The development of land-based amphibians derived from tetrapods, all of whom were labyrinthodonts. The term labyrinthodont ("labyrinth-tooth") refers to the folded, interlocking layers of enamel in their teeth (fig.1). This primitive trait was originally inherited from the Devonian ancestors of tetrapods, the sarcoptygerians or lobe-finned fish.
Fig.1: Labyrinthodont tooth, shown in cross section.
During the early Carboniferous (Mississippian) period, lasting from 358.9 to 323.2 mya, three primary lines of labyrinthodont amphibians arose: the temnospondyls, the reptilomorphs, and the lepospondyls. The temnospondyls were a highly successful branch of amphibians who eventually became extinct during the Jurassic period. Reptilomorphs led both to the Sauropsid reptiles, ancestors of all later reptiles, and to the synapsids, who led eventually to early mammals. Lepospondyls were mainly eel-like and lizard-like forms inhabiting rivers or wetlands. Some are considered to have led to present-day amphibians. The largest genus, Diplocaulus, reached 1 m in length, but most were much smaller.
The Mississippian period, when amphibians developed, is named for exposed rock formations from this period in the
One of the earliest tetrapods known
from the Early Mississippian, Whatcheeria deltae,
was found in a limestone
Fig.3: Skull of Whatcheeria deltae, with bones labelled (after Lombard and Bolt 1995, fig.3)..
The first proto-temnospondyl - like species were the Colosteids, who were lizard-like aquatic predators, partly covered with scales. One of the first discoveries of a colosteid was Pholidogaster pisciformes, ("scaly stomach [with a] fish form"), identified by Thomas H. Huxley in 1862 from a nearly complete skeleton found in a coal deposit in Gilmerton, Scotland. These deposits date from the Visean and early Serpukhovian stages of of the Middle Carboniferous (ca. 338-320 mya). The Philodigaster fossil had been originally grouped with fish fossils. Huxley (1862) identified it as a thin, elongated aquatic amphibian about 44 cm. long, with with weak, undeveloped limbs. It also had hard stomach scales, which allowed it to crawl on land, and fang-like canines indicating it was a carnivore. P. pisciformes provided an early illustration of the tetrapod features shown in amphibians, representing the transition to terrestial habits.
medium to large-sized swamp dwellers known as Baphetids;
in turn, the first true temnospondyls, perhaps arising from aquatic
such as Eucritta.
Early temnospondyls had elongated,
crocodile-like forms up to 1.5 m in length. During the Late
Visean epoch (340-330 mya), these early temnospondyls expanded into
forms, with many
tropical swamps and rivers. Other had become
semi-terrestial amphibians, including the large Eryops,
and the smaller Dendrerpeton.
The Paleozoic temnospondyls saw their greatest expansion during the late Carboniferous and early Permian periods. Some, such as Cacops, who had huge, armor-plated heads and shoulders, led a terrestrial existence, returning to water only to lay their eggs. Others, including Trematops and Eryops, who had strong, thick limbs and large, somewhat frog-like heads, maintained a semi-aquatic existence. Still others, like Trimerorhachis, were fully aquatic, witth long, crocodile-like bodies armoured with protective coatings of scales. Others, like Branchiosaurus, retained external gills even as adults and were unable to leave the water.
Most temnospondyls had relatively large heads of varying forms. Some, such as Laidleria, had flat, triangular skulls which are distinctive among vertebrates in general. Others, like Gerrothorax, had bizarre, wide, parabolic heads. Still others, such as Archegosaurus, had very elongated mouths in a form something like that of gharial (i.e. Nile) crocodiles, and which is associated with reptiles which hunt small fish.
The Temnospondyl post-cranial skeleton, meanwhile, varied in proportions related to swimming or walking specializations. The vertebral structure was essentially that of the primitive, late Devonian tetrapods and osteolepiform lobe-finned fish, with intercentra and pleurocentra. Regarding the limbs and digits, temnospondyls did not develop limb specializations, such as occurred in the reptilomorph lineage. While the temnospondyl hind foot had five digits or toes, their forefoot had only four.
Class: Amphibia Order: Temnospondyli Family: Dendrepetontidae
arcadianum was a land-based temnospondyl found in
Carboniferous (Pennsylvanian) period. They were first discovered in
fossil stump deposits in the Joggins Formation in Nova Scotia by
Dawson (1861, 1862; Owen 1862; Carroll 1967). These
date from the Westphalian B stage of the Pennsylvanian period (314-311
mya; Milner 1980).
