Temnospondyli
Temnospondyli (from Greek τέμνειν (temnein, "to cut") and σπόνδυλος (spondylos, "vertebra")) is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few species continued into the Cretaceous. Fossils have been found on every continent. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including fresh water, terrestrial, and even coastal marine environments. Their life history is well understood, with fossils known from the larval stage, metamorphosis, and maturity. Most temnospondyls were semiaquatic, although some were almost fully terrestrial, returning to the water only to breed. These temnospondyls were some of the first vertebrates fully adapted to life on land. Although temnospondyls are considered amphibians, many had characteristics, such as scales, claws, and armour-like bony plates, that distinguish them from modern amphibians.
| Temnospondyli | |
|---|---|
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| Skeleton of Eryops megacephalus in the National Museum of Natural History, Washington, D.C. | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Superclass: | Tetrapoda |
| Clade: | Batrachomorpha |
| Order: | †Temnospondyli Zittel, 1888 |
| Subgroups | |
Temnospondyls have been known since the early 19th century, and were initially thought to be reptiles. They were described at various times as batrachians, stegocephalians, and labyrinthodonts, although these names are now rarely used. Animals now grouped in Temnospondyli were spread out among several amphibian groups until the early 20th century, when they were found to belong to a distinct taxon based on the structure of their vertebrae. Temnospondyli means "cut vertebrae", as each vertebra is divided into several parts.
Experts disagree over whether temnospondyls were ancestral to modern amphibians (frogs, salamanders, and caecilians), or whether the whole group died out without leaving any descendants. Different hypotheses have placed modern amphibians as the descendants of temnospondyls, another group of early tetrapods called lepospondyls, or even as descendants of both groups (with caecilians evolving from lepospondyls and frogs and salamanders evolving from temnospondyls). Recent studies place a family of temnospondyls called the amphibamids as the closest relatives of modern amphibians. Similarities in teeth, skulls, and hearing structures link the two groups.
Description
Many temnospondyls are much larger than living amphibians, and superficially resemble crocodiles. Others are smaller and resemble salamanders.[1] Most have broad, flat heads that are either blunt (brevirostrine) or elongated (longirostrine). The skulls are rounded or triangular in shape when viewed from above, and are usually covered in pits and ridges. The rugged surfaces of bones may have supported blood vessels, which could transfer carbon dioxide to the bones to neutralize acidic build up in the blood (early semiaquatic tetrapods would have had difficulty expelling carbon dioxide from their bodies while on land, and these dermal bones may have been an early solution to the problem).[2] Many temnospondyls also have canal-like grooves in their skulls called sensory sulci. The sulci, which usually run around the nostrils and eye sockets, are part of a lateral line system used to detect vibrations in water.[1] As semiaquatic animals, all known temnospondyls have small limbs with no more than four toes on each front foot and five on each hind foot.[3] Terrestrial temnospondyls have larger, thicker limbs, and some even have claws.[4] One unusual terrestrial temnospondyl, Fayella, has relatively long limbs for its body, and probably lived as an active runner able to chase prey.[5]
Homologues of most of the bones of temnospondyls are also seen in other early tetrapods, aside from a few bones in the skull, such as interfrontals, internasals, and interparietals, that have developed in some temnospondyl taxa.[1] Most temnospondyls have tabular horns in the backs of their skulls, rounded projections of bone separated from the rest of the skull by indentations called otic notches; in some temnospondyls, such as Zatrachys, they are pointed and very prominent. Among the most distinguishing features of temnospondyls are the interpterygoid vacuities, two large holes in the back of the palate. Another pair of holes, choanae, are present in front of these vacuities, and connect the nasal passage with the mouth. Temnospondyls often have teeth on their palates, as well as in their jaws. Some of these teeth are so large, they are referred to as tusks. In some temnospondyls, such as Nigerpeton, tusks in the lower jaw pierce the palate and emerge through openings in the top of the skull.[6]
Very little is known of the soft tissue of temnospondyls. A block of sandstone, described in 2007 from the Early Carboniferous Mauch Chunk Formation of Pennsylvania, included impressions of the bodies of three temnospondyls. These impressions show, when alive, they had smooth skin, robust limbs with webbed feet, and a ridge of skin on their undersides.[7] Trackways referable to small temnospondyls have also been found in Carboniferous and Permian rocks. The trackways, called batrachichni, are usually found in strata deposited around freshwater environments, suggesting the animals had some ties to the water.[8]
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Unlike modern amphibians, many temnospondyls are covered in small, closely packed scales. The undersides of most temnospondyls are covered in rows of large ventral plates. During early stages of development, they first have only small, rounded scales. Fossils show, as the animals grew, the scales on the undersides of their bodies developed into large, wide ventral plates. The plates overlap each other in a way that allows a wide range of flexibility. Later semiaquatic temnospondyls, such as trematosaurs and capitosaurs, have no evidence of scales. They may have lost scales to make movement easier under water or to allow cutaneous respiration, the absorption of oxygen through the skin.[9]
