April 16, 2013 | 33
Good news: I’ve decided to treat you all to yet another article on temnospondyls. This time we look at several (mostly) Permian groups – the sclerocephalids, archegosaurids and their relatives – that have sometimes (but not consistently) been grouped together as the Archegosauroidea or Archegosauriformes. As before, I have to note that there are strongly competing views on temnospondyl phylogeny. Archegosauroids have been included within Limnarchia by some (Yates & Warren 2000) and Euskelia by others (McHugh 2012). This group includes some of the most spectacular and gigantic of temnospondyls. Before talking about anything that might be interesting, however, we need to address some boring taxonomic stuff, sorry.
Archegosauroidea and Archegosauriformes are named for Archegosaurus decheni, named in 1847. However, the taxonomy of these temnospondyls is somewhat confusing. The names Actinodontidae Lydekker, 1885 and Sclerocephalidae Jaeckel, 1909 have both been used for the group that includes Sclerocephalus, one of the best known members of this assemblage. Actinodontidae is named for Actinodon: however, rather than being close to Sclerocephalus, Actinodon was argued by Werneburg & Steyer (1999) to be an eryopid (a temnospondyl group not covered here: I’ll get to them some other time). Sclerocephalidae is thus the correct name here. However, while some studies find Archegosaurus – and hence Archegosauridae – to be well separated in phylogenetic terms from Sclerocephalidae, others have found Archegosaurus to be closely related to the sclerocephalids Cheliderpeton and Sclerocephalus (Klembara & Steyer 2012, McHugh 2012), in which case Sclerocephalidae and Archegosauridae could be considered synonymous.
Ruta et al.’s (2007) temnospondyl phylogeny suggests that the best course of action for these animals might be to recognise Sclerocephalidae, Intasuchidae (for an Intasuchus + Cheliderpeton clade), Melosauridae, and Archegosauridae within Archegosauroidea/Archegosauriforms. This is actually pretty much the scheme used by Schoch & Milner (2000) (I think. I still haven’t seen this volume).
Archegosauroids are long-snouted to very long-snouted. The teeth in the posterior part of the premaxilla are larger than those in the maxilla; other distinctive features include the presence of temporal fossae and broad uncinate processes on the ribs. Numerous, well preserved specimens of Sclerocephalus have been described (most from Germany); members of the group are also ‘famous’ due to the enormous size and massively elongate snouts of some species. One in particular has been mentioned in such august publications as the Guinness Book of Records.
Thanks to those numerous German fossils, we have good information on the palaeobiology and ontogeny of Sclerocephalus in particular [image above by Günter Bechly]. Juveniles, less than 100 mm in total length, had external gills, poorly ossified limbs and a tail accounting for about half of the total length. They were apparently slender-bodied active hunters that preyed on crustaceans and plankton. The gills had been lost by the time the animals had reached 300 mm in length. Individuals became morphologically adult at a total length of about 700 mm, and at this time their tail, though still rudder-like, became proportionally shorter and their ribs and vertebrae became more ossified (Lohmann & Sachs 2001).
Only in ‘late adults’ – specimens in which the skull alone might be 200 mm long – were all the bones fully ossified. The lateral line canals present in young specimens became shallower and eventually disappeared in adults, and while this might indicate that they had lost their sensory organs and made the full transition to terrestrial life, the possibility that the sensory organs had migrated out of the periphery of the bone has been raised (Schoch 2001). If this is true then absence of lateral line canals doesn’t necessarily imply loss of the associated sensory organs, and indeed some temnospondyls with a morphology apparently well suited for aquatic life lack lateral line canals entirely.
While juvenile sclerocephalids are known from sediments deposited in large, deep lakes, adults are unknown from such locations and probably only visited them to breed: their well ossified limbs suggest that they were amphibious, and it might be that they were predators of shallow ponds and lakes and frequently moved from one water body to the next. Stomach contents show that adult individuals of Sclerocephalus were predominantly piscivorous, but we also know that they sometimes ate micromelerpetontids, and even other Sclerocephalus individuals (Lohmann & Sachs 2001). In some places Sclerocephalus lived alongside archegosaurids.
Incidentally, the Sclerocephalus species S. haeuseri is unusual among fossil tetrapods in having been divided into two subspecies: S. h. haeuseri and S. h. jeckenbachensis (Boy 1988). This is not without precedent among temnospondyls, as Sulej (2002) split the metoposaurid species Metoposaurus diagnosticus into two subspecies as well (M. d. diagnosticus and M. d. krasiejowensis). Nameable subdivisions must, of course, exist among fossil species just as they do in living ones; it’s just that our convention has also been to erect new species when sufficient differences are found. As usual, it always comes back to the argument over what the hell a ‘species’ is (still too few people realize that species concepts across animal groups are in no way comparable or equal).
