"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
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". Otic notch 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/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.R. L. Paton, T. R. Smithson and J. A. Clack, "An amniote-like skeleton from the Early Carboniferous of Scotland", (abstract), Nature 398, 508–513 (8 April 1999) 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 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 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.Laurin, M. (1996): Hearing in Stegocephalians, from the Tree of Life Web Project The tympanum in anurans and amniotes appear to have evolved separately, indicating most, if not all, labyrinthodonts were unable to pick up airborne sound.Lombard, R. E. & Bolt, J. R. (1979): Evolution of the tetrapod ear: an analysis and reinterpretation. Biological Journal of the Linnean Society No 11: pp 19–76 Abstract
Respiration The early labyrinthodonts possessed well developed internal gills as well as primitive lungs/swim bladders inherited from 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 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 adults with feathery external gills similar to those found in modern lissamphibian tadpoles and in the fry of lungfish 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, 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 1202–1213 PDF 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.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.Foer, J. (2005): Pushing PhyloCode: What if we decide to rename every living thing on Earth?, Discovery Magazine, April issue
Ichthyostegalia