramerica to Gondwanaland.


Decline

From the middle of the Permian, the climate dried up, making life difficult for the amphibians. The terrestrial reptiliomorphs disappeared, though aquatic crocodile-like Embolomeri continued to thrive until going extinct in the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
. The diverse lepospondyl inhabitants of the undergrowth disappear from the fossil record, among them the snake-like Aïstopoda. With the close of the
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
, most of the Permian groups disappeared, with the exceptions of the Mastodonsauroidea,
Metoposauridae Metoposauridae is an extinct family of trematosaurian temnospondyls. The family is known from the Triassic period. Most members are large, approximately long and could reach 3 m long.Brusatte, S. L., Butler R. J., Mateus O., & Steyer S. J. (201 ...
and
Plagiosauridae Plagiosauridae is a clade of temnospondyl amphibians of the Middle to Late Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the begi ...
, who continued into the Triassic. In the early Triassic these groups enjoyed a brief renaissance in the waterways of continental shallows, with large forms like ''
Thoosuchus ''Thoosuchus'' (meaning "active crocodile") is an extinct genus of basal trematosauroid trematosaurian temnospondyl. Fossils have been found from Russia and date back to the Early Triassic. It is the type genus of the family Thoosuchidae, fo ...
'', ''
Benthosuchus ''Benthosuchus'' (meaning "deep water crocodile") is an extinct genus of temnospondyl amphibian from the Early Triassic of Russia. It was primarily aquatic, living in rivers and lakes. Multiple species are known, with the largest reaching ab ...
'' and ''
Eryosuchus ''Eryosuchus'' is an extinct genus of capitosauroid temnospondyl amphibian from the Middle Triassic of northern Russia. It was a very large predator: the largest specimen known could reach up to 3.5 m (11.5 ft) in length, with a skull ...
''. Their ecological niches were probably similar to those of modern-day crocodiles, as fish hunters and riverside carnivores. All groups developed progressively weaker vertebrae, reduced limb ossification and flatter skulls with prominent lateral line organs, indicating the late Permian/early Triassic temnospondyls rarely if ever left the water. An extremely large
brachyopid Brachyopidae is an extinct family of temnospondyl labyrintodonts. They evolved in the early Mesozoic and were mostly aquatic. A fragmentary find from Lesotho, Africa is estimated to have been long, the largest amphibian ever known to have li ...
(likely a plagiosaur or a close relative) is estimated to have been 7 meters long, and probably just as heavy as the Permian '' Prionosuchus''. With the rise of the real crocodiles in the middle Triassic, even these Temnospondyli went into decline, though some hung on to at least early
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
on the southern Gondwanaland, in regions too cold for crocodiles.


Origin of modern amphibians

There is today a general consensus that all modern amphibians, the Lissamphibia, have their origin in labyrinthodont stock, but this is where consensus ends. The fragile bones of the lissamphibians are extremely rare as fossils, and the modern amphibians are highly derived, making comparison with fossil labyrinthodonts difficult. Traditionally, the Lepospondyli has been favored as lissamphibian ancestors. Like the modern amphibians, they were mostly small with simple vertebrae, resembling lissamphibians in many aspects of external anatomy and presumably ecological niches. At a subclass level, it was thought that labyrinthodonts gave rise to lepospondyls, and lepospondyls to lissamphibians. Several cladistic studies also favour the lepospondyl link, though placing Lepospondyli as close relatives or even derived from reptile-like amphibians. One problem with this position is the question of whether Lepospondyli actually is monophyletic in the first place. Temnospondyl affinity for the Lissamphibia is suggested by other works. The temnospondyl family Amphibamidae has been considered a possible candidate for the ancestors of lissamphibians. The amphibamid '' Gerobatrachus'', described in 2008, was proposed to be a
transitional form A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross a ...
between temnospondyls and anurans (frogs and toads) and caudatans (salamanders). It possessed a mixture of anuran and caudatan features, including a broad skull, short tail, and small
pedicellate teeth Pedicellate teeth are a tooth morphology today unique to modern amphibians, but also seen in a variety of extinct labyrinthodonts "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major ...
. Complicating the picture is the question of whether Lissamphibia itself may be polyphyletic. Though a minority view, several variants have been forwarded through history. The "Stockholm school" under Gunnar Säve-Söderbergh and Erik Jarvik argued during much of the 20th century that Amphibia as a whole is biphyletic, based on details of the nasal capsule and cranial nerves. In their view lepospondyls are ancestors of frogs, while
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s and caecilians have evolved independently from porolepiform fish. Robert L. Carroll suggested the tailed amphibians (salamanders and caecilians) are derived from lepospondyl microsaurs and frogs from temnospondyls. The cladistic analysis of ''Gerobatrachus'' suggested salamanders and frogs evolved from temnospondyl stock and caecilians being the sister group of the reptile-like amphibians, rendering Lissamphibia itself an evolutionary grade relative to the remaining tetrapod classes. The cladistic analysis of ''Chinlestegophis'' by Pardo et al. (2017) does recover Lissamphibia as polyphyletic, but with all groups of lissamphibians falling within Temnospondyli; Batrachia is recovered in the analysis as part of Dissorophoidea, while Gymnophonia falls within Stereospondylomorpha.