The skulls of Dendrepeton arcadianum adult specimens are generally about 95-100 cm in length, and their bodies are estimated to have been about 1 meter long. A number of immature skulls were also found in the Joggins formation, measuring about 70 cm. Among the diagnostic anatomical traits for D. arcadianum are a short snout; and large, deep, ovoid otic notches in the back of the skull. The large, deeply concave otic notches indicate the presence of a large, circular tympanic membranes or ear drums (Milner 1980). The position of the suture between the lachrymal and jugal bone (located beside and behind the eyes) is also considered a diagnostic feature for this species. A pineal "eye" foramen is also found in the suture of the parietal bones (Milner 1980).
A related species, Dendrepeton rugosum, has also been found in coal deposits in Jarrow, Ireland (Huxley and Wright 1867). These date from the slightly earlier Westphalian A stage (316-314 mya) of the Upper Carboniferous period (equivalent to the Pennsylvanian Period in North America). D. rugosum has a shorter snout, and a larger otic notch, than D. arcadianum (Milner 1980). Dendrepeton fossils in the Jarrow deposits are often found associated with the Carboniferous tree genera Lepidodendron and Sigillaria, within which the Dendrerpeton fossils may be contained. In this condition, their remains are often disarticulated and flattened.
Class: Amphibia Order: Temnospondyli Family: Europidae
Class: Amphibia Order: Temnospondyli Family: DissorophidaeCacops aspideformus was a relativedly small amphibian, about 40 cm in length, that lived in North America during the Early Permian Kungurian phase (290-280 mya). Its fossils were found initially in the Clear Fork Permian strata in north Texas (Williston 1911). Recently the taxon has also been identified in Oklahoma (Reisz et al. 2009). It was adapted to a terrestrial lifestyle, with a heavily built skull, strong legs, a short tail, and a row of armor plates along its back.
amphibians (Reptilomorphs or
Reptiles developed from amphibians in the later part of the Carboniferous era (Late Mississippian and Pennsylvanian periods, about 335-295 mya). The first transitional forms, called Reptilomorphs (Säve-Söderbergh 1934; Panchen 1970) have also been called Anthracosaurs. This alternative name for reptile-like amphibians, used by the influential paleontologist Alfred Sherwood Romer (1956), derived from the coal deposits in which some amphibian fossils were first discovered.
These two (largely synonomous) major groupings of amphibians (reptilomorph and anthracosaurs) are used inconsistently in the literature partly because when originally defined, they included groups now considered less closely related. There is currently no real consensus on which term is preferable. The name Reptiliomorpha was first used by Gunnar Säve-Söderbergh in 1934 to designate various types of late Paleozoic reptile-like amphibians with labyrinthodont teeth. In 1956 Friedrich von Huene assigned both amphibians and anapsid reptiles to the Reptiliomorpha. This included eight orders, now considered to belong to distinct lineages, whose similarities are the result of convergences or shared primitive features (Palaios 2014d)
Romer (1956), in his widely-read textbooks on vertebrate paleontology, used the name Anthracosauria instead. In 1970, meanwhile, the German paleontologist Panchen (1970) reverted to Säve-Söderbergh's use of the term Reptilomorphia. Romer's term Anthracosauria, however, is still in use, both by evolutionary taxonomists (e.g. Carroll 1988 and 2002, and Hildebrand & Goslow 2001), and some cladistic taxonomists (Gauthier et al 1988). Another likely reason for confusion of the terms is the tendency for cladistic analyses to make improvisational rearrangements of Linnean ranks of classification ( Palaios 2014d). Michael Benton (2000, 2005) made Reptilomorpha the sister-clade to Batrachomorpha, ancestors of all current amphibians. In this solution, Reptilomorpha is made into a superorder, and Anthrocosauria into an order (Palaios 2014d). This inclusive approach, in an attempt at clarity, will be followed here.
Superorder: Reptiliomorpha Order: Anthracosauria suborder: Embolomeri Family: Proterogyrinidae
Fig.1: ProterogyrinusProterogyrinus was similar in shape to other reptiliomorphs, such as Crassigyrinus. It was about two and a half meters (7–8 feet) long, and showed a mixture of primitive (tetrapod-like) and derived traits. Like other reptiliomorphs, such as Seymouria, Proterogyrinus could move further away from water than most amphibians. Like tetrapods and early amphibians, Proterogyrinus had labyrinthodont teeth with folded enamel. Its rounded skull was amphibian, still retaining a fish-like hinged skull, with a hinge at the neck. In its postcranial anatomy, Proterogyrinus retained amphibian ankle bones, and had small shoulder girdle bones. Yet it also had reptilian vertebrae, pelvis, humerus, and digits, with both five toes and five fingers, and a 2-3-4-5-3 phalangeal count, which is more reptilian than amphibian.
Class Amphiba; order Reptilomorpha; suborder Diadectomorpha
More developed reptilomorph amphibians with additional reptilian features arose in the late Pennsylvanian through Early Permian periods. A suborder of these more advanced forms,, called the Diadectomorphs, include the type species Diadectes, Limnoscelis and Tseajaia.
