Temnospondyls the early years (part I)

Like plethodontid salamanders, Wealden dinosaurs, and rhinogradentians, the remarkably successful and diverse tetrapods known as temnospondyls have been riding the Tet Zoo wagon right since the earliest posts of ver 1. But, to my shame, I've never gotten round to completing one of the ten or so posts that I plan to publish on them. If you're interested in tetrapod evolutionary history and haven't heard of temnospondyls before, it's time to get learning, as they were one of the most diverse, abundant and ecologically significant tetrapod groups of the Palaeozoic and early Mesozoic [adjacent pic shows random compilation of different temnospondyls, including the palate of the immense Brazilian archegosaurid Prionosuchus plummeri]. Furthermore, they survive to the present: if, that is, you agree with the conclusions of some experts...

It's easy to become frustrated with the fact that people aren't more familiar with the amazing diversity of Palaeozoic life: most people think that vertebrate evolutionary history begins and ends with dinosaurs, mammoths and Neanderthals, and it's always been a problem that so few sources provide an adequate overview of the diversity that existed before the dinosaurs. As a child it bugged me that the same few Palaeozoic animals were featured in every single prehistoric animal book, as if they were the only ones that ever existed: the lepospondyl Diplocaulus, the anthracosaur Pholiderpeton (then known as Eogyrinus), the seymouriamorph Seymouria, the diadectomorph Diadectes, and that most famous of temnospondyls, Eryops. In reality, these are just the tips of many icebergs. To make matters worse, those few temnospondyls that feature in the popular or semi-technical literature are consistently portrayed inaccurately, and that includes good old Eryops: take Maurice Wilson's painting shown below - nice painting, but not really an accurate depiction (neither morphologically nor ecologically).

So where do you go if you want to learn about the full and wondrous diversity of temnospondyls, or indeed of any Palaeozoic tetrapods? That's a very good question, as there just aren't readily accessible texts devoted to these animals. Publishers only want dinosaurs. Dinosaurs dinosaurs bloody dinosaurs, although Cenozoic mammals and pterosaurs have also been the subjects of a few recent non-technical volumes. A major revision of temnospondyls has recently appeared: Schoch and Milner's Handbuch der Paläoherpetologie volume (Schoch & Milner 2000). However, it only covers the stereospondyls, an advanced temnospondyl sub-group. Furthermore, like all volumes in the handbuch series, it is horrendously expensive (I did at least, once, see a copy).

Anyway, enough of the preamble. In this and future posts I'll be providing a group-by-group overview of temnospondyls, concentrating on those groups and taxa that are particularly interesting in terms of palaeobiology, evolution or life appearance. I am, obviously, not attempting a full technical review or anything, and if you're seriously interested in that sort of thing it's mandatory that you get hold of Schoch & Milner (2000) as well as a ton of the primary literature. Temnospondyl phylogeny still has its grey patches and areas of argument, but several studies have been devoted to analyzing the relationships of large chunks of the temnospondyl tree (e.g. Yates & Warren 2000) and supertree studies devoted to the phylogeny of Palaeozoic tetrapods have also incorporated temnospondyls (Ruta et al. 2003a, b), so we do have agreement on the main branching order.

What are temnospondyls? They're a tetrapod clade (traditionally - and erroneously - lumped together with several other groups in an assemblage termed the labyrinthodonts), and they're perhaps, but perhaps not, ancestral to extant lissamphibians (we'll come back to that issue later). Their closest relatives appear to be the Carboniferous baphetids and colosteids, and the Carboniferous-Permian anthracosaurs, and if you want to know more about those groups I'm afraid you'll have to want for quite a bit longer. Characteristic temnospondyl features include particularly large openings on the palate (termed interpterygoid vacuities), palatal tusks, a diamond-shaped interclavicle, scapulae that possess ornamented surfaces, and distinctive vertebrae and pelvic bones. Their most distinctive features - the interpterygoid vacuities - remain of unknown function (so far as I can tell from the literature). Lateral line canals on the skull bones of some species suggest that they possessed neuromasts: the specialised cells that fish use to detect pressure changes in water. The absence or presence of lateral line canals in temnospondyls have therefore been widely used to help inform guesses about lifestyle, but as we'll see when we get to some groups this is not always reliable. Note that modern amphibians (like the giant salamander shown below) are atypical compared to temnospondyls and other Palaeozoic forms in being naked-skinned: with a few exceptions, you should imagine temnospondyls and their kin as scaly. Again, more on this in the future.

Which phylogenetic definition should be favoured for Temnospondyli is a bit tricky, and I want to avoid that issue for now. They include terrestrial, amphibious and wholly aquatic forms, brackish-water and marine forms, small generalist predators, large and formidable super-newts, giant small-limbed pseudo-crocodiles, frog-headed lurkers, the famous tusked toilet-bowl heads [like gigantic Mastodonsaurus giganteus, shown in image above. That skull is 60 cm long], sail-backed and armoured forms.. and others. The smallest were less than 30 cm long; the biggest may have approached 10 m in length (Cox & Hutchinson 1991). They are so diverse that it is difficult to pick a representative member. While many lineages become devoted to life in water, others were strongly, or entirely, terrestrial.

We begin our tour with taxa right down at the base of the group. These animals mostly lived in the Carboniferous (some survived to as late as the Late Permian), at a time when the southern continents were clustered together deep in the south and partly covered by an ice cap, and a conjoined Europe and North America were located on the equator.

Often regarded as the most primitive temnospondyl group is Edopoidea (previously known as Edopsoidea). Unlike more advanced kinds they exhibited an archaic pattern of palatal bones, and still possessed various additional bones at the back of the skull. Edopoids also had particularly big premaxillae (the bones that form the tip of the snout) and proportionally small external nostrils. Within the clade, the most basal member seems to be Edops from the Lower Permian of the USA, a broad-skulled animal with large palatal teeth. It was fairly big, at 2 m in length. Fragmentary remains from the Viséan (one of the early sections of the Carboniferous) of Scotland appear to come from Edops or a close relative and hence predate the type Edops material of the Permian.

Cochleosaurids were long-snouted edopoids, ranging in length from c. 30 cm to perhaps 3 m, known from swamp and lacustrine habitats of Nova Scotia, Ohio, the Czech Republic, Ireland and Africa. Lateral line canals are absent (with one exception: read on), so they are thought to have been mostly terrestrial (although it is known that they produced aquatic gilled larvae). While their skulls are decorated by a sculpturing of pits and furrows, a distinctive feature of the group is that a zone along the skull midline is only lightly sculptured (Sequeira 1996, 2004, Milner & Sequeira 1998). Cochleosaurids might have been amphibious predators and they have typically been imagined as rather crocodile-like in basic lifestyle [life restoration of Cochleosaurus above from here]. The broader-skulled forms were perhaps similar to Edops in being amphibious ambush predators of fish and smaller tetrapods, while some of the more narrow-snouted cochleosaurids, like the cochleosaurine Chenoprosopus milleri, may have foraged on land for arthropods and small tetrapods. However, even Edops is interpreted as terrestrial by some workers (Schoch 2001, p. 341).

Nigerpeton, the only African cochleosaurid, is closely related to Chenoprosopus but is in many ways a highly unusual member of the group. It was gigantic compared to some of its relatives, with a long, flattened skull 45-56 cm long, and its lateral line system contrasts with their absence in other edopoids. Like many temnospondyls, Nigerpeton possessed particularly large fang-like palatal teeth as well as enlarged teeth in the lower jaw, but the degree of heterodonty it possessed is extraordinary: the teeth at its premaxillary tips were large, and tooth size then declined posteriorly before increasing again in the maxilla, before decreasing again further posteriorly. Particularly big fangs - bigger than those lining the jaws - were present in patches on parts of the palate, while huge fangs near the lower jaw tip fitted through special openings in the skull roof when the mouth was closed [in the adjacent image, the big holes near the snout-tip are not the nostrils (those are much further back), but are instead the openings for the fangs of the lower jaw]. Protruding lower jaw teeth are also seen in a much later group of temnospondyls, the mastodonsauroids (and are also present in some living crocodilians). These specialized teeth suggest that Nigerpeton was an effective carnivore, presumably capable of grabbing large tetrapods (Steyer et al. 2006). While other cochleosaurids are Carboniferous and from the Northern Hemisphere, Nigerpeton is unusual in being from the Upper Permian of Niger. Discovered alongside captorhinids, pareiasaurs, and the late-surviving basal temnospondyl Saharastega, Nigerpeton provides further support for the idea that Late Permian west African was home to a strongly endemic fauna (Sidor et al. 2005, Steyer et al. 2006).

Edopoids - perhaps the most basal temnospondyl clade we know of - therefore included mid-sized terrestrial and amphibious taxa as well as late-surviving big-toothed macropredators. And - already - we have we stop as I need to go get a haircut. Much much more on temnospondyls to come later, honest.

Refs - -

Cox, C. B. & Hutchinson, P. 1991. Fishes and amphibians from the Late Permian Pedra de Fogo Formation of northern Brazil. Palaeontology 34, 561-573.

Milner, A. C. & Sequeira, S. E. K. 1998. A cochleosaurid temnospondyl amphibian from the Middle Pennsylvanian of Linton, Ohio, U.S.A. Zoological Journal of the Linnean Society 122, 261-290.

Ruta, M., Coates, M. I. & Quicke, D. L. J. 2003a. Early tetrapod relationships revisited. Biological Reviews 78, 251-345.

- ., Jeffery, J. & Coates, M. I. 2003b. A supertree of early tetrapods. Proceedings of the Royal Society of London B 270, 2507-2516.

Schoch, R. R. 2001. Can metamorphosis be recognised in Palaeozoic amphibians? Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 220, 335-367.

- . & Milner, A. C. 2000. Stereospondyli. Handuch der Paläoherpetologie, Teil 3B. Verlag Dr. Friedrich Pfeil, München.

Sequeira, S. E. K. 1996. A cochleosaurid amphibian from the Upper Carboniferous of Ireland. Special Papers in Palaeontology 52, 65-80.

- . 2004. The skull of Cochleosaurus bohemicus Fric, a temnospondyl from the Czech Republic (Upper Carboniferous) and cochleosaurid interrelationships. Transactions of the Royal Society of Edinburgh 94, 21-43.

Sidor, C. A., O'Keefe, F. R., Damiani, R., Steyer, J. S., Smith, R. M. H., Larsson, H. C. E., Sereno, P. C., Ide, O. & Maga, A. 2005. Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea. Nature 434, 886-889.

Steyer, J. S., Damiani, R., Sidor, C. A., O'Keefe, R., Larsson, H. C. E., Maga, A. & Ide, O. 2006. The vertebrate fauna of the Upper Permian of Niger. IV. Nigerpeton ricqlesi (Temnospondyli: Cochleosauridae), and the edopoid colonization of Gondwana. Journal of Vertebrate Paleontology 26, 18-28.

Yates, A. M. & A. A. Warren. 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.