Temnospondyls the early years (part II)

At last, I fulfill those promises of more temnospondyls. Last time we looked at....

  At last, I fulfill those promises of more temnospondyls. Last time we looked at the edopoids, perhaps the most basal temnospondyl clade: here we look at the rest of the basal forms. Scary predators, marine piscivores, late-surviving relics, and some unfortunate beasts burned alive in forest fires...Studies on temnospondyl phylogeny mostly agree that 'post-edopoid' temnospondyls form a clade, the most basal members of which include Capetus, Dendrerpeton and Balanerpeton (Milner & Sequeira 1994, 1998, Holmes et al. 1998, Ruta et al. 2003a, b) [though some workers have found some of these taxa to be more basal than edopoids (Steyer et al. 2006)]. In contrast to the condition in edopoids, the interpterygoid vacuities of 'post-edopoids' are rounded at their anterior ends, the jugal (the bone that forms the cheek region) is shortened, and the configuration of skull bones is overall less archaic and fish-like. Among these basal 'post-edopoids', Dendrerpeton (from the Upper Carboniferous of Nova Scotia and Ireland) had a rather large skull with laterally facing orbits, a short body, and well-developed, robust limbs. It lacked lateral line canals and grew to c. 1 m. These features suggest that it was predominantly terrestrial (Holmes et al. 1998). Several species have been named (Milner 1996). Studies of a well-preserved, three-dimensional ear indicate that Dendrerpeton had a frog-like tympanum (ear drum) suited for the perception of airborne sounds (Robinson et al. 2005). This discovery provides support for the idea that lissamphibians descend from temnospondyls (a subject we'll come back to much later), and indicates that at least some basal temnospondyls were listening to noises. But what were they listening to? Were they vocalizing, perhaps during the breeding season? It's previously been argued that the temnospondyl stapes was too massive to support a tympanum (see Laurin & Soler-Gijón 2006), so note that we have to be cautious in inferring the presence of a tympanum in all temnospondyls.The Canadian Dendrerpeton specimens come from the famous Joggins Tree Stump Locality where the fossils of several tetrapod taxa (and the earliest land snails) have been discovered inside the hollowed trunks of lycopsid trees. It used to be thought that the animals had fallen into these natural traps and eventually died there of starvation, but the presence of abundant charcoal within the deposits now raises the possibility that the animals were taking refuge from forest fires, and that at least some of these unfortunate creatures were roasted alive (Falcon-Lang 1999, Scott 2001) [the adjacent picture depicts the early reptile Hylonomus, trapped in a hollow tree stump and about to be killed as a forest fire advances overhead].Another basal temnospondyl is Balanerpeton woodi from the Viséan of East Kirkton in Scotland, a famous locality that has yielded a phenomenal diversity of Carboniferous invertebrates, fishes and early tetrapods. With a length of about 50 cm, Balanerpeton was superficially like a big salamander but it was odd in that, while the 40-42 teeth lining each half of the upper jaw were small, the 25-30 in each half of the lower jaw were much longer. The absence of lateral line canals, ossified wrist and ankle bones, and the apparent presence of eyelids indicate that it also was a terrestrial animal, although its larvae were aquatic (Milner & Sequeira 1994) [Balanerpeton skeletal reconstruction and life restoration immediately below is from Milner & Sequeira (1994), and borrowed from here].Also somewhere around the base of Temnospondyli was Capetus palustris from the famous Upper Carboniferous Nýrany* desposits of the Czech Republic: this is one of the richest Upper Carboniferous fossil sites in the world, yielding at least 700 tetrapod fossils, discovered from the 1870s onwards. Previously, Capetus was regarded by some authors as an edopoid, close either to Edops or to the cochleosaurids, but recent studies have shown that it is not an * There is meant to be a caron (wedge or inverted circumflex) on the 'r' in Nýrany, but I cannot find the html code for this symbol.Capetus was a fairly scary looking, broad-headed predator with a deep posterior lower jaw: its skull was about 40 cm long, suggesting a total length of c. 1.5 m. As Sequeira & Milner (1993) noted, this makes Capetus one of the largest tetrapods in the Nýrany