Chapter 4 Devonian

During the Devonian, there was a dramatic change in terrestrial vegetation. From the humble stock of rhyniophytes, rhyniophytoids and primitive lycopsids in the Silurian, there was a rapid radiation of plant groups, as vegetation evolved to fill the newly-available, terrestrial ecological niches (Figure 4.1). This new plant cover included the first appearance of equisetes, 'advanced' lycopsids (e.g. protolepidodendrids), trimerophytes, fern-like plants (e.g. rhacophytids, cladoxylids), progymnosperms and, eventually, fully gymnospermous plants. By the end of the period, all of the major groups of vascular plants except the angiosperms were present. During the period, there was also a number of major morphological innovations in the Plant Kingdom, including laminate foliage, the seed as a means of propagation, and the arborescent habit (Figure 1.1). Together, these phylogenetic and morphological developments provided the spring-board from which the luxuriant Carboniferous floras could develop.

The Devonian plant fossil record is particularly good in Britain, especially in the lower part of the system. The flora found at Rhynie stands preeminent, but there are also a number of other key sites in southern Scotland, Wales and the Welsh Borders. As a consequence, British Devonian palaeobotany has played a central role in developing our ideas on early land plant evolution.

Palaeogeographical setting

During the Devonian, most of Britain was on the south-east margins of Laurussia and thus very near the equator (Figure 4.2). Apart from the marine environments of south-west England, sedimentation was mainly non-marine, in fluvio-deltaic or lacustrine environments (Allen, 1979; Trewin, 1985), and resulted in red-beds of the Old Red Sandstone Inagnafacies' (Erben, 1964). The proximity of Britain to the equator indicates tropical temperatures but the evidence for precipitation rates is equivocal. Clearly, in the upland regions there must have been considerable precipitation, in order to produce large enough rivers to generate the extensive deltas. On the deltas themselves, however, there is little direct sedimentological evidence of precipitation, and there is indication that, at least periodically, the climate was comparatively dry (e.g. the 'Psammosteus Limestone' in the Welsh Borders — Allen, 1985).

The geographical distribution of Devonian plant fossils has been discussed by Edwards (1973, 1990), Raymond et al. (1985), Raymond (1987), Allen and Dineley (1988), and Edwards and Berry (in Cleat, 1991). Through much of the Devonian, the general pattern seen in the Silurian continues to be recognizable. Devonian plant fossils have been most widely found in the low palaeolatitudes, particularly in Laurussia (which includes the British localities) and thus represent tropical and subtropical vegetation. Within these low palaeolatitudes, some provincialism can be recognized, such as in northern Gondwana (e.g. Malone, 1968; Tims and Chambers, 1984), Kazakhstania (Yurina, 1969; Senkevich, 1975) and Cathaysia (Li and Cai, 1978; Li and Edwards, 1992).

Northern high palaeolatitudes, as represented in present-day Siberia, had a quite distinctive vegetation throughout the Devonian (Petrosyan, 1968; Stepanov, 1975). However, the plant fossils from here have not been studied to the same extent as those of the low palaeolatitudinal Laurussia, and a detailed comparative analysis will be needed to establish the exact extent of the differences.

Fossils representing the southern high palaeo-latitude vegetation occur in South Africa, South America and Antarctica. Their stratigraphically lowest occurrence is in the Middle Devonian (Anderson and Anderson, 1983). In contrast to the low palaeolatitude vegetation, especially of Laurussia, lycopsids such as Archaeosigillaria and Leptophloeum predominated, and this remained the situation through into the Early Carboniferous.

Edwards (1973) argued that, in the Upper Devonian, plant fossil provincialism is much less extreme, and that most assemblages are dominated by taxa such as Archaeopteris, Cyclostigma and Rhacophyton (see also Edwards and Berry in Cleal, 1991). However, the Upper Devonian palaeobotanical record (especially in the Frasnian) is much poorer than that of the Lower and Middle Devonian (Banks, 1980), and this may be the real reason for the apparent reduction in provincialism. That the reduction is artificial is supported by the similarity of the palaeophytogeography of the Middle Devonian and the Lower Carboniferous, suggesting that there was an essential floristic continuity from the Middle Devonian to Early Carboniferous.

There has been disagreement as to the degree of global provincialism in Devonian plant fossil assemblages. Raymond (1987), using statistical techniques, such as polar ordination analysis, claims to recognize considerable differentiation, especially in the upper Lower Devonian. In contrast, Allen and Dineley (1988) argue that, other than in Kazakhstan (the Kazakhstania Palaeocontinent) little provincialism can be recognized until the Lower Carboniferous. An important factor may be that the Laurussian assemblages have been subject to considerably more study than those of the other continents. It cannot be ruled out, therefore, that at least some of the apparent provincialism may be an artifact introduced by palaeobotanists working in partial isolation in distant parts of the globe.

Stratigraphical background

(Figure 4.9) (p. 60) summarizes the chronostratigraphy used in this chapter, which essentially follows House et al. (1977). There have been proposed changes in the Lower Devonian, with the introduction of the Lochkovian and Pragian as the two lower stages of the system (Ziegler and Klapper, 1985). However, there are practical difficulties with identifying accurately the boundaries of these two stages in non-marine sequences and so, following Edwards (1990), the traditional stage divisions (Gedinnian and Siegenian) have been retained. Although all of the stages shown in (Figure 4.9) are defined in marine sequences, it is now possible to use palynology to establish reasonably detailed correlations with the continental sequences where most Devonian plant fossils are found in Britain (Streel et al, 1987).

The lithostratigraphy of the British Devonian sequences is reviewed by House et al. (1977), where details will be found of the formational intervals described herein (see also Simpson, 1959).

The most recently published biostratigraphical scheme for Devonian plant macrofossils is given by Edwards and Berry (in Cleal, 1991), which essentially follows that of Banks (1980); this is summarized in (Figure 4.9). It provides a sequence of assemblage zones, whose boundaries are identified by the tops and bottoms of the ranges of particular species. However, the zones are also defined in terms of significant morphological developments, such as the appearance of stems with centrarch primary xylem in the Psilophyton Zone and of the arborescent habit in the Svalbardia Zone. This departs from traditional biostratigraphical methodology and makes it perhaps less robust for establishing detailed stratigraphical correlations. Nevertheless, it makes it a particularly useful framework around which the evolutionary history of terrestrial vegetation during the Devonian can be fitted.

Devonian vegetation

The Early Devonian vegetation of Britain appears to have been dominated by small, morphologically primitive plants. In Gedinnian times, rhyniophyte and rhyniophytoid plants (e.g. Cooksonia, Salopella) continue to be dominant (see Chapter 3). Perhaps the single most important locality for these plants is in the Siegenian of Britain (Rhynie), where petrified fossils show details of their cell structure. However, this represents a rather atypical, volcanically-influenced habitat and elsewhere in