Origin of the amniotes

The fossil sequence leading from the early Carboniferous labyrinthodonts to the amniotes has traditionally been seen as fairly well mapped out since the early 20th century, mainly leaving only the question of the demarcation line between the amphibian and reptilian grade of reproduction. Work by Carroll and
Laurin Laurin is both a surname and a given name. Notable people with the name include: Surname * Anna-Lena Laurin (born 1962), Swedish composer * Camille Laurin (1922–1999), psychiatrist and politician in Quebec, Canada * Dan Laurin (born 1960), Swed ...
around the turn of the millennium has greatly helped in pinpointing the transition. Carroll R.L. (1991): The origin of reptiles. In: Schultze H.-P., Trueb L., (ed) ''Origins of the higher groups of tetrapods — controversy and consensus''. Ithaca: Cornell University Press, pp 331-353. The early reptile-like amphibians were mostly aquatic, the first highly terrestrially adapted groups being the Seymouriamorpha and the Diadectomorpha. The seymouriamorphs were small to medium-sized animals with stout limbs, their remains are sometimes found in what has been interpreted as dry environments, indicating their skin had a water-tight epidermal horny overlay or even scales as evident in ''
Discosauriscus ''Discosauriscus'' was a small seymouriamorph which lived in what is now Central and Western Europe in the Early Permian Period. Its best fossils have been found in the Broumov and Bačov Formations of Boskovice Furrow, in the Czech Republic. ...
''. Their skeletons are very similar to those of early reptiles, though finds of seymouriamorph tadpoles have shown they retained an amphibian reproduction. The diadectomorph families are generally considered to be the closest known relatives of modern amniotes. They too are thought to have been on the amphibian side of the divide, despite no known diadectomorph fossil tadpoles. Analysis of new finds and composition of larger trees do however indicate the phylogeny may not be as well understood as traditionally thought. Several authors have suggested that terrestrial eggs evolved from amphibian eggs laid on land to avoid predation on the eggs and competition from other labyrinthodonts. The amniote egg would necessarily have had to evolve from one with an anamniote structure, as those found in fish and modern amphibians. In order for such an egg to excrete CO2 on land without the specialized membranes to aid in respiration, it would have to be very small, 1 cm in diameter or smaller. Such very small eggs with direct development would severely restrict the adult size, thus the amniotes would have evolved from very small animals. A number of small, fragmentary fossils of possibly diadectomorph affinity has been proposed as the first amniote, including ''
Gephyrostegus ''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 tee ...
'', '' Solenodonsaurus'', '' Westlothiana'' and '' Casineria''. Fossilized footprints found in
New Brunswick New Brunswick (french: Nouveau-Brunswick, , locally ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. It is the only province with both English and ...
indicate the first reptiles were established by 315 million years ago.


History of classification

The term ''labyrinthodont'' was coined by Hermann Burmeister in reference to the tooth structure. Labyrinthodontia was first used as a systematic term by Richard Owen in 1860, and assigned to
Amphibia Amphibians are four-limbed and ectothermic 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, arbo ...
the following year. It was ranked as an
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
under class Amphibia by Watson in 1920 and as a superorder by Romer in 1947. An alternative name, ''Stegocephalia'' was created in 1868 by American palaentologist Edward Drinker Cope, from Greek ''stego cephalia''—"roofed head", and refer to
anapsid An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolv ...
skull and the copious amounts of dermal armour some of the larger forms evidently had. This term is widely used in 19th and early 20th century literature. Classification of the earliest finds was attempted on the basis of the skull roof, often the only part of the specimen preserved. With the frequent convergent evolution of head shape in labyrinthodonts, this led to
form taxa Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of sc ...
only. The relationship of the various groups to each other and to the lissamphibians (and to some degree the first reptiles) is still a matter of some debate. Several schemes have been forwarded, and at present there is no consensus among workers in the field.


