Box The Scottish Old Red fishes

The Old Red Sandstone Continent of northern Europe and Canada lay close to the equator, in hot tropical conditions, during the Devonian. Fishes lived in the shallow seas and in landlocked lakes around this continent. One of the best collecting areas is in the north of Scotland, where the first specimens came to light nearly 200 years ago.

The fish beds were laid down in large deep lakes (Trewin 1986). Bulky armored ostracoderms and placoderms fed on the bottom in shallow waters, while shoals of silvery acanthodians darted and swirled near the surface. Bony fishes, such as the actinopterygian Cheirolepis, the "rhipidistian" Osteolepis and the lungfish Dipterus, moved rapidly through the plants near the water's edge seeking prey, and sometimes swam out through the deeper waters to new feeding grounds.

The fishes are usually found, beautifully preserved and nearly complete (Fig. 16.9a), in dark-colored siltstones and fine sandstones. These rocks were deposited in the deepest parts of the lake, probably in anoxic (low oxygen) conditions (Fig. 16.9b). There were repeated cycles of deposition, perhaps controlled by the climate. During times of high rainfall, great quantities of sand were washed into the lakes from the surrounding Scottish Highlands. Lake levels then fell during times of aridity, and in places the lakes dried out, leaving mud cracks and soils. Then flooding occurred, together with mass kills of fishes, perhaps as a result of eutrophication (oxygen starvation caused by decaying algae after an algal bloom) or following storms. In all, a pile of lake deposits some 2-4 km thick accumulated over the 40-50 myr of the Devonian, and there are dozens of fish beds throughout this thickness.

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Figure 16.9 The Old Red Sandstone lake in northern Scotland: (a) typical preservation of two specimens of Dipterus; and (b) model of environmental cycles in the lake. Sediment is fed in from the surrounding uplands during times of heavy rainfall. Fishes inhabit shallow and surface waters, but carcasses may sink below the thermocline into cold, relatively anoxic waters, where they sink to the bottom and are preserved in undisturbed condition in dark grey laminated muds. (Courtesy of Nigel Trewin.)

Figure 16.9 The Old Red Sandstone lake in northern Scotland: (a) typical preservation of two specimens of Dipterus; and (b) model of environmental cycles in the lake. Sediment is fed in from the surrounding uplands during times of heavy rainfall. Fishes inhabit shallow and surface waters, but carcasses may sink below the thermocline into cold, relatively anoxic waters, where they sink to the bottom and are preserved in undisturbed condition in dark grey laminated muds. (Courtesy of Nigel Trewin.)

shark fossils because the bulk of the skeleton is cartilaginous and rots away before fossiliza-tion. The apatite teeth and scales are more commonly found isolated, and these and other fish teeth and scales, sometimes called ich-

thyoliths, have proved useful in biostratigra-phy (Box. 16.6).

Modern sharks, called collectively neosela-chians, are faster swimmers and more ferocious flesh eaters than their precursors.

dorsal fin dorsal fin

20 mm

20 mm

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