Devonian

The most significant evolutionary development during the Devonian was the colonization of the land. Although the first land plants appeared in the Late Silurian, it was during the Devonian that they developed in abundance to provide the first true soils, and to support the first land animals. At first the plants would have existed without much assistance from the surface materials. In the Lower Devonian, the dominant plants were rootless and leafless psilo-phytes, but by the end of the period many different types had evolved: ferns, seed ferns, horsetails (e.g. calamites) and club mosses (lycopods), many of them reaching a very large size. Most of these more advanced plants developed roots, and thus obtained nutrients from the soil.

It is difficult to imagine the landscape before these soils developed. Although many areas would have had a high rainfall, there could be no soils without land plants. Much of the ground would be reddened by ferric iron oxides, as there would be an insufficient quantity of organic matter to reduce the iron to its green ferrous state. The lack of organic matter is the main reason why many Devonian (and earlier) river deposits are red in colour, like much of the Old Red Sandstone of north-west Europe.

The first land plants were followed almost immediately by land animals. Many of these were arthropods (spider-like mites and wingless insects) which had a covering of chitin. Although this covering had developed originally in their marine ancestors, it served these first land animals both by giving support to their bodies and by protecting the body tissues from drying up in the terrestrial environment. Many of these arthropods were sap-sucking and spore-eating forms similar to some living animals. Most of them were ground dwellers, but insect flight may have evolved during the Late Devonian (when tall plants first appeared), though the first fossil winged insects are not known until the Carboniferous.

In the Devonian there is a close correlation between the increase in complexity of spore structures and the diversity of terrestrial arthropods. Although some of these early land animals were predators, and others were probably detritus feeders (e.g. the myriapods), many of the early mites, chelicerates and wingless insects may have fed on spores and dispersed them as a result; some of the complex ornament on Devonian spores may have been a direct adaptation to assist this dispersion (Kevan et al, 1975).

The Devonian also saw large evolutionary radiations in fish, culminating in the emergence of the first amphibians in the Late Devonian. The Early Devonian is renowned for the heavily armoured benthic ostracoderms. These had no jaws, but by the Middle Devonian the fish included groups of large predatory placoderms (arthrodires) which were the dominant carnivores in both marine and freshwater environments.

In north-west Europe and north-eastern North America, there are large areas of river and lake deposits: the Old Red Sandstone. This continental area extended continuously from northern Germany, England and eastern Canada to Scandinavia and north Greenland and much of the North-west Territories (Fig. g) after the close of the Iapetus Ocean sometime in the Early Devonian. Marine deposits were laid down around the margins of this continent in Germany, south-west England, the United States and much of western Canada.

Marine faunas in the basal Devonian are similar to those present in Silurian beds. Brachiopods and corals continued to be among the commonest benthos throughout the period; but there was a major change in pelagic faunas towards the end of the Lower Devonian, when the monograptids became extinct and several families of ammonoids developed. After Early Devonian times, the ammonoids become the best indicators of stratigraphic time zones. Because they had calcareous skeletons and not chitinous skeletons like the graptolites, cephalopod-rich limestones formed in areas receiving little terrigenous sediment instead of graptolitic shales (Tucker, 1974).

In shallow marine sandy and muddy areas, the brachiopods continued to be the most commonly preserved benthos. Spire-bearers (spiriferoids, atrypoids and athyroids) are commonest, but stropho-menoids, chonetoids and rhynchonelloids also continue from the Silurian. Towards the end of the Devonian the productellids made their appearance; these brachiopods were anchored by spines on the surface of very convex pedicle valves, a new development that was to see its full expression in the productids of the Carboniferous. Bivalves and trilobites also continued from the Silurian; they show some new forms, especially those associated with reefs (bio-herms). In general, however, the trilobites decreased in diversity during the Devonian, with only one family (the proetids) surviving into the Carboniferous.

In Devonian carbonate shelf environments (see House, 1975), the tabulate corals and the stromatoporoids reached their acme; rugose corals (which became more common in the Carboniferous) were also more common than during the Silurian. These groups

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