Several groups of temnospondyls have large bony plates on their backs. One temnospondyl, Peltobatrachus, has armour-like plating that covers both its back and underside.[10] The temnospondyl Laidleria also has extensive plating on its back. Most members of the family Dissorophidae also have armor, although it only covers the midline of the back with two narrow rows of plates.[11] Other temnospondyls, such as Eryops, have been found with small, disc-like bony scutes that were in life probably embedded in the skin. All of these temnospondyls were adapted to a terrestrial lifestyle. Armor may have offered protection from predators in the case of Peltobatrachus.[10] The scutes may have provided stability for the spine, as they would have limited flexibility and may have been connected by strong ligaments.[12] Temnospondyls, such as Sclerothorax and Eryops, that may have been at least partly terrestrial also have long neural spines on top of their vertebrae that would have stabilized the spine.[13] Bony scutes are also seen in plagiosaurs, but unlike Peltobatrachus, Laidleria, Eryops, and dissorophids, these animals are thought to have been fully aquatic. Plagiosaurs may have inherited their armor from a terrestrial ancestor, as both Peltobatrachus and Laidleria have been considered close relatives of the group.[10]
Temnospondyls' vertebrae are divided into several segments. In living tetrapods, the main body of the vertebra is a single piece of bone called the centrum, but in temnospondyls, this region was divided into a pleurocentrum and intercentrum. Two types of vertebrae are recognized in temnospondyls: stereospondylous and rhachitomous vertebrae. In rhachitomous vertebrae, the intercentra are large and wedge-shaped, and the pleurocentra are relatively small blocks that fit between them. Both elements support a spine-like neural arch, and well-developed interlocking projections called zygapophyses strengthen the connections between vertebrae. The strong backbone and strong limbs of many ratchitomous temnospondyls allowed them to be partially, and in some cases fully, terrestrial. In stereospondylous vertebrae, the pleurocentra have been lost entirely, with the intercentra enlarged as the main body of the vertebrae. This weaker type of backbone indicates that stereospondylous temnospondyls spent more time in water.[14]
History of study
Temnospondyli was named by the German palaeontologist Karl Alfred von Zittel in his second edition of Handbuch der Palaeontologie, published in 1888. However, temnospondyl remains have been known since the early part of the 19th century. The earliest described temnospondyl was Mastodonsaurus, named by Georg Friedrich Jaeger in 1828 from a single tooth that he considered to belong to a reptile. Mastodonsaurus means "breast tooth lizard" after the nipple-like shape of the tip of the tooth.[15]
The naming of these first specimens was disputed. Leopold Fitzinger named the animal Batrachosaurus in 1837. In 1841, the English palaeontologist Richard Owen referred to the genus as Labyrinthodon to describe its highly folded or labyrinthine teeth. Owen thought that the name Mastodonsaurus "ought not to be retained, because it recalls unavoidably the idea of the mammalian genus Mastodon, or else a mammilloid form of the tooth... and because the second element of the word, saurus, indicates a false affinity, the remains belonging, not to the Saurian, but to the Batrachian order of Reptiles."[16] Owen recognized that the animal was not a "saurian" reptile,[a] yet he also referred Jaeger's Phytosaurus to the genus. Although the two genera have similarly sized conical teeth, Phytosaurus was later found to be a crocodile-like reptile. Additional material, including skulls, firmly placed Labyrinthodon as an amphibian. Jaeger also named Salamandroides giganteus in 1828, basing it on partial occiput, or back portion of the skull. In 1833, he described a complete skull of S. giganteus that had the same teeth as his Mastodonsaurus, making it the first known complete skull of a temnospondyl. Because Mastodonsaurus was named first, it has precedence over the other names as a senior subjective synonym.[17] Batrachosaurus is still used as the name of an unrelated brachyopid temnospondyl.
Mastodonsaurus and other similar animals were referred to as labyrinthodonts, named like Labyrinthodon for teeth that were highly folded in cross section. Owen's "Labyrinthodon Jaegeri" was later found at Guy's Cliffe, England by paleontologist William Buckland. Other specimens were found in the red sandstone of Warwickshire. As more fossils were uncovered in England, Owen depicted these labyrinthodonts as the "highest" form of batrachian and compared them to crocodiles, which he considered the highest form of reptiles. He also noted the large labyrinthodonts of the Keuper (a unit of rocks that dates to the Late Triassic) were younger than more advanced reptiles in the Magnesian and Zechstein, which are Late Permian in age. Owen used these fossils to counter the notion that reptiles evolved from a sequential progression from early amphibians (what he called "metamorphosed fishes").[18]
In addition to Mastodonsaurus, some of the earliest named genera included Metopias and Rhombopholis in 1842, Zygosaurus in 1848, Trematosaurus in 1849, Baphetes and Dendrerpeton in 1853, Capitosaurus in 1858, and Dasyceps in 1859.[19] Baphetes is now placed as an early tetrapod outside Temnospondyli, and Rhombopholis is now considered a prolacertiform reptile.[20][21]
Later in the 19th century, temnospondyls were classified as various members of Stegocephalia, a name coined by the American paleontologist Edward Drinker Cope in 1868. Cope placed stegocephalians in the class Batrachia, the name then used for Amphibia. Stegocephalia means "roof-headed" in Greek, a reference to the wide, flat heads of temnospondyls and other early tetrapods. During this time, palaeontologists considered temnospondyls to be amphibians because they possessed three main features: gill arches in juvenile skeletons, indicating they were amphibious for at least the first part of their lives; ribs that do not connect at the underside of the rib cage; and deep pits in the skull that were interpreted as space for mucous glands.[22]
