Crassigyrinus

Crassigyrinus scoticus  is an aquatic stem-tetrapod from the Carboniferous period of Scotland, notable for combining bizarre specializations such as tiny forelimbs, with a number of primitive features. These have given rise to arguments about its taxonomic status. Not only is it taxonomically bizzare, it must also have been a remarkable and formibable animal when alive.

Appearance
Crassigyrinus is one of those Carboniferous creatures that throws a wrench into the carefully planned taxonomies of paleontologists: this bizarre tetrapod had small, thin, but noticeable hind limbs and vestigial forelimbs, making it look a bit like an elongated frog (or a giant tadpole, hence its Greek name). This eel-like creature seems to have spent most, if not all, of its time in the water, and its double rows of sharp teeth point to an unmistakably carnivorous diet. Experts still don't know where to assign Crassigyrinus on the tetrapod evolutionary tree, if it in fact belongs there at all!

Behaviour
Crassigyrinus was a large, long-bodied, permanently aquatic predator, with fearsome-looking teeth in a heavily reinforced skull. The snout in particular was consolidated and buttressed, and with a kinetic inertial jaw mechanism, would have produced a bone-smashing snap-trap. Its large eyes were probably adapted for use in murky coal-swampy water. Behaviourally, it was somewhat similar to a Moray eel. It was aggressive, as shown when one attacked Nigel.

In Prehistoric Park
On site, Nigel wades through a swamp. Something big moves about underwater and makes bubbles. As Nigel makes a move to catch a Meganeura, something in the water bites his right ankle. He says "Animal bites for us wildlife folks are just a badge of courage.

Later, in the water he sees a big amphibian. He passes the net with the Meganeura in to a companion and swims underwater (without a diving mask) and catches the amphibian after a struggle, as it is very strong and slippery. He shows that it is an underwater ambush predator. It has two rows of teeth in its upper jaw and one in its lower jaw. He sees that it is a Crassigyrinus, whose fossils have only been found in Scotland; he nicknames it a "swamp monster" as it has no common name. That is what bit his ankle earlier. He has to let it go, as he has no way to transport it safely.