Devonian Environments

The early Palaeozoic world was very different from today, largely because of an entirely different continental layout. Oceans have come and gone, and continents have drifted from tropical regions to the north and south. Precise details of former continental positions (see Chapter 2) are less certain for the Palaeozoic than they are for the Mesozoic and Cenozoic, so palaeogeographical maps of the Silurian and Devonian worlds are controversial in some respects.

3.8.1 Siluro-Devonian faunal provinces

It is possible to distinguish faunal provinces among early fishes. For example, there was a Scotto-Norwegian fauna of thelodonts in the Silurian that differed in many respects from the Acadian-Anglo-Welsh fauna (Figure 3.17(a)). In other words, Silurian fish fossils from Wales and southern England are more like those from the eastern parts of North America and Greenland (Acadia) than those from the central parts of Scotland or from Norway.

The extraordinary distributions ofthelodont faunas make sense when Silurian geography is considered (Figure 3.17(a)). Scotland, Norway, Greenland and Canada formed a single land mass, located largely south of the equator. A separate Canadian Arctic-Russian thelodont fauna straddles the northern boundaries, and the Acadian-Anglo-Welsh fauna is typical of the southern region. The rest of Europe was separated from this continent by the Iapetus Ocean, and it was also located largely south of the equator. The Scotto-Norwegian thelodont fauna is restricted to the western end of this land mass. Siberia, with the Angaran th-elodont fauna, was another land mass. Thelodonts do not appear to have reached the southern continents, Gondwana, at this time.

The Silurian thelodont faunas were kept apart by barriers to mixing, major land masses and wide oceans. Many of these barriers disappeared in the Early Devonian, and a single thelodont assemblage, the Turinia fauna, occurs nearly worldwide (Figure 3.17(b)). One barrier was lost when the North

American-Greenland-Scotland continent fused fully to the rest of Europe with the closure of the Iapetus Ocean. Thelodonts were able to spread worldwide, and they reached Australia and other parts of Gondwana. Only Siberia remained isolated to some extent, and the separate Angaran thelodont fauna survived there, although Turinia invaded southern regions.

Some other fish groups, such as the armoured jaw-less fishes and placoderms, do not show such uniform global distributions in the Devonian. Indeed, most 'ostracoderms', except conodonts, thelodonts and pituriaspids, were absent from Gondwana after the Ordovician. They divide into distinctive faunas in Euramerica, Siberia, eastern Siberia (Tuva), South China and eastern Gondwana (Australia and Antarctica) (Young, 1993). The galeaspids, for example, are found only in Vietnam and South China, whereas the camuropiscid arthrodires and others are restricted to Australia.

3.8.2 Siluro-Devonian environments

Silurian and Devonian seas and freshwaters were warm, and fish fossil localities are clustered in the equatorial and tropical belt (Figure 3.17). Important environmental changes took place on land during the Silurian and Devonian, and these affected vertebrate evolution. The first land plants appeared in the Mid- to Late Silurian. They were small and reed-like, and probably grew around ponds and lakes with their tuberous roots partly in the water. Early Devonian terrestrial rocks very rarely contain fossils of land plants or animals, but by Mid- and Late Devonian times, large horsetails and scale trees (lycopods) became quite common.

The first land animals were scorpions, millepedes and spider-like arthropods, all of which could live in water and on land. They first appear in the Late Silurian, and they crept ashore presumably to exploit the new green plants around the waters' edge. In the Early Devonian, fossils of spiders, mites and wingless insects have been found, and the diversity of insects increased in the Late Devonian. These plants and animals provided new sources of food for animals that could exploit the shallow waters of the lakes and the land around the edges.

30°

Fig. 3.17 Thelodont faunal provinces and palaeogeography in the Silurian (a) and Devonian (b). Continental outlines for those times are shown with heavy lines, and modern continental margins are shown with fine lines. The five thelodont provinces are indicated by shading. (After Halstead, 1985.)

Angaran | Acadian-

Anglo Welsh

Canadian Arctic-Russian

| Scotto-Norwegian

Angaran | Acadian-

Anglo Welsh

Canadian Arctic-Russian

| Scotto-Norwegian

Turinia

Fig. 3.17 Thelodont faunal provinces and palaeogeography in the Silurian (a) and Devonian (b). Continental outlines for those times are shown with heavy lines, and modern continental margins are shown with fine lines. The five thelodont provinces are indicated by shading. (After Halstead, 1985.)

Early fish evolution has been studied most on the Euramerican continent, sometimes called the Old Red Sandstone continent (ORC), because the Devonian rocks of Scotland, first studied in the 1820s (see Box 3.4), were termed the Old Red Sandstone. This continent was a large tropical land mass, characterized by hot, arid climates in its core, and monsoonal climates around the edges. The land surface was probably bare rock with limited soil cover, as very few plants ventured far from the watersides. Periodic rainfall would have eroded the interior of the ORC at a prodigious rate, and transported the debris down rivers in flash floods to the margins. Fishes are found in rivers, freshwater lakes and marine lagoons around the margins of the ORC, and there is some uncertainty about how much of early fish evolution took place in the sea, and how much in fresh waters.

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