Several suborders of stegocephalians were recognized in the late 19th and early 20th centuries. Animals now regarded as temnospondyls were primarily labyrinthodonts, but some were classified in the Branchiosauria. Branchiosaurs were small-bodied and had simple conical teeth, while labyrinthodonts were larger and had complex, folded dentin and enamel in their teeth. Branchiosauria included only a few forms, such as Branchiosaurus from Europe and Amphibamus from North America, that had poorly developed bones, external gills, and no ribs. Some skeletons of Amphibamus were later found with long ribs, prompting its reassignment to Microsauria (although more detailed studies found it to be a temnospondyl).[23] Soft tissue, such as scales and external gills, were found in many well-preserved branchiosaur fossils from Germany. In the early 20th century, branchiosaurs would be recognized as larval forms of temnospondyls lacking many of the typical features that define the group, and is no longer recognized as a distinct group.[24]
Other animals that would later be classified as temnospondyls were placed in a group called Ganocephala, which was characterized by plate-like skull bones, small limbs, fish-like scales, and branchial arches. Unlike labyrinthodonts, they did not have parietal foramina, small holes in their skulls behind their eye sockets. Archegosaurus, Dendrerpeton, Eryops and Trimerorhachis were placed in this group and were considered to be the most primitive members of Reptilia. Their rhachitomous vertebrae, notochord, and lack of occipital condyles (which attached the head to the neck) were features that were also shared with fishes. Thus, they were considered a link between early fishes and more advanced forms such as stegocephalians.[25]
Another group was called Microsauria by Cope in 1868. He classified Microsauria as a subgroup of Labyrinthodontia, placing many small, amphibian-like animals within it. Among them was Dendrerpeton, once placed in Ganocephala. Dendrerpeton was later placed as a labyrinthodont with other temnospondyls, but confusion existed for many years over the classification of small amphibians.[26]
By the end of the 19th century, most of what are today regarded as temnospondyls were placed in the suborder Labyrinthodonta. The American paleontologist Ermine Cowles Case called it Labyrinthodonta vera or "true labyrinthodonts".[27] The names Stegocephalia and Labyrinthodontia were used interchangeably to refer to the order in which it belonged. The labyrinthodontian suborders Microsauria and Branchiosauria, both of which contain temnospondyls, were distinct from Labyrinthodonta. Within Labyrinthodonta were the groups Rhachitomi, Labyrinthodonti, and Embolerimi. Members of Rhachitomi, such as Archegosaurus and Eryops, had rhachitomous vertebrae with enlarged intercentra that displaced the pleurocentra. Labyrinthodonti, such as Mastodonsaurus, Trematosaurus, and Micropholis, had lost their pleurocentra, and the intercentra made up the entire body of the vertebrae. Embolerimi had intercentra and pleurocentra that were of equal size. Embolomeres are now identified as reptiliomorphs distantly related to temnospondyls.
In 1888, von Zittel divided stegocephalians among three taxa: Lepospondyli, Temnospondyli, and Stereospondyli. He placed microsaurs in Lepospondyli, a group which he characterized as having simple, spool-shaped vertebral centra. Temnospondyli included forms with the centra divided into pleurocentra and intercentra. All members of Stereospondyli had amphicoelous centra composed only of the intercentra. Cope objected to von Zittel's classification, considering the vertebrae of lepospondyls and stereospondyls indistinguishable because each had a simple spool shape. He continued to use Ganocephala and Labyrinthodonta (which he alternatively referred to as Rhachitomi) to distinguish animals based on the absence or presence of occipital condyles.[28]
Temnospondyli became a commonly used name at the turn of the 20th century.[29] Paleontologists included both embolomeres and rhachitomes in the group. Cope's Ganocephala and Labyrinthodonta fell out of use. In 1919, British paleontologist D. M. S. Watson proposed that the evolutionary history of these large amphibians could be seen through changes in their vertebrae. Embolomerous forms in the Carboniferous graded into rhachitomous forms in the Permian, and finally into stereospondyls in the Triassic. More importantly, Watson began using the term Labyrinthodontia to refer to these groups.[30] The name Temnospondyli was rarely used in the decades that followed. Swedish paleontologist Gunnar Säve-Söderbergh removed embolomeres from the group, narrowing its scope to rhachitomes and stereospondyls. His classification of labyrinthodonts was based heavily on characteristics of the skull rather than the vertebrae.[29]
The American paleontologist Alfred Romer brought the name Temnospondyli back into use in the later 20th century. Säve-Söderbergh used the name Labyrinthodontia in a strict sense (sensu stricto) to refer to Rhachitomi and Stereospondyli, excluding Embolomeri. Romer agreed with this classification, but used the name Temnospondyli to avoid confusion with Labyrinthodontia in its wider sense (sensu lato). Unlike modern temnospondyl classification, however, Romer included the primitive Ichthyostegalia in the group.[29]
Evolutionary history
Carboniferous and Early Permian
Temnospondyls first appeared in the Early Carboniferous around 330 million years ago (Mya). During the Carboniferous, temnospondyls included basal medium-sized forms such as Dendrerpeton or large semiaquatic forms such as Cochleosaurus. Other, more derived temnospondyls, such as the amphibamids, were smaller and more terrestrial. They resembled salamanders, and some taxa, such as the genus Branchiosaurus, even retained external gills like the modern-day axolotl. During the latest Carboniferous and Early Permian around 300 Mya, several groups, such as the dissorophids and trematopids evolved strong, robust limbs and vertebrae and became adapted to life on land while others such as the eryopids, developed into large semiaquatic predators. The dvinosaurs, a group of small aquatic temnospondyls, evolved from terrestrial ancestors in the Late Carboniferous.[31]
Late Permian
During the Late Permian, increasing aridity and the diversification of reptiles contributed to a decline in terrestrial temnospondyls, but semiaquatic and fully aquatic temnospondyls continued to flourish, including the large Melosaurus of Eastern Europe. Other temnospondyls, such as archegosaurids, developed long snouts and a close similarity to crocodiles, although they lacked the armor characteristic of the latter group. These temnospondyls included the largest known batrachomorph, the 9-m-long Prionosuchus of Brazil.[32]
