It is generally thought that the fishes found at Dura Den were fresh water lake dwellers. The large numbers found in so small an area, and their very close groupings suggest that they died as a result of the shallow lake drying up during a period of drought. It may be that more frequently occurring drought periods led to the necessity to be able to breathe air directly, and the subsequent development of lungs.
Holoptychius is the commonest fish found at Dura Den. It grew to just under one meter in length. Isolated scales from around the world show that it was widely dispersed, and capable of crossing the seas to invade new freshwater regions.
It was a plump, deep bodied fish with well developed teeth, small eyes and large nasal capsules. The shape of its body and the position of its fins suggest that it was an ambush predator which lay in wait for passing smaller fish, catching them with a quick forward lunge, rather like the pike does today. It had long, slender, trailing pectoral fins which would have been very flexible and mobile. It may have used them to propel itself along the lake floor by 'walking under water'.
Holoptychius was a crossopterigian - lobe-finned - fish, a group which was common in later Devonian times, from 390 - 450 million years ago. Today only one type survives: the coelacanth Latimeria which is found off Madagascar and Indonesia. However, crossopterygians are in the group which is thought to have given rise to amphibians and reptiles so they are, ultimately, our own ancestors.
Phaneropleuron was a lungfish. Modern lungfish can burrow and aestivate - same as hibernate, but in hot weather - in times of drought when pools dry up, until the rains come. However, it seems that this ability did not develop until Permian times, some 70 million years too late for this Phaneropleuron. Its lung was a modified swim bladder which allowed it to gulp air at the water surface for short periods. The lung developed at the same time when lungfish were moving into fresh water from the sea. This seems to have occurred in the middle Devonian period, about 380 million years ago.
It probably had tooth plates like many other lungfish. This would enable it to eat hard food such as shell fish, as well as softer things, like worms. Lungfish were very common in Devonian and Carboniferous times, but now only three species are known - one lives in Australia, one in Africa, and one in South America. Phaneropleuron is one of the rarer fish found at Dura Den.
Bothriolepis was a strange-looking fish whose head and trunk were completely encased in boxes of bone while its tail was naked. Its front fins were also encased in bone, giving them a stilt-like appearance. A joint at the shoulder allowed the animal to rotate them.. Its eyes and nostrils were on the top of its head, and the lower surface of the head and trunk was flattened. This suggests that it spent a lot of time on the bottom of the lake bed, perhaps propped up on its stilt-like 'arms', with its vulnerable tail buried.. It had very weak jaws, and probably ate mud and sand, digesting out anything nutritious.
Bothriolepis belonged to a now-extinct group of fossil fishes called placoderms - plated skins. Nothing like it is alive today so it is difficult to say exactly how it lived. Its fossils are known from Upper Devonian rocks all over the world and it was the most successful of all placoderms.
Line drawings from J. Saxon The Fossil Fishes of the North of Scotland, 1975