Archegosaurus and the melosaurines
Archegosaurids are Permo-Carboniferous archegosauroids, initially described from Europe but later discovered in Russia and Brazil. The type taxon for the group, Archegosaurus decheni, is known from numerous specimens belonging to different growth stages. They show that the skull increased markedly in length during ontogeny, with adults (skull length as much as 280 mm) having slender, semi-spatulate snouts where the lateral margins (as seen from above) are slightly concave posterior to the nostrils. In contrast, younger animals (skull length as short as 18 mm) have blunter, shorter snouts without the semi-spatulate form (Witzmann 2006a, Witzmann & Scholz 2007). Ossification is substantially delayed in Archegosaurus, with its scales, pectoral girdle bones and hyoid bones all ossifying much later in development than is normal for temnospondyls. This is almost certainly an evolutionary response to an aquatic lifestyle and hence an advanced feature, not a ‘primitive’ one (Witzmann 2006a).
A supposed second species of Archegosaurus, A. dyscriton, was originally named Memonomenos and thought for a while to be an anthracosaur or anthracosaur relative (and hence as nowhere close to temnospondyls in phylogenetic terms). Intriguingly, A. dyscriton does not group together with A. decheni in recent phylogenies (Ruta et al. 2007) and hence the name Memonomenos may warrant resurrection.
A mostly Russian archegosauroid group termed Melosaurinae includes taxa that possess snouts where the anterior edges of the premaxillae are far broader than those of Archegosaurus. They also possess the ‘pinched-in’ part of the snout just posterior to the nostrils.
Melosaurus Meyer, 1857, Konzhukovia Gubin, 1991, Tryphosuchus Konzhukova, 1955, Koinia Gubin, 1993 and Uralosuchus Gubin, 1994 are all members of this group, though an alternative classification scheme where Melosauridae is recognized as a ‘family’ that contains a Melosaurinae and a Tryphosuchinae is used by some (e.g., Golubev 1995, Schoch & Milner 2000).
The largest melosaurines were about 3 m long. They seem to have been aquatic ambush predators of smaller temnospondyls and other prey, though I wonder if that semi-duckbilled snout shape played some unusual role in foraging or prey capture.
I’ve discussed Archegosaurus together with the melosaurines here since both groups are ‘short-snouted’ compared to the gharial-snouted archegosaurids we’re about to look at. However, Ruta et al. (2007) found a sister-group relationship between Archegosaurus and the gharial-snouted forms; Melosaurinae being the sister-group to this clade.
While the members of several temnospondyl groups – such as the trematosaurids looked at previously and the rhinesuchoids – possess elongate jaws, some archegosaurids possess really elongate jaws where the snout is slender and gharial-like and the mandibular symphysis is strikingly long. Gubin (1991) united the gharial-snouted forms in the new group Platyoposaurinae.
This gharial-like condition in present in Platyoposaurus, Baschkirosaurus and the famous Prionosuchus, but the most extreme mandibular symphysis is that present in Bageherpeton longignathus from the Late Permian Rio do Rasto Formation of Brazil (Dias & Barbarena 2001): in this animal, even the precoronoid bones (ordinarily not involved in the symphysis) are elongated and form part of the symphysis. Another peculiarity of Bageherpeton is that the bone along the dorsal midline of the symphysis is raised upwards relative to the jaw edges (Dias & Barbarena 2001). This is presumably related in some way to prey capture, but your guess is as good as mine.
Associated skeletons of Platyoposaurus from Russia and Archegosaurus from Germany show that archegosaurids had long bodies and long tails, but rather small limbs, and they almost certainly swam by way of lateral undulations (Witzmann & Schoch 2006). Here’s an image I found online of what appears to be a reconstructed Platyoposaurus skeleton: unfortunately, I don’t know anything about where it comes from, who took the photo, or how much of it represents extrapolation or guesswork. Anybody know anything about it? And that tail is not proportionally long, despite what I just said.
One of these animals, Prionosuchus plummeri from Middle Permian Brazil, was enormous, with one specimen possessing a skull estimated at 1.6 m in length. Associated postcrania show that it was similar in overall body shape to smaller archegosaurids, and on the basis of this, combined with data from modern gharials (which seem to have been similarly proportioned to long-snouted archegosaurids), Cox & Hutchinson (1991) implied that Prionosuchus might have had a total length of around 10 metres (in which case it’s the world’s largest temnospondyl and – I think – largest non-amniote tetrapod). Long-snouted archegosaurids including Prionosuchus were almost certainly piscivores that caught prey by using rapid lateral snapping of their long jaws. That’s about all we know as goes the palaeobiology of these remarkable temnospondyls; at this stage, anything else is speculation.