Mesozoic
As temnospondyls continued to flourish and diversify in the Late Permian (260.4 - 251.0 Mya), a major group called Stereospondyli became more dependent on life in the water. The vertebrae became weak,[33] the limbs small, and the skull large and flat, with the eyes facing upwards. During the Triassic period, these animals dominated the freshwater ecosystems, evolving in a range of both small and large forms. During the Early Triassic (251.0 - 245.0 Mya) one group of successful long-snouted fish-eaters, the trematosauroids, even adapted to a life in the sea, the only known batrachomorphs to do so with the exception of the modern crab-eating frog. Another group, the capitosauroids, included medium- and large-sized animals 2.3 to 4 m (7.5 to 13.1 ft) in length, with large and flat skulls that could be over a meter long in the largest forms such as Mastodonsaurus. These animals spent most or all their lives in water as aquatic predators, catching their prey by a sudden opening of the upper jaw and sucking in fish or other small animals.[34]
In the Carnian stage of the Late Triassic (228.0 - 216.5 Mya), capitosauroids were joined by the superficially very similar Metoposauridae. Metoposaurids are distinguished from capitosauroids by the positioning of their eye sockets near the front of their skulls. Another group of stereospondyls, the plagiosaurs, had wide heads and gills, and adapted to life at the bottom of lakes and rivers. By this time, temnospondyls had become a common and widespread component of semiaquatic ecosystems. Some temnospondyls, such as Cryobatrachus and Kryostega, even inhabited Antarctica, which was covered in temperate forests at the time.[35][36]
Triassic temnospondyls were often the dominant semiaquatic animals in their environments. Large assemblages of metoposaurs with hundreds of individuals preserved together have been found in the southwestern United States. They have often been interpreted as mass death events caused by droughts in floodplain environments. Recent studies show these dense assemblages were instead probably the result of currents accumulating dead individuals in certain areas. These environments seem to have had little diversity, as they were inhabited almost exclusively by metoposaurs.[37]
The Triassic-Jurassic extinction event around 199.6 Mya led to the extinction of most Mesozoic temnospondyls. The brachyopoids survived, as well as a few capitosauroids and trematosauroids. While the latter two groups soon became extinct, brachyopoids persisted and grew to large sizes during the Jurassic. Among brachyopoids, the brachyopids flourished in China and the chigutisaurids became common in Gondwana. The most recent known temnospondyl was the giant chigutisaurid Koolasuchus, known from the Early Cretaceous of Australia. It survived in rift valleys that were too cold in the winter for pseudosuchians that normally would have competed with them. Koolasuchus was one of the largest of the brachyopoids, with an estimated weight of 500 kg (1,100 lb).[38]
Classification
Originally, temnospondyls were classified according to the structure of their vertebrae. Early forms, with complex vertebrae consisting of a number of separate elements, were placed in the suborder Rachitomi, and large Triassic aquatic forms with simpler vertebrae were placed in the suborder Stereospondyli. With the recent growth of phylogenetics, this classification is no longer viable. The basic rhachitomous condition is found in many primitive tetrapods, and is not unique to one group of temnospondyls. Moreover, the distinction between rhachitomous and stereospondylous vertebrae is not entirely clear. Some temnospondyls have rhachitomous, semirhachitomous, and sterospondylous vertebrae at different points in the same vertebral column. Other taxa have intermediate morphologies that do not fit into any category. Rachitomi is no longer recognized as a group, but Stereospondyli is still considered valid.[39][40] Below is a simplified taxonomy of temnospondyls showing currently recognized groups:
Class Amphibia
- Order Temnospondyli
- Superfamily Edopoidea
- Family Cochleosauridae (Chenoprosopidae)
- Family Edopidae
- Family Dendrerpetontidae
- Suborder Euskelia
- Superfamily Dissorophoidea
- Family Amphibamidae
- Family Branchiosauridae
- Family Dissorophidae
- Family Micromelerpetontidae
- Superfamily Eryopoidea
- Family Eryopidae
- Family Parioxyidae
- Family Zatrachydidae
- Superfamily Dissorophoidea
- Clade Limnarchia
- Clade Stereospondylomorpha
- Superfamily Archegosauroidea
- Family Actinodontidae
- Family Archegosauridae
- Family Intasuchidae (placement is uncertain)
- Family Sclerocephalidae
- Suborder Stereospondyli
- Family Peltobatrachidae
- Family Lapillopsidae
- Family Rhinesuchidae
- Family Lydekkerinidae
- Clade Capitosauria
- Superfamily Mastodonsauroidea (Capitosauroidea)
- Family Heylerosauridae
- Family Mastodonsauridae
- Family Stenotosauridae
- Superfamily Mastodonsauroidea (Capitosauroidea)
- Infraorder Trematosauria
- Superfamily Trematosauroidea
- Superfamily Metoposauroidea
- Superfamily Plagiosauroidea
- Superfamily Brachyopoidea
- Superfamily Rhytidosteoidea
- Superfamily Archegosauroidea
- Clade Stereospondylomorpha
- Superfamily Edopoidea
Phylogeny
In one of the earliest phylogenetic analyses of the group, Gardiner (1983) recognized five characteristics that made Temnospondyli a clade: a bone at the back of the skull, the parasphenoid, is connected to another bone on the underside of the skull, the pterygoid; large openings called interpterygoid vacuities are present between the pterygoids; the stapes (a bone involved in hearing) is connected to the parasphenoid and projects upward; the cleithrum, a bone in the pectoral girdle, is thin; and part of the vertebra called the interdorsal attaches to the neural arch.[41] Additional features were given by Godfrey et al. (1987), including the contact between the postparietal and exoccipital at the back of the skull, small projections (uncinate processes) on the ribs, and a pelvic girdle with each side having a single iliac blade.[42] These shared characteristics are called synapomorphies.
