TETRAPODA |--+--LEPOSPONDYLI | `--REPTILOMORPHA | Temnospondyli |--Edopoidea `--+--Euskelia | `--LISSAMPHIBIA `--Limnarchia |--Dvinosauria `--Stereospondyli |--Rhinesuchidae `--+--Capitosauria `--Trematosauria |--Trematosauroidea `--+--Metoposauroidea `--+--Plagiosauroidea `--+--Rhytidosteidae `--Brachyopoide
Temnospondyls had, for the most part, relatively large heads, with rather flat, akinetic skulls. The shape of the skull was quite variable, and often distinctive. Some, like Archegosaurus, had the mouth drawn out into the classic "longirostrine" shape associated with reptiles which hunt small fish. Others, like Gerrothorax, had bizarre, wide, parabolic heads like galeaspids or other "cornuate" jawless fish. Some, Laidleria for example, had very flat, triangular heads with no easy parallels in any other vertebrate group, before or since.
In contrast, the temnospondyl post-cranial body plan was conservative and fairly unspecialized. The body proportions varied considerably from one group to another. As a result, we may suspect that some tended toward an eel-like style of swimming or an undulating, almost legless type of locomotion on land. Others had relatively stiff, massive bodies and presumably used their limbs for both land and water locomotion. However, no temnospondyl group developed any limb specializations as in the reptilomorph lineage. The temnospondyl fore-foot had only four toes, the hind-foot five. The vertebral structure was essentially of the primitive, intercentra and pleurocentra sort, as in the early Devonian tetrapods and osteolepiform lobe-finned fish. However there was also some variation in vertebral structure, a distinction being made between the ancestral, ratchitomous condition that characterized the majority of temnospondyls, and the more specialized sterospondylous condition of the later, aquatic, Triassic types.
|Early to late Carboniferous||Late Carboniferous-early Permian||Triassic|
The heyday of the Paleozoic temnospondyls was during the late Carboniferous and early Permian periods. During this time there was a rich diversity of these creatures, some with huge heads and armour plates, and adapted to a terrestrial existence, such as the Dissorophid Cacops, others with strong robust limbs and large heads, but still semi-aquatic and rather like a cross between a crocodilian and an enormous frog (Trematops, Eryops), others aquatic with long bodies and armoured with a protective coating of scales (Trimerorhachis) while others again retained external gills even as adults and were unable to leave the water (Branchiosaurus).
The increasing aridity of the late Permian world, brought about the single landmass (the Pangean supercontinent) and disruption of rainfall patterns, and the sucess of the reptiles, was not the best news for these large tetrapods, and they retreated to the swamps, ponds and river courses. Originally these Late Permian temnospondyls had a crocodile-like apperance and were probably still quite capable of climbing out on land (Actinodon, Archegosaurus, Platyoposaurus), but these transitional types were soon replaced by their descendents, the large to very large, completely aquatic stereospondyls. One lineage of stereospondyls, the Trematosauridae, actually took up a life in the oceans, the only stem tetrapods ever to have done so. The Trematosaurs underwent quite an evolutionary radiation, and the various genera are distinguished by the proportions and shape of their skulls.
During this same period, other Stereospondyl lines continued to thrive, but also evolved in more extreme directions. Because these creatures lived their entire lives in water (although they were probably able to climb out on land for short periods) their skeletal structure became increasingly cartiliginous, and their heads increasingly large and flat. Among the Capitosaurids and Mastodonsaurids the head became so large and flat in fact that it is difficult to see how the animals could have moved their lower jaws. Presumably, they only opened their mouths by titling back the whole head.
Both large and small aquatic Temnospondyls continued as an important part of the freshwater ecosystem right up until the end of the Triassic period, even co-existing with the formidable crocodile-like phytosaurid reptiles. Important at this time were the Metoposaurs of Laurasia (Europe and America), which evolved from completely different ancestors to look strikingly like the Capitosaurs. Indeed, the only easy distinction is that the Metoposaurs had eyes further forward on the skull.
The terminal Triassic extinction killed off all the big temnospondyls, along with the phytosaurs. Only a few, short-headed stragglers making it through to the Jurassic, the Brachyopoids. These survived in China and Australia (the easternmost parts of north and south Pangea respectively), perhaps protected by geographical isolation. Safe from enemies, they grew to be as big as Capitosaurs. The last temnospondyls were five meter long giants who sought refuge in the Cretaceous polar rift valleys of south-east Gondwana, where the climate was too cold to support crocodiles, their main predators.
Traditionally, there were only two temnospondyl suborders, the Ratchitomi and the Stereospondyli, which were determined according to vertebral type, and these are mentioned in the older books. Romer provided a third suborder, the fish-eating marine-going trematosaurs. Other paleo-tetrapod authorities such as Nilsson and Panchen removed the short-headed Peltobatrachidae and Plagiosauridae from the Brachyopoids and placed them in a seperate order. Later writers like Carroll and Benton did away with the ratchitome - sterospondyl classification altogether, arguing that the sterospondyls were an artificial (polyphyletic) group, the stereospondylous condition having evolved seperately a number of times.
However, the relationships between the different temnospondyl families, and sometimes even the question of which family goes in which superfamily, remained very controversial. Many of these problems in temnospondyl taxonomy were addressed by Yates & Warren (2000) in comprehensive cladistic analysis from which the following cladogram is taken.
In this diagram, the geological period is shown on the top. The known occurance of taxa (genera and families) in the fossil record is indicated by solid bars. Thin lines indicate hypothetical ancestry (cladistic techniques forbid deriving one known monophyletic genus or family from another, rather, both are derived from a hypothetical "most recent common ancestor", which gives the branching pattern as shown above).
Several things need to be said about this diagram. First, not every temnospondyl taxon is included. Many known types are not shown here. However, most of the important forms are shown. Secondly Yates and Warren revive the monophyletic Trematosauria and Stereospondyli, but replaced the Ratchitomi with the clades Euskelia and Limnarchia.