I haven’t mentioned all of the archegosauroid taxa here, let alone discussed them all. But consider this a basic introduction to one of the most fascinating and remarkable of temnospondyl clades. Note that they are not especially close relatives of the similarly long-snouted trematosauroids we looked at recently – the similarity is convergent. Furthermore, there are other gharial-snouted temnospondyl groups too…
For previous Tet Zoo articles on temnospondyls, see…
Refs – -
Boy, J. A. 1988. Über einige Vertreter der Eryopidea (Amphibia : Temnospondyli) aus dem europäischen Rotliegend (?höchstes Karbon – Perm). 1. Sclerocephalus. Paläontologische Zeitschrift 62, 107-132.
Cox, C. B. & Hutchinson, P. 1991. Fishes and amphibians from the Late Permian Pedra de Fogo Formation of northern Brazil. Palaeontology 34, 561-573.
Dias, E. V. & Barbarena, M. C. 2001. A temnospondyl amphibian from the Rio do Rasto Formation, Upper Permian of southern Brazil. Anais da Academia Brasileira de Ciências 73, 135-143.
Golubev, V. K. 1995. New species of Melosaurus (Amphibia, Labyrinthodontia) from the Kazanian of the Kama River drainage Area. Paleontological Journal 29, 107-119.
Gubin. Y. M. 1991. (Permian archegosauroid amphibians of the USSR). Trudy Pal Inst Akad Nauk SSSR 249, 1-138.
Klembara, J. & Steyer, J. S. 2012. A new species of Sclerocephalus (Temnospondyli: Stereospondylomorpha) from the Early Permian of the Boskovice Basin (Czech Republic). Journal of Paleontology 86, 302-310.
Lohmann, U. & Sachs, S. 2001. Observations on the postcranial morphology, ontogeny and palaeobiology of Sclerocephalus haeuseri (Amphibia: Actinodontidae) from the Lower Permian of southwest Germany. Memoirs of the Queensland Museum 46, 771-781.
McHugh, J. B. 2012. Temnospondyl ontogeny and phylogeny, a window into terrestrial ecosystems during the Permian-Triassic mass extinction. University of Iowa, dissertation (available at http://ir.uiowa.edu/etd/2942).
Price, L. I. 1948, Um anfibio Labirinthodonte da formacao Pedra de Fogo, Estado do Maranhao. Ministerio da Agricultura, Departamento Nacional da Producao ineral Divisao de Geologia e Mineralogia, Boletim 124, 7-32.
Ruta, M., Pisani, D., Lloyd, G. T. & Benton, M. J. 2007. A supertree of Temnospondyli: cladogenetic patterns in the most species-rich group of early tetrapods. Proceedings of the Royal Society of London B 274, 3087-3095.
Schoch, R. R. 2001. Can metamorphosis be recognised in Palaeozoic amphibians? Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 220, 335-367.
- . & Milner, A. R. 2000. Handbuch der Paläoherpetologie: Teil 3B, Stereospondyli. Pfeil, Munich.
Sulej, T. 2002. Species discrimination of the Late Triassic temnospondyl amphibian Metoposaurus diagnosticus. Acta Palaeontologica Polonica 47, 535-546.
Werneburg, R. & Steyer, J.-S. 1999. Redescription of the holotype of Actinodon frossardi (Amphibia, Temnospondyli) from the Lower Permian of the Autun basin (France). Geobios 32, 599-607.
Witzmann, F. 2006a. Developmental patterns and ossification sequence in the Permo-Carboniferous temnospondyl Archegosaurus decheni (Saar-Nahe Basin, Germany). Journal of Vertebrate Paleontology 26, 7-17.
- . 2006b. Cranial anatomy and ontogeny of the Permo-Carboniferous temnospondyl Archegosaurus decheni from the Saar-Nahe Basin, Germany. Transactions of the Royal Society of Edinburgh: Earth Sciences 96, 131-162.
- . & Schoch, R. R. 2006. The postcranium of Archegosaurus decheni and a phylogenetic analysis of temnospondyl postcrania. Palaeontology 49, 1211-1235.
- . & Scholz, H. 2007. Morphometric study of allometric skull growth in the temnospondyl Archegosaurus decheni from the Permian/Carboniferous of Germany. Geobios 40, 541-554.
Yates, A. M. & Warren, A. A. 2000. The phylogeny of the “higher” temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli. Zoological Journal of the Linnean Society 128, 77-121.