"Labyrinthodontia" (from Ancient Greek λαβύρινθος (labúrinthos, meaning "maze", and ὀδούς (odoús), meaning "tooth") is an informal group of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic 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 constitute an evolutionary grade (a paraphyletic group), 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 fossilize. 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 redefined the previously obsolete term Stegocephalia ("roof heads") as a cladistic alternative to "Labyrinthodontia" or "Tetrapoda".
Labyrinthodont traits
thumb|left|Cross-section of a labyrinthodont tooth
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 (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" 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".
- Complex vertebrae made of multiple components: an intercentrum (wedge-shaped front lower piece), two pleurocentra (upper rear piece), and a vertebral arch/spine (upper projection). The relative development and shape of the elements is highly variable.
The labyrinthodonts in life
thumb|left|Early ("[[ichthyostegalian") labyrinthodont]]
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. Some lineages remained waterbound or became secondarily fully aquatic with reduced limbs and elongated, eel-like bodies.
Skull
thumb|right|Skeletal reconstruction of [[Acanthostega, an early (ichthyostegalian) labyrinthodont]]
thumb|right|Reconstruction of [[Branchiosaurus, a temnospondyl tadpole or paedomorph form with external gills]]
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. This would have made most labyrinthodonts slow and clumsy on land.
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 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/swim bladders inherited from their ancestors.
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.
Groups of labyrinthodonts
The systematic placement of groups within Labyrinthodontia is notoriously fickle. Several groups are identified, but there is no consensus of their phylogenetic 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") when most of the groups appear to have evolved. 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.
Ichthyostegalia
thumb|[[Acanthostega, a fish-like early ichthyostegalian]]
The early labyrinthodonts are known from the Devonian and possibly extending into the Romer's Gap of the early Carboniferous. These labyrinthodonts are often grouped together as the order Ichthyostegalia, though the group is an evolutionary grade rather than a clade. Ichthyostegalians were predominantly aquatic and most show evidence of functional internal gills 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
thumb|[[Seymouria, a terrestrial reptiliomorph from the Permian]]
An early branch was the terrestrial reptile-like amphibians, variously called Anthracosauria or Reptiliomorpha. Tulerpeton 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. The first reptiles likely evolved from reptile-like forms in the Early Carboniferous or at the end of the Devonian, possibly during the fossil-poor Romer's Gap.
Temnospondyli
thumb|right|[[Platyoposaurus, an advanced crocodile-like temnospondyl from the Permian]]
The most diverse group of labyrinthodonts was the Temnospondyli. Temnospondyls appeared in the early Carboniferous and came in all sizes, from small salamander-like Stereospondyli that scurried along the waters edge and undergrowth, to giant, well armoured Archegosauroidea 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.
thumb|right|[[Eryops, a well known euskelian temnospondyl]]
A temnospondyl's fore-foot had only four toes, and the hind-foot five, similar to the pattern seen in modern amphibians.
Lepospondyli
thumb|[[Hyloplesion, a salamander-like lepospondyl]]
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. Some microsaur 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. 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 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.
Swamp predators
By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems 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 and were capable of functioning as lungs (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.
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 early Carboniferous or possibly earlier, smaller forms gave rise to the first reptiles. 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. 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 evolved with some groups having crocodile-like skulls with slender snouts, and presumably had a similar life-style (Archegosauridae, Melosauridae, Cochleosauridae and Eryopidae, and the reptile-like suborder Embolomeri). 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 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.
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 on the southern Gondwanaland, in regions too cold for crocodiles.
Origin of modern amphibians
thumb|right|The [[Amphibamidae|amphibamid temnospondyl Gerobatrachus from the Permian, proposed ancestor of lissamphibians]]
There is today a general consensus that all modern amphibians, the Lissamphibia, have their origin in labyrinthodont stock, but this is where consensus ends. 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 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.
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 salamanders 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.
Origin of the amniotes
thumb|right|[[Tseajaia, an advanced, very reptile-like amphibian]]
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 around the turn of the millennium has greatly helped in pinpointing the transition.
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. Their skeletons are very similar to those of early reptiles, though finds of seymouriamorph tadpoles have shown they retained an amphibian reproduction. Analysis of new finds and composition of larger trees do however indicate the phylogeny may not be as well understood as traditionally thought. The amniote egg would necessarily have had to evolve from one with an anamniote structure, as those found in fish and modern amphibians. Solenodonsaurus, and Casineria. Labyrinthodontia was first used as a systematic term by Richard Owen in 1860, and assigned to Amphibia the following year.
It was ranked as an order 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 paleontologist Edward Drinker Cope, from Greek stego cephalia—"roofed head", and refer to anapsid 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 only. The classification presented here is from Watson, 1920: and followed by several subsequent authors with minor variations: Colbert 1969, Daly 1973, Carroll 1988 and Hildebrand & Goslow 2001. The classification cited here is from Romer & Parson, 1985: Thus it remains in use as an informal term of convenience by some modern scientists. Dashed lines indicate relationships that commonly vary between authors.
A summary (with diagram) of characteristics and main evolutionary trends of the above three orders is given in Colbert 1969 pp. 102–103, but Kent & Miller (1997) have an alternative tree: