Box Old Red Sandstone Fishes Of Scotland

Some of the most prolific collections of Devonian jawless fishes, placoderms, acanthodians and lobefinned fishes, have come from the Old Red Sandstone of the Orcadian Lake, a large subtropical lake in the north of Scotland that covered much of Caithness, the Moray Firth, Orkney and Shetland. This lake lay on the southern margins of the Old Red Continent, and sediment was fed in by erosion of the uplands round about (see illustration I). The region was affected also by annual seasons of dramatic rainfall.

Lake levels rose and fell as a result of the seasonal wet and dry climatic conditions, some following annual cycles, others longer-term Milankovitch cycles of 20,000 and 90,000 years. The fluctuations in lake level affected the oxygen content and salinity of the water. The sediments frequently occur in repeated cycles that occupy thicknesses of about 10 m of the rock column, and repeat through a total thickness of 2-4km of rock (Trewin and Davidson, 1999). In places, annual varves, generally less than 1 mm thick, may be detected.

Fossil fishes occur in the Scottish Old Red Sandstone both as scattered fragments and in great concentrations within 'fish beds'. Mortality horizons, single layers containing high concentrations of fish carcasses, seem to have formed during deoxy-genation events that may have occurred every 10 years or so when the lake was deepest. Repeated mortality events of this kind occurred over thousands of years, and built up major fish beds in several places. These could have either followed an algal bloom, when decaying algae removed oxygen from the water, or a severe storm that stirred up deep anoxic waters to the surface. Other likely causes of fish kills in the Old Red Sandstone lakes include rapid changes in salinity and cold shock. The carcasses floated for some time near the surface, buoyed up by gases of decay. After a few days the gas escaped, possibly by rupturing the body walls, and the carcasses fell to the anoxic lake-floor where they were buried by fine sediments. This process yields extensive beds of fish remains representing several species, and the carcasses are often in good condition (see illustration II) because they have not been scavenged, and because of the low-energy bottom conditions.

The Old Red Sandstone food chains are based on lakeside plants (mosses, reedy horsetails and scale trees) and phyto-plankton, which were eaten by shrimps and molluscs, which in turn were eaten by lobefins such as Dipterus (Figure 3.20) and Osteolepis (Figure 3.23(b, c)). There is also evidence for small arthropods around the lake margins, and these may have been a source of food for these fishes as well. The smaller fishes were preyed on by carnivorous forms such as Coccosteus (Figure 3.13) and the bony fish Cheirolepis (Figure 3.19) that have been found with remains of acanthodians and of Dipterus in their stomachs. The heavier placoderms such as Pterichthyodes (Figure 3.14(b)) scavenged for organic matter—decaying plant and animal remains—on the shallower oxygenated parts of the lake-bed. The top carnivore seems to have been the lobefin

Glyptolepis, which reached lengths of over 1 m. It may have been a lurking predator like the modern pike, hiding among water plants and launching itself rapidly at passing prey.

I The Old Red Sandstone lakes of the north of Scotland: topographic sketch showing sediment source from alluvial fans and plains derived from erosion of the uplands, and the cycle of life, death, and fossilization of the fish fauna; from left to right: fishes living in shallow areas of the lake, carcasses float out to the middle of the lake, and sink into the cold anoxic conditions beneath the thermocline where they are preserved in laminated muds on the deep lake-floor. (After Trewin, 1985, courtesy of Blackwell Science Ltd.)

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II Typical Old Red Sandstone fishes from Achanarras Quarry, Caithness: (a) juvenile Pterichthyodes; (b) Dipterus, showing slight separation of head elements on fossilization; (c) Palaeospondylus, a possible larval lungfish. Scales: 20 mm in (a) and (b); 10 mm in (c). (Courtesy of Nigel Trewin.)

Fig. 3.18 The fins of (a) an actinopterygian, Amia,to show the simple basal skeleton, (b) the lobefin Eusthenopteron,an osteolepiform, and (c) the lobefin Neoceratodus, a lungfish, to show the more complex skeleton that supports a muscular lobe in the middle of the fin. (Modified from Zittel, 1932.)

Fig. 3.18 The fins of (a) an actinopterygian, Amia,to show the simple basal skeleton, (b) the lobefin Eusthenopteron,an osteolepiform, and (c) the lobefin Neoceratodus, a lungfish, to show the more complex skeleton that supports a muscular lobe in the middle of the fin. (Modified from Zittel, 1932.)

3.9.1 Devonian actinopterygians

The oldest actinopterygians, represented by scales, are Late Silurian in age, but the group began to diversify only in the Late Devonian. An early form is Cheirolepis from the Mid-Devonian of Scotland, typically 250 mm in length (Pearson and Westoll, 1979). The body is slender and elongate (Figure 3.19(a)), and the tail is strongly heterocercal, although the tail fin beneath makes it nearly symmetrical. There are large triangular dorsal and anal fins and paired pectoral and pelvic fins.

The body is covered with small overlapping lozenge-shaped scales (Figure 3.19(b)) that articulate with each other by means of a peg and socket arrangement in the tail region. The scales are arranged in sweeping diagonal rows that run backwards and downwards. There are larger ridge scales on the dorsal edge of the tail that act as a cutwater. The fin rays (actinotrichia) are covered with jointed dermal bones, the lepidotrichia.These provide

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