Crassigyrinus, or... How I'd love a giant killer Carboniferous tadpole for a pet

Today I submitted another one of those long-delayed manuscripts. Yay. I also got to work preparing one of the three conference talks I'm supposed to be giving this year - how the hell I'm going to pull off all three I'm not sure. Anyway, leaving well alone the whole picture-of-the-day debacle, it's time for a proper post. Last time we looked at the edopoids, one of the most basal clades of temnospondyls, and in the next post I plan to write about some of the other basal temnospondyls.

Ever trying to recycling old text that sits, un-used, on disks, drives and memory-sticks, here I'm going to avoid temnospondyls, yet stay within the world of Palaeozoic tetrapods. We're going to look at one of the most peculiar and mysterious of tetrapods from its time: the bizarre Crassigyrinus scoticus - a large freshwater predator shaped something like a giant tadpole...

Crassigyrinus was a truly bizarre animal, known only with certainty from sediments deposited in ponds and lakes of Early Carboniferous Scotland (Godfrey (1988) reported a possible specimen from Greer, West Virginia). During the Carboniferous, Scotland was near to the equator and covered with swamps and lakes that were inhabited by diverse fishes and tetrapods. These included the giant swimming anthracosaur Proterogyrinus, snake-like aïstopods, and small, terrestrial temnospondyls. Large scorpions, eurypterids and millipedes were also present.

Crassigyrinus is all but absent from textbooks published prior to 1990, so you might think that it's a new discovery. But that's not so: the first good specimen (a partial skull) was collected in the 1850s by the famous Scottish geologist, journalist, social campaigner and folklorist Hugh Miller (1802-1856). This wasn't officially named and described until 1929 however when David M. S. Watson (1886-1973) described and named it: the specimen consisted only of the lower half of the skull, and Watson mistakenly thought that Crassigyrinus was shallow-skulled, hence the generic name he chose (Crassigyrinus means 'shallow wriggler', or something like that). During the 1970s and 80s, two additional specimens were discovered (the skull of one of these is shown, in dorsal view, in the adjacent image: borrowed from here). The most exciting of these was a nearly complete skeleton discovered near Fife by Stan Wood, a well known professional fossil collector. This specimen provided much new information on the body of this bizarre beast, revealing its tiny limbs and primitive spinal column for the first time. We also now know that the skull of Crassigyrinus, while long, was not really shallow.

Macromerium scoticum, named in 1890 for a lower jaw from the same locality as the more recently discovered Crassigyrinus specimens (Lydekker 1890), is now regarded as the type specimen of Crassigyrinus (Panchen 1985), despite previous arguments that it was quite different and more likely to belong to a baphetid (the generic name Macromerium is not available for Crassigyrinus as the type species belongs to an anthracosaur [I think]). Anyway, the reason that Crassigyrinus only appears in textbooks post-1990 is that 1990 is the year when the first good life restoration was published.

Crassigyrinus was long-bodied, with a large, blunt-nosed head equipped with huge eyes and a ferocious array of teeth. Thickened bony ridges ran along the dorsal midline of the snout and between the eyes, and it is inferred that they helped the skull to withstand stress when the animal bit prey. Its massive palatal teeth and wide gape (it could probably open its jaws as wide as 60º) further suggest that it was a powerful predator with a strong bite. Its peculiar stunted forelimbs were tiny and the humerus was only 35 mm long (the whole animal was about 1.5 m long). Various foramina on the humeral surfaces recall those seen in Ichthyostega, Acanthostega and lobed-finned fishes like Eusthenopteron (Panchen 1985, 1991). The hindlimbs were much larger than the forelimbs, and in the pelvis the ilium lacked a bony connection to the vertebral column (a classic feature of aquatic tetrapods). The tail is unknown but is assumed to have been long and laterally compressed.