Temnospondyls are placed as basal tetrapods in phylogenetic analyses, with their exact positioning varying between studies.[43] Depending on the classification of modern amphibians, they are either included in the crown group Tetrapoda or the stem of Tetrapoda. Crown-group tetrapods are descendants of the most recent common ancestor of all living tetrapods and stem tetrapods are forms that are outside the crown group. Modern amphibians have recently been suggested as descendants of temnospondyls, which would place them within crown Tetrapoda. Below is a cladogram from Ruta et al. (2003) placing Temnospondyli within crown Tetrapoda:[44]
| Tetrapoda |
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Other studies place modern amphibians as the descendants of lepospondyls and place temnospondyls in a more basal position within the stem of Tetrapoda. Below is a cladogram from Laurin and Reisz (1999) placing Temnospondyli outside crown Tetrapoda:[45]
| Tetrapoda |
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Most phylogenetic analyses of temnospondyl interrelationships focus on individual families. One of the first broad-scale studies of temnospondyl phylogeny was conducted by paleontologist Andrew Milner in 1990.[46] A 2007 study made a "supertree" of all temnospondyl families, combining the family-level trees of previous studies. The following cladogram is modified from Ruta et al. (2007):[47]
|
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1 Temnospondyli, 2 Edopoidea, 3 Dvinosauria, 4 Euskelia, 5 Eryopoidea, 6 Dissorophoidea, 7 Limnarchia, 8 Archegosauroidea, 9 Stereospondyli, 10 Rhytidostea, 11 Brachyopoidea, 12 Capitosauria, 13 Trematosauria, 14 Metoposauroidea
The most basal group of temnospondyls is the superfamily Edopoidea. Edopoids have several primitive or plesiomorphic features, including a single occipital condyle and a bone called the intertemporal that is absent in other temnospondyls. Edopoids include the Late Carboniferous genus Edops and the family Cochleosauridae. Dendrerpetontidae has also been included in Edopoidea, and is the oldest known temnospondyl family. Balanerpeton woodi is the oldest species, having been present over 330 million years ago during the Viséan stage of the Early Carboniferous. Recent analyses place Dendrerpetontidae outside Edopoidea in a more derived position.[48][49] Other primitive temnospondyls include Capetus and Iberospondylus. Saharastega and Nigerpeton, both described in 2005 from Niger, are also primitive yet come from the Late Permian. They are almost 40 million years younger than other basal temnospondyls, implying a long ghost lineage of species that are not yet known in the fossil record.[50]
In 2000, paleontologists Adam Yates and Anne Warren produced a revised phylogeny of more derived temnospondyls, naming several new clades.[40] Two major clades were Euskelia and Limnarchia. Euskelia includes the temnospondyls that were once called rhachitomes and includes two subfamilies, the Dissorophoidea and the Eryopoidea. Dissorophoids include small, mostly terrestrial temnospondyls that may be the ancestors of modern amphibians. Eryopoids include larger temnospondyls like Eryops. The second major clade, Limnarchia, includes most Mesozoic temnospondyls, as well as some Permian groups. Within Limnarchia are the superfamily Archegosauroidea and the most derived temnospondyls, the stereospondyls.