Vertebral classification

A systematic approach based on the relative size and shape of the elements of the complex labyrinthodont vertebrae was favored in the early 20th century. This classification quickly fell into disuse as some forms of backbones appear to have appeared more than once and different types are found in close relatives, sometimes even in the same animal, and already by the middle of the 20th century several of the small-bodied groups were suspected of being larval or neotenic forms. The classification presented here is from Watson, 1920: * Order
Stegocephalia Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...
(= Labyrinthodontia) ** "Grade"
Rachitomi The Rachitomi were a group of extinct Palaeozoic labyrinthodont amphibians, according to an earlier classification system.Kent, G.C. & Miller, L. (1997): Comparative anatomy of the vertebrates. 8th edition. Wm. C. Brown Publishers. Dubuque. 487 p ...
(Primitive complex vertebrae, all Ichthyostegalia, most large Temnospondyli and some reptile-like amphibians) ** "Grade" Embolomeri ( Intercentrum and pleurocentrum cylinders of equal size, today considered a suborder of secondarily aquatic reptile-like amphibians) ** "Grade" Stereospondyli (Simplified backbones with only intercentrum and
vertebral arch The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
, still recognized as a valid group) * Order Phyllospondyli (Small, flimsy vertebrae, today considered to represent tadpoles and paedomorphic forms) * Order Lepospondyli (Hourglass-shaped or cylindrical vertebrae, mid Carboniferous to mid Permian, phylogeny uncertain) * Order Adelospondyli (Cylindrical vertebrae with conical depressions at each end meeting in the middle, now considered a Lepospondyl group) * Order Gymnophiona ( extant) * Order Urodela ( extant) * Order Anura ( extant)


Traditional classification

The traditional classification was initiated by Säve-Söderbergh in the 1930s. He believed that Amphibia was biphyletic, and that salamanders and
caecilians 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 ...
had evolved independently from porolopiform fish. Few shared Säve-Söderbergh's view of a biphyletic Amphibia, but his scheme, either with the Lepospondyli as a separate subclass or sunk into Temnospondyli, was continued by Romer in his much used Vertebrate Paleontology of 1933 and later editions, Romer, A. S., (1947, revised ed. 1966) Vertebrate Paleontology, University of Chicago Press, Chicago and followed by several subsequent authors with minor variations: Colbert 1969, Colbert, E. H., (1969), ''Evolution of the Vertebrates'', John Wiley & Sons Inc (2nd ed.) Daly 1973, Carroll 1988 and Hildebrand & Goslow 2001. The classification cited here is from Romer & Parson, 1985: * Subclass Labyrinthodontia ** Order Ichthyostegalia (primitive ancestral forms, e.g., '' Ichthyostega''—Middle to late Devonian only). ** Order Temnospondyli (Late Devonian to Cretaceous, e.g., '' Eryops'', possibly ancestral to modern amphibians) ** Order Anthracosauria (Carboniferous and Permian, e.g., '' Seymouria'', ancestral to early reptiles) * Subclass Lepospondyli (Carboniferous and Permian, e.g., '' Diplocaulus'', small group, possibly ancestral to modern amphibians) * Subclass Lissamphibia (Permian to present) ** Order Gymnophiona ( extant) ** Order Urodela ( extant) ** Order Anura ( extant)


Phylogenetic classification

Labyrinthodontia has fallen out of favor in recent taxonomies because it is paraphyletic: the group does not include all the descendants of their most recent common ancestor. Various groups that have traditionally been placed within Labyrinthodontia are currently variously classified as stem tetrapods, basal tetrapods, non-amniote Reptiliomorpha and as a monophyletic or paraphyletic Temnospondyli, according to various cladistic analysis. This reflects the emphasis of ascertaining lineage and ancestral-descendant relatedness in modern-day cladistics. The name does however linger as a handy reference for the early amphibian tetrapods, and as an apt anatomical description of their distinct tooth pattern. Thus it remains in use as an informal term of convenience by some modern scientists. The largely synonymous name
Stegocephalia Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...
has been taken up by Michel Laurin and defined cladistically for all traditional labyrinthodonts (including their descendants), so that the name with the largely traditional meaning is still employed. An informal term with a broader meaning is stem tetrapod, a stem group consisting of all species more closely related to modern tetrapods than to
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
, but excluding the crown group. This group includes both traditional "labyrinthodonts" as well as more basal tetrapodomorph fish, though its total content is a matter of some uncertainty, as the relationships of these animals are not well understood. Below is a suggested evolutionary tree of Labyrinthodontia, from Colbert 1969 and Caroll 1997. Carroll, R. L. (1997): Patterns and Processes of Vertebrate Evolution. Cambridge University Press, Cambridge. 464 pages Dashed lines indicate relationships that commonly vary between authors. A good summary (with diagram) of characteristics and main evolutionary trends of the above three orders is given in Colbert 1969 pp. 102–103, but see Kent & Miller (1997) for an alternative tree.


See also

* Amphibian * Batrachosauria *
Prehistoric amphibian This list of prehistoric amphibians is an attempt to create a comprehensive listing of all genera from the fossil record that have ever been considered to be amphibians, excluding purely vernacular terms. The list includes all commonly accepted g ...
*
Prehistoric life The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''gigaannum'') and evide ...
* Reptiliomorpha * Temnospondyli * Tetrapod


References


External links


Palaeos.com Cladogram of Reptiliomorpha


{{Taxonbar, from=Q753235 Mesozoic amphibians Paleozoic amphibians Paraphyletic groups Taxa named by Richard Owen