Crassigyrinus has often been argued to combine a perplexing array of fish and tetrapod features, with the result being that its phylogenetic affinities have proved controversial. However, our views on how fishy it was have changed in recent years, as claims that it possessed several bones not normally seen in tetrapods - including a preopercular at the back of the skull and an anterior tectal and lateral rostral near the nostril - now seem erroneous (Clack 1998). The bones identified as anterior tectals and lateral rostrals seem instead to be misplaced, misinterpreted nasals and vomers.

It's not just the bones of the nasal region that have proved problematical, but the nostril and its supposed associated openings as well. Three Crassigyrinus skulls are now known, but for a long time there was just the Hugh Miller specimen, and this was always somewhat confusing as it seemed to have two nostril openings. Panchen (1967) argued that the lower of the two - the one closer to the edge of the mouth - was the true nostril, and he also thought that it was continuous with a groove that connected this opening with the jaw margin (which would then make this groove a nasolabial groove). However, he later regarded the lower opening as some sort of sensory structure (Panchen & Smithson 1990), and this explains why the most oft-reproduced life restoration of Crassigyrinus - an excellent stippled drawing produced by Michael Coates* - equips Crassigyrinus with a barbel located at the same place as the supposed sensory pit (see above for that pic: borrowed from here).

* Whom, thanks to Carl Zimmer, I always associate with Spot the dog (Zimmer 1998).

Alas, it was not to be. Clack (1998) described how further examination of both the original specimen and one of the new ones demonstrated how the supposed sensory pit was merely an artifact of preservation; not a real anatomical feature. No barbels in Crassigyrinus, no second nostril, no nasolabial groove [psst - want to know more about stem-group tetrapods? Then buy Clack (2002), shown in adjacent pic].

At the back of the skull, prominent notches just behind the eyes might once have been interpreted as having accommodated ear drums. However, it now seems that in many stem-group tetrapod lineages, these notches actually housed spiracles (small remnants of the gill system), and it is likely that this was their function in Crassigyrinus (Panchen 1985, Clack 2002).

The huge eye sockets of Crassigyrinus were diamond-shaped, and placed quite high up on the skull and close together. The inferred large size of the eyes suggests that it was able to see well in dark or murky water. Its cheek region bears canals that presumably housed a lateral line system and a distinctive sort of surface ornament on its skull bones suggested to Clack (1998) that it might have had weed-mimicking skin flaps like those seen on matamatas and wobbegongs. These have yet to be shown in any life restoration, with the exception of a yet-to-be-published piece that I advised [clue: it's in Tom Holtz's new dinosaur encyclopedia]. A bowl-shaped depression - the interpremaxillary fenestra - connected the dorsal surface of the snout tip with the palate, but it doesn't serve any obvious function except as an exit point for an enlarged dentary fang pair: another tetrapod where teeth from the lower jaw pierce the skull roof. Special notches on the lower jaws seem to have housed the enlarged palatal fangs.

The rest of the skeleton of Crassigyrinus is quite primitive in many details. Its vertebrae are simple and lack enlarged interlocking processes (zygapophyses), and the back of its skull does not have the distinct ball-shaped component called the occipital condyle. However, this is thought not to be a primitive character (because a properly formed occipital condyle is present in many fishes), but a paedomorphic one (Panchen 1991). The ribs, while well-developed, lack distinct facets for articulation with the vertebrae, and hence also appear primitive or degenerate compared to those of most other tetrapods. A consequence of this combination of features is that Crassigyrinus has even been regarded by some as the most primitive of tetrapods*, or even as a non-tetrapod: Panchen (1991) reported this (and contested it), noted that 'it is a tetrapod, contrary to the opinion of some of our colleagues' (p. 116). The idea that Crassigyrinus was a late-surviving relict was considered by Ahlberg & Milner (1994) who noted that 'Instead of being the first tetrapod to 'return to the water', [Crassigyrinus] may be the last survivor of the primitive tetrapods that never left the water' (p. 512).

* Incidentally, there are two different understandings of the term Tetrapoda: the conventional/traditional version - the one I use here at Tet Zoo - and the crown-group version, where Tetrapoda in the conventional sense is replaced by Stegocephali. Guess which I prefer. No time to discuss this now.