Yates and Warren also named Dvinosauria, a clade of small aquatic temnospondyls from the Carboniferous, Permian, and Triassic.[40] They placed Dvinosauria within Limnarchia, but more recent studies disagree on their position. For example, a 2007 study places them even more basal than euskelians, while a 2008 study keeps them as basal limnarchians.[47][51]
Within Stereospondyli, Yates and Warren erected two major clades: Capitosauria and Trematosauria. Capitosaurs include large semiaquatic temnospondyls like Mastodonsaurus with flat heads and eyes near the back of the skull. Trematosaurs include a diversity of temnospondyls, including large marine trematosauroids, aquatic plagiosaurs, brachyopoids that survived into the Cretaceous, and metoposauroids with eyes near the front of their heads. In 2000, paleontologists Rainer Schoch and Andrew Milner named a third major clade of stereospondyls, the Rhytidostea.[52] This group included more primitive stereospondyls that could not be placed in either Capitosauria or Trematosauria, and included groups like Lydekkerinidae, Rhytidosteidae, and Brachyopoidea. While Capitosauria and Trematosauria are still widely used, Rhytidostea is not often supported as a true clade in recent analyses. Rhytidosteids and brachyopoids are now grouped with trematosaurians, but lydekkerinids are still considered to be a primitive family of stereospondyls.[53][54]
A new phylogeny of temnospondyls was offered by paleontologist Rainer Schoch in 2013. It supported many of the clades that were found by Yates and Warren, but it did not find support for their division of derived stereospondyls into Euskelia and Limnarchia. Eryopids were found to be more closely related to stereospondyls than to dissorophoids, which were grouped with dvinosaurs. The clade including Eryopidae and Stereospondylomorpha was named Eryopiformes. In addition, Schoch named the clade containing all temnospondyls except edopoids Eutemnospondyli and reinstated the name Rhachitomi for the clade containing all temnospondyls except edopoids and dendrerpetontids. Below is the cladogram from Schoch's analysis:[55]
| Temnospondyli |
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Relationship to modern amphibians
Modern amphibians (frogs, salamanders, and caecilians) are classified in Lissamphibia. Lissamphibians appear to have arisen in the Permian. Molecular clock estimates place the first lissamphibian in the Late Carboniferous, but the first member of Batrachia (frogs and salamanders, but not caecilians) is estimated to have appeared in the Middle Permian using the same technique.[56][57]
Using fossil evidence, there are three main theories for the origin of modern amphibians. One is that they evolved from dissorophoid temnospondyls.[58][59] Another is that they evolved from lepospondyls, most likely the lysorophians.[60] A third hypothesis is that caecilians descended from lepospondyls and frogs and salamanders evolved from dissorophoids.[61]
Recently, the theory that temnospondyls were the ancestors of all lissamphibians has gained wide support. The skull morphology of some small temnospondyls has been compared to those of modern frogs and salamanders, but the presence of bicuspid, pedicellate teeth in small, paedomorphic or immature temnospondyls has been cited as the most convincing argument in favor of the temnospondyl origin of lissamphibians.[62] Seen in lissamphibians and many dissorophoid temnospondyls, pedicellate teeth have calcified tips and bases. During the development of most tetrapods, teeth begin to calcify at their tips. Calcification normally proceeds downward to the base of the tooth, but calcification from the tip stops abruptly in pedicellate teeth. Calcification resumes at the base, leaving an area in the center of the tooth uncalcified.[63] This pattern is seen in living amphibians and fossils.
The dissorophoid family Amphibamidae is thought to be most closely related to Lissamphibia. In 2008, an amphibamid called Gerobatrachus hottoni was named from Texas and was nicknamed the "frogamander" for its frog-like head and salamander-like body. It was thought to be the most closely related temnospondyl to lissamphibians and was placed as the sister taxon of the group in a phylogenetic analysis. Another species of amphibamid called Doleserpeton annectens is now thought to be even more closely related to lissamphibians. Unlike Gerobatrachus, Doleserpeton was known since 1969, and the presence of pedicellate teeth in its jaws has led some paleontologists to conclude soon after its naming that it was a relative of modern amphibians. It was first described as a "protolissamphibian", and the specific name annectens means "connecting" in reference to its inferred transitional position between temnospondyls and lissamphibians.[62] The structure of its tympanum, a disk-like membrane that functions like an ear drum, is similar to that of frogs and has also been used as evidence for a close relationship.[64][65] Other features including the shape of the palate and the back of the skull, the short ribs, and the smooth skull surface also point to it being a closer relative of lissamphibians than is Gerobatrachus. Below is a cladogram modified from Sigurdsen and Bolt (2010) showing the relationships of Gerobatrachus, Doleserpeton, and Lissamphibia:[66]
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Chinlestegophis, a putative Triassic stereospondyl considered to be related to metoposauroids such as Rileymillerus, has been noted to share many features with caecilians, a living group of legless burrowing amphibians. If Chinlestegophis is indeed both an advanced stereospondyl and a relative of caecilians, this means that although all lissamphibians are descended from temnospondyls, the different groups would have descended from different branches of the temnospondyl family tree. Anurans and urodelans would therefore be surviving dissorophoids while apodans (caecilians) are surviving stereospondyls.[67]
Paleobiology
Metabolism and gas exchange
A study on the fully aquatic Archegosaurus shows that its heat balance, gas exchange, osmoregulation, and digestion were more similar to those of fish than those of modern aquatic amphibians like salamanders.[68]
Feeding