assemblage. Its teeth were mostly subconical, but those in the anterior part of the lower jaw seem to have been slightly laterally compressed, and possibly with weakly developed keels. As is the case in edopoids and some other basal temnospondyls, its skull bones lack lateral line canals. Sequeira & Milner (1993) suggested that Capetus was an alligator-like amphibious predator specializing on slow-moving tetrapod prey, and that it exploited a different lifestyle from that pursued by its contemporaries, the cochleosaurid Cochleosaurus, and the baphetids Baphetus and Megalocephalus. It was apparently rare in the fauna, with only eight fossils out of 700 from the Nýrany assemblage belonging to this taxon [the life restoration at the top of the article depicts Capetus. As is always the case with Palaeozoic tetrapods, life restorations of the animals being discussed here are few and far between (see previous lamentations on aetosaurs), so I had to resort to knocking one up myself. It's not bad, but it's not good either. Freely available for use, so long as Tet Zoo is credited].Among the most surprising of the basal temnospondyls is the recently described Saharastega moradiensis from Niger. The big deal is that - while Saharastega is apparently way down near the base of Temnospondyli (and hence close to the Carboniferous taxa Balanerpeton and Capetus) - is it from the Upper Permian Moradi Formation, and hence was very much a 'late-survivor', hanging on for long, long after other basal forms had bit the dust. As we saw in the edopoid article, the Moradi Formation seems to contain a strongly provincial, relictual fauna. Saharastega had a fairly nondescript, flattish and subtriangular skull with widely separated and laterally-facing orbits located close to the skull margins. The tabular horn - a pointed projection, growing from the tabular bone at the rear corner of the skull - was particularly odd in this taxon in being directed laterally, rather than posteriorly, and the jaw joint was positioned unusually anteriorly. These peculiarities suggest that Saharastega was doing something interesting, but we don't know what that was. Although originally argued to belong to Edopoidea (Sidor et al. 2005), restudy has shown it to be outside of this clade (Steyer et al. 2006) [adjacent image shows Christian Sidor with Saharastega skull].Another basal temnospondyl, the Upper Carboniferous Spanish taxon Iberospondylus schultzei, is interesting in that it was discovered in sediments deposited in coastal marine waters (Laurin & Soler-Gijón 2001, 2006). In contrast to the taxa we've looked at so far, it possesses lateral line canals, and therefore was very likely to have been aquatic (though we'll return later to how reliable lateral line canals are in demonstrating aquatic habits). Furthermore, the articulated condition of one of the specimens indicates that little post-mortem transport had occurred, so Iberospondylus was local to the environment in which it was preserved. It's been known for some time that at least some temnospondyls were marine animals (and we'll look at these other marine temnospondyls in a later post), but the basal position of Iberospondylus within temnospondyl phylogeny suggests that members of the group were able to inhabit the marine environment very early on in the group's history. As Laurin & Soler-Gijón (2001) argued, there are indications that this might have been true of even more basal tetrapods: if so, this would explain how Devonian tetrapods became near-globally distributed so early on in their evolution [Iberospondylus skull shown in adjacent image].This ends our look at the most basal members of Temnospondyli - or, at least, it does according to the phylogenetic schemes I've decided to follow. You will note that at least some of these animals, including the edopoids, Dendrerpeton and Balanerpeton, were apparently terrestrial or mostly terrestrial. It is inferred that a total length of perhaps 40 cm or so was primitive for the group, but a size of 1.5 m or more was evolved within Edopoidea, and also exhibited by broad-skulled Capetus. Some basal temnospondyls were aquatic, and even marine, and some hung on until as late as the Late Permian: about 40 million years longer than we'd thought prior to 2005.Later, more 'advanced' temnospondyls can be imagined to form a 'higher temnospondyl' clade and, when we come back to temnospondyls in the future, it's members of these groups that we'll be looking at.Falcon-Lang, H. J. 1999.