However, new evidence from other Carboniferous tetrapods, and new phylogenetic studies, have shown that Crassigyrinus is more likely to have been secondarily primitive, rather than a basal relict: it was fairly close to the base of Tetrapoda, but its alleged fish-like characters are either erroneous or secondarily reacquired. It also used to be argued (e.g. Panchen & Smithson 1988) that Crassigyrinus was close to the anthracosaurs and perhaps more closely related to amniotes* than to stem-group tetrapods, but the evidence used to support this is generally regarded as erroneous nowadays. Because of its uniqueness, Crassigyrinus has at times been given its own special new taxonomic groups: Panchen (1973) made it the sole member of a new 'family', Crassigyrinidae, within its own 'order', Palaeostegalia [adjacent life restoration by M. Collins borrowed from here. Note the barbel].

* Amniota is the tetrapod clade that includes synapsids and reptiles, and hence excludes all the tetrapods that used to be called 'amphibians'.

How did Crassigyrinus live? Compared by various authors to moray eels, amphiumas and sirenids, it perhaps lurked among waterweed and submerged branches, remaining concealed, partly thanks to its weed-like skin flaps. It would have detected prey with its huge eyes and lateral line system and, bursting from cover with undulations of its tail and body, it would have grabbed fishes and other animals with its deep, toothy skull. Large fangs allowed it to keep hold of slippery prey and it could open its jaws extremely wide to grab and swallow large prey. Crassigyrinus was probably a major predator of smaller vertebrates and may have been one of the dominant predators in its environment. Its proportionally tiny limbs indicate that it was unable to move on land. All in all - a pretty surreal Carboniferous beast, and I wish we knew more about it.

Finally.. what's with the picture at the top? I recently learnt that the Impossible Pictures series Prehistoric Park (which, like Primeval, kinda went under my radar) included an episode where Nigel Marven captured a live Crassigyrinus. Oh.

Refs - -

Ahlberg, P. E. & Milner, A. R. 1994. The origin and early diversification of tetrapods. Nature 368, 507-514.

Clack, J. A. 1998. The Scottish Carboniferous tetrapod Crassigyrinus scoticus (Lydekker) - cranial anatomy and relationships. Transactions of the Royal Society of Edinburgh: Earth Sciences 88, 127-142.

- . 2002. Gaining Ground: the Origin and Evolution of Tetrapods. Indiana University Press, Bloomington and Indianapolis.

Godfrey, S. J. 1988. Isolated tetrapod remains from the Carboniferous of West Virginia. Kirtlandia 43, 27-36.

Lydekker, R. 1890. On two new species of labyrinthodonts. Quarterly Journal of the Geological Society, London 46, 289-294.

Panchen, A. L. 1967. The nostrils of choanate fishes and early tetrapods. Biological Reviews 42, 374-420.

- . 1973. On Crassigyrinus scoticus Watson, a primitive amphibian from the Lower Carboniferous of Scotland. Palaeontology 16, 179-193.

- . 1985. On the amphibian Crassigyrinus scoticus Watson from the Carboniferous of Scotland. Philosophical Transactions of the Royal Society of London B 309, 505-568.

- . 1991. The early tetrapods: classification and the shapes of cladograms. In Schultze, H.-P. & Trueb, L. (eds) Origins of the Higher Groups of Tetrapods, Controversy and Consensus. Comstock/Cornell University Press (Ithaca and London), pp. 110-144.

- . & Smithson, T. R. 1988. The relationships of the earliest tetrapods. In Benton, M. J. (ed) The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Clarendon Press (Oxford), pp. 1-32.

- . & Smithson, T. R. 1990. The pelvic girdle and hind limb of Crassigyrinus scoticus (Lydekker) from the Scottish Carboniferous and the origin of the tetrapod pelvic skeleton. Transactions of the Royal Society of Edinburgh, Earth Sciences 81, 31-44.

Zimmer, C. 1998. At the Water's Edge: Macroevolution and the Transformation of Life. Free Press, New York.