Although the earliest temnospondyls were primarily semiaquatic, they had the ability to feed on land. Later, eryopoids and dissorophoids, some well adapted to terrestrial life, also fed on land. Some eryopoids became better adapted toward life in water and shifted their diets toward aquatic organisms. The first primarily aquatic feeders were archegosaurs in the Permian. Trematosaurs and capitosaurs became independently aquatic and also returned to this type of feeding.[69]
Most aquatic stereospondyls have flattened heads. When feeding, they probably opened their mouths by lifting their skulls instead of lowering their lower jaws. The jaw mechanics of the plagiosaurid Gerrothorax is well known, and is one of the most highly adapted. Gerrothorax is thought to have lifted its skull to around 50° above horizontal through the flexing of the atlanto-occipital joint between the occipital condyles of the skull and the atlas vertebra of the neck. As the skull is raised, the quadrate bone pushes forward and causes the lower jaw to protrude outward.[70] Other stereospondyls probably also lifted their skulls, but they are not as well adapted for such movement. D.M.S. Watson was the first to suggest skull lifting as a means of feeding in temnospondyls. He envisioned that Mastodonsaurus, a much larger temnospondyl than Gerrothorax, was able to make the same movement.[71][72] Paleontologist A.L. Panchen also supported the idea in 1959, suggesting that Batrachosuchus also fed in this way.[10] At the time it was thought that these temnospondyls lifted their heads with strong jaw muscles, but it is now thought that they used larger muscles in the neck that were attached to the large pectoral girdle. Plagiosuchus, a close relative of Gerrothorax, also has a hyobranchial skeleton that muscles may have attached to. Plagiosuchus has very small teeth and a large area for muscle attachment behind the skull, suggesting that it could suction feed by rapidly opening its mouth.[34]
Unlike semiaquatic temnospondyls, terrestrial temnospondyls have skulls that are adapted for biting land-living prey. The sutures between the bones of the skull in the dissorophoid Phonerpeton are able to withstand a high degree of compression. Compressive forces would have been experienced when biting down on prey.[73] Earlier aquatic tetrapods and tetrapod ancestors differ from temnospondyls like Phonerpeton in that their skulls were also built to withstand tension. This tension would have been experienced during suction feeding underwater. Temnospondyls like Phonerpeton were among the first tetrapods that were almost exclusively terrestrial and fed by biting.[74]
Reproduction
Temnospondyls, like all amphibians, reproduced in aquatic environments. Most temnospondyls probably reproduced through external fertilization. Like most living frogs, female temnospondyls would have laid masses of eggs in water while males released sperm to fertilize them. Several fossils were described from the Early Permian of Texas in 1998 that may be egg masses of dissorophoid temnospondyls. They were the first known fossils of amphibian eggs. The fossils consist of small disks with thin membranes that are probably vitelline membranes and halo-like areas surrounding them that are most likely mucous coatings. They are attached to plant fossils, suggesting that these temnospondyls laid eggs on aquatic plants much like modern frogs. The mucous membranes show that the eggs were laid by amphibians, not fish (their eggs lack mucous), but the type of amphibian that laid them cannot be known because no body fossils are preserved with the eggs. The eggs are thought to be from dissorophoids because they are likely to be close relatives of modern amphibians, and probably had similar reproductive strategies. They are also the most common amphibians from the deposit in which the eggs were found.[75]
One temnospondyl, the dvinosaur Trimerorhachis, may have brooded young in an area between the gills called the pharyngeal pouch. Small bones belonging to younger Trimerorhachis individuals have been found in these pouches. The living Darwin's Frog is also a mouth brooder and would be the closest modern analogue to Trimerorhachis if it cared for its young in this way. An alternative possibility is that Trimerorhachis was cannibalistic, eating its young like many amphibians do today. If this was the case, the bones of these smaller individuals were originally located in the throat and were pushed into the pharyngeal pouch as the animal fossilized.[76]
Body impressions of Early Carboniferous temnospondyls from Pennsylvania suggest that some terrestrial temnospondyls mated on land like some modern amphibians. They reproduced through internal fertilization rather than mating in water. The presence of three individuals in one block of sandstone shows that the temnospondyls were gregarious. The head of one individual rests under the tail of another in what may be a courtship display.[77] Internal fertilization and similar courtship behavior are seen in modern salamanders.[7]
Growth
While most types of temnospondyls are distinguished on the basis of features in mature specimens, several are known from juvenile and larval specimens. Metamorphosis is seen in dissorophoids, eryopids, and zatrachydids, with aquatic larvae developing into adults capable of living on land. Several types of dissorophoids, such as branchiosaurids, do not fully metamorphose, but retain features of juveniles such as external gills and small body size in what is known as neoteny.[78] Dvinosaurians and the plagiosaurid Gerrothorax also retained gills,[79] although recent studies found that (at least as adults) their gills were internal like those of fish, rather than external like those of salamanders.[80]
Temnospondyl larvae are often distinguished by poorly developed bones and the presence of a hyobranchial apparatus, a series of bones that gills would attach to in life. However, some fully mature temnospondyls also possess hyobranchial bones but did not have external gills.[81] A dense covering of scales is also seen in larvae and adults. Major body changes occur in metamorphosis, including the reshaping and strengthening of skull bones, the thickening of postcranial bones, and an increase in body size.