Diadectes sideropelicus ("cross-biter of iron clay"), was a reptilomorph dating from the Early Permian Cisuralian stage (290–272 mya). It was first found in the Permian red beds of north Texas, in the Wichita and Clear Fork group near Wichita Falls. The initial identification was made in 1878 by Copes, who collected extensively there.
The anatomy of Diadectes shows a mixture of reptilian and amphibians traits, combining a robust, reptile-like skeleton with a more primitive, thick-boned skull resembling that of a Seymourian amphibian. Its heavy vertebrae and ribs, massive limb girdles and short, robust limbs indicate that D. sideropelicus was a fully terrestrial animal. Diadectes was one of the very earliest herbivorous tetrapods. Its teeth show advanced specializations for herbivorous diet. Eight front teeth are spatulate and peg-like,serving as incisors to clip off mouthfuls of vegetation. The broad, blunt cheek teeth, used for grinding, show extensive wear associated with occlusion.
A major difference between these ancient, large amphibians and the first reptiles, who were small in stature, is the amniotic egg which developed in reptiles. Here the embryo is contained in an egg with nutrient fluid and a protective sheath, as opposed to amphibians and fish whose larvae are hatched in open water.
Additional differences developed by reptiles from their amphibian ancestors include stronger legs and girdles, different vertebrae, and stronger jaw muscles (cf. Carroll 1988; Gauthier et al. in Benton, 1988). The reptilian skull was also much stronger. The ancestral amphibians had a relatively weak skull inherited from the fish-like tetrapods, and paired aortas or systemic arches.
Class: Amphibia Order: Reptilomorphs Suborder: Chroniosuchia
Chroniosuchians, first defined by Kuhn (1970) were a suborder of reptiliomorphs that lived from the Middle Permian to Late Triassic in what is now Eastern Europe, Kyrgyzstan, China and Germany. They were all short limbed, with a strong tail and elongated snout somewhat resembling that of modern crocodiles. The group is traditionally considered to be a suborder of labyrinthodonts. Chroniosuchians likely existed as riverine predators.
Most forms bore a heavy armour of scutes along the back, possibly for protection against land predators. This is illustrated by a species, Chroniosaurus dongusensis from the upper Tatarian to Early Wuchiapingian levels from the Late Permian period in the Orenburg Region of the South Cis-Urals. C. dongsensis had diagostic rows of interlocking bony plates called osteoderms along their backs, reaching from head to tail (Tverdokhlebova, 1972). These osteoderms constitute the most frequently found fossilized remain of Chroniosaurus. The flat osteoderm plates are connected to the neural arches of vertebra by an extension of bone on their undersurfaces. Each osteoderm thus is paired with a single vertebra. The front margin of each osteoderm also has a pair of "anterior wings" that slip into a notch in the posterior margin of the osteoderm in front of it.Chroniosuchians are also distinguished from other early reptiliomorphs by the lack of intertemporal bones in the skull, as well as the presence of holes in front of the eye sockets, called antorbital fenestrae. Like many early tetrapods, chroniosuchians have vertebrae that are divided into three parts: a pleurocentrum and an intercentrum on the bottom, and a neural arch on top. Chroniosuchians have vertebrae whose pleurocentrum makes up most of the body, while the intercentrum is small and wedge-like (called shizomerous vertebrae).
Gephyrostegus bohemicus was a Late Carboniferous, lizard-like reptilomorph from the Pennsylvanian period (about 310 mya), found at Nyran in the Czech Republic (Jaeckel 1902). It is thought to have been derived from a form similar to Silvanerpeton. Gephyrostregus is considered directly ancestral to early reptiles, including Casineria and Cephalerpeton.
Gephyrostegus, with an overall length of about 22 cm., was smaller than other basal tetrapods and slightly smaller than its more primitive sister, Silvanerpeton. The top and sides of the body of Geprostegus was covered by tiny circular scales, while on its underside, there were large V-shaped scales. Juveniles had a relatively larger head and orbit (eye socket). In the rear of its skull, the otic notch was reduced. Some postcranial bones of Geprostegus were relatively shorter than those of Silvanerpeton, including the pelvis, and the four and fifth hand digits. In its ankles, some bones had fused together to create a larger bone, the astragalus, otherwise found only on reptiles. Further, an elbow or olecranon process had developed on the ulna, one of the two lower arm bones. The humerus (upper arm bone) was more also more slender or gracile.
While most early tetrapods lived their lives in water, Gephyrostegus preferred land. Its eggs would have been laid in or near water to prevent dessication because, presumeably, no amniotic membrane or shell surrounded the embryo.
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