Labyrinthodontia
"Labyrinthodonts" Amniotes and Lissamphibians survive to present.
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Artist's conception of a Proterogyrinus, an anthracosaur | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Batrachomorpha |
Subclass: | †Labyrinthodontia Owen, 1860 |
Groups included | |
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Cladistically included but traditionally excluded taxa | |
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"Labyrinthodontia" (
"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.
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.[2]
- 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".[2]
- 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).[3][4]
- Complex 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.[5] 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.[6] 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.[7] All teeth were labyrinthodont. The sole exception were the chisel-like teeth of some of the advanced herbivorous diadectomorphs.[6] The skull had prominent otic notches behind each eye and a parietal eye.
Post-cranial skeleton
The
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
Respiration
The early labyrinthodonts possessed well developed internal gills as well as primitive lungs, derived from the swim bladders of their ancestors.[2][dubious ] 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.[6] 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.[6] 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.[13]
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.[14] 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.[15] 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.[6]
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.[16]
Groups of labyrinthodonts
The systematic placement of groups within Labyrinthodontia is notoriously fickle.[17][18] Several groups are identified, but there is no consensus of their phylogenetic relationship.[19] 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.[17][20] Further complicating the picture is the amphibian larval-adult life cycle, with physical changes throughout life complicating phylogenetic analysis.[21] The Labyrinthodontia appear to be composed of several nested clades.[22] The two best understood groups, the Ichthyostegalia and the reptile-like amphibians have from the outset been known to be paraphyletic.[2] Tellingly, labyrinthodont systematics was the subject of the inaugural meeting of International Society for Phylogenetic Nomenclature.[23]
Ichthyostegalia
The early labyrinthodonts are known from the Devonian and possibly extending into the
Reptile-like amphibians
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.[27] 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.[28]
With the exception of the
Temnospondyli
The most diverse group of labyrinthodonts was the
A temnospondyl's fore-foot had only four toes, and the hind-foot five, similar to the pattern seen in modern amphibians.[9] 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.[2] 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, and even marine forms such as the Trematosauridae. The Temnospondyli may have given rise to the modern frogs and salamanders in the late Permian or early Triassic.[22]
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.[33] Most were aquatic and external gills 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.[34] 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
Evolutionary history
The labyrinthodonts 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, the sister group of the landliving vertebrates. Earliest traces of the land-living forms are fossil trackways from Zachełmie quarry, Poland, dated to 395 million years ago, attributed to an animal with feet very similar to Ichthyostega.[37][38]
Swamp predators
By the late Devonian, land
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
While the body shape and proportions of the ichthyostegalians went largely unchanged throughout their evolutionary history, the limbs underwent a rapid evolution. The
Onto land
The end of the Devonian saw the late Devonian extinction event, followed by a gap in the fossil record of some 15 million years at the start of the Carboniferous, called "Romer's gap". The gap marks the disappearance of the ichthyostegalian forms as well as the origin of the higher labyrinthodonts.[5][10] Finds from this period found in East Kirkton Quarry includes the peculiar, probably secondarily aquatic Crassigyrinus, which may represent the sister group to later labyrinthodont groups.[40]
Early Carboniferous saw the radiation of the family
While most labyrinthodonts remained aquatic or semi-aquatic, some of the reptile-like amphibians adapted to explore the terrestrial
Heyday of the labyrinthodonts
The herbivorous Diadectidae reached their maximum diversity in the late Carboniferous/early
Several adaptations to
In
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.[2] The diverse lepospondyl inhabitants of the undergrowth disappear from the fossil record, among them the snake-like Aïstopoda.
With the close of the Paleozoic, most of the Permian groups disappeared, with the exceptions of the Mastodonsauroidea, Metoposauridae and Plagiosauridae, 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, Benthosuchus and Eryosuchus. Their ecological niches were probably similar to those of modern-day crocodiles, as fish hunters and riverside carnivores.[33] 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 (likely a plagiosaur or a close relative) is estimated to have been 7 meters long, and probably just as heavy as the Permian Prionosuchus.[44]
With the rise of the real
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.[22] The fragile bones of the lissamphibians are extremely rare as fossils, and the modern amphibians are highly derived, making comparison with fossil labyrinthodonts difficult.[10]
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.[30][46] Several cladistic studies also favour the lepospondyl link, though placing Lepospondyli as close relatives or even derived from reptile-like amphibians.[47][48][49] One problem with this position is the question of whether Lepospondyli actually is monophyletic in the first place.[20][50]
Temnospondyl affinity for the Lissamphibia is suggested by other works.
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.[55] Robert L. Carroll suggested the tailed amphibians (salamanders and caecilians) are derived from lepospondyl microsaurs and frogs from temnospondyls.[56] 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.[54] 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.[57]
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 around the turn of the millennium has greatly helped in pinpointing the transition.[58][59]
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.[60] Their skeletons are very similar to those of early reptiles, though finds of seymouriamorph tadpoles have shown they retained an amphibian reproduction.[16] 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.[61] Analysis of new finds and composition of larger trees do however indicate the phylogeny may not be as well understood as traditionally thought.[18]
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.[62][63] The amniote egg would necessarily have had to evolve from one with an anamniote structure, as those found in fish and modern amphibians.[59] 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.[58] A number of small, fragmentary fossils of possibly diadectomorph affinity has been proposed as the first amniote, including Gephyrostegus,[64] Solenodonsaurus,[61] Westlothiana[65] and Casineria.[29] Fossilized footprints found in New Brunswick indicate the first reptiles were established by 315 million years ago.[66]
History of classification
The term labyrinthodont was coined by
Classification of the earliest finds was attempted on the basis of the
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.[35] 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.[72] The classification presented here is from Watson, 1920:[69]
- Order Stegocephalia(= Labyrinthodontia)
- "Grade" Rachitomi (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" vertebral arch, 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 extant)
- Order extant)
- Order extant)
Traditional classification
The traditional classification was initiated by
- 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 extant)
- Order extant)
- Order extant)
- Order
Phylogenetic classification
Labyrinthodontia has fallen out of favor in recent taxonomies because it is
The largely synonymous name
Below is a suggested evolutionary tree of Vertebrates including the Labyrinthodontia, from Colbert 1969 and Caroll 1997.[73][78] Dashed lines indicate relationships that commonly vary between authors.
From lobe-finned fish
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"Labyrinthodontia" | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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.[35]
See also
- Amphibian
- Batrachosauria
- Prehistoric amphibian
- Prehistoric life
- Reptiliomorpha
- Temnospondyli
- Tetrapod
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External links
Labyrinthodontia.