Temnospondyls like Sclerocephalus are known from both large adult specimens and small larvae, showing an extreme change in body shape. In these species, the shape and proportions of skull bones change in the early stages of development. The ornamentation on the surface of the skull roof also develops at this time. Small, regularly spaced pits are the first to form, followed by larger ridges. As development continues, the external gills disappear. Small teeth that once covered the palate are lost. The postcranial skeleton does not develop at the same rate as the skull, with ossification (the replacement of cartilage by bone) happening more slowly.[81] Vertebrae and limb bones are poorly developed, ribs and fingers are absent in the early stages, and the scapulocoracoid and ischium are entirely absent through most of development.[82] Once maturity is reached, most bones have fully formed and growth rate slows. The bones of some temnospondyls like Dutuitosaurus show growth marks, possibly an indication that growth rate varied with the change in seasons.[83] Fossils of temnospondyls like Metoposaurus and Cheliderpeton show that individuals grew larger past maturity. The oldest individuals usually have more pitting on their skulls with deeper sulci.[84]
One group of temnospondyls, the Branchiosauridae, is also known from larval specimens. Branchiosaurids like Branchiosaurus and Apateon are represented by many fossils preserving skin and external gills. An entire growth series is exhibited in the wide range of sizes among specimens, but the lack of terrestrially adapted adult forms suggests that these temnospondyls were neotenic. Unlike other temnospondyls, their postcranial skeletons developed quickly but were still partly cartilaginous when fully mature. Adults likely had an aquatic lifestyle similar to juveniles. Recently, large specimens of Apateon gracilis were described with adaptations toward a terrestrial lifestyle, indicating that not all branchiosaurs were neotenic.[81]
Studies of temnospondyl development have reached differing conclusions regarding what forms of gills were present in temnospondyls which possessed the organs. Although some species possessed external gills which were preserved as soft tissue, for many groups the type of gill can only be inferred from the structure of the bones which would have supported them. Scientists have disagreed on what the these bones imply. Scientists who compare temnospondyls to fish find that the bones correlate with internal gills, while those who compare them closely to salamanders consider the bones to correlate with external gills. This conundrum, known as Bystrow's paradox, has made it difficult to assess gills in temnospondyls.
Bystrow's paradox was finally resolved by a 2010 study. This study found that grooved ceratobrachnial structures (components of the branchial arches) are correlated with internal gills. Ancient tetrapods which preserved grooved ceratobranchials, such as the dvinosaur Dvinosaurus, probably only had internal gills as adults. Nevertheless, external gills are known to have been conclusively present in at least some temnospondyls. However, these situations only occur in larval specimens or members of specialized groups such as the branchiosaurids. One living species of lungfish (Lepidosiren) has external gills as larvae which transform into internal gills as adults. Despite adult dvinosaur specimens having skeletal features correlated with internal gills, some larval specimens of another dvinosaur, Isodectes preserved soft tissue external gills. Thus, the gill development of dvinosaurs (and presumably other temnospondyls) mirrored that of Lepidosiren. Despite this feature likely being an example of convergent evolution (as other lungfish exclusively possessed internal gills), it still remains a useful gauge for how temnospondyl gills developed. Thus, even temnospondyls which were aquatic and possessed gills as adults (such as dvinosaurs) only had external gills as larvae.[80]
While most temnospondyls are aquatic in early stages of life, most metoposaurids appear to have been terrestrial in their juvenile stage. Like other Mesozoic temnospondyls, adult metoposaurids were adapted to a semiaquatic lifestyle. Their bones are not highly developed for movement on land. The cross-sectional thickness of limb bones in adult metoposaurids shows that they could not withstand the stress of terrestrial locomotion. Juvenile individuals have bones that are thick enough to withstand this stress, and could probably move about on land. To maintain a terrestrial lifestyle, a temnospondyl's limb bones would have to thicken with positive allometry, meaning that they would grow at a greater rate than the rest of the body. This is not the case in metoposaurids, meaning that as their bodies grew larger they became less adapted toward a terrestrial lifestyle.[85]
Hearing
Temnospondyls and other early tetrapods have rounded otic notches in the back of the skull that project into the cheek region. In life, the otic notch would have been covered by a membrane called the tympanum, which is seen as a disk-like area in living frogs. The tympanum is involved in hearing, and is similar to the ear drum of more advanced tetrapods. It was traditionally thought that the tympanum developed very early in tetrapod evolution as a hearing organ and progressed to form the eardrum of amniotes. Thus, temnospondyls possessed a hearing system supposedly ancestral to that of living amphibians and reptiles.[86]
Frogs and all other living tetrapods have a rod-like bone called the stapes that aids in hearing by transferring vibrations from the ear drum —or homologous tympanum— to the inner ear. Temnospondyls also have a stapes, which projects into the otic cavity. The stapes likely evolved from the hyomandibula of lobe-finned fishes. The positioning of the stapes and the shape of the otic region suggests that the tympani of temnospondyls and frogs are homologous, but the tympani of these amphibians are no longer considered homologous with the hearing systems of reptiles, birds, and mammals. Therefore, ear structures in temnospondyls were not ancestral to those of all other tetrapods.[86]
The ability of the tympanum and stapes to effectively transmit vibrations is called impedance matching. Early tetrapods like temnospondyls have thick stapes with poor impedance matching, so it is now thought that they were not used for hearing. Instead, these thick stapes may have functioned to support the tissue that covers the otic notch.[42] Early temnospondyls, like Dendrerpeton, could not hear airborne sound but would have been able to detect vibration in the ground.[87] Later temnospondyls like Doleserpeton had otic regions adapted to hearing. Doleserpeton has a structure in the inner ear called the perilymphatic duct, which is also seen in frogs and is associated with hearing. Its stapes is also a better transmitter of sound. The hearing system of Doleserpeton and related temnospondyls was able to detect airborne sound and may have been ancestral to that of living amphibians.[64][65]
Notes
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External links
- Temnospondyli Tree of Life project page on temnospondyls.
- Temnospondyli Palaeos page on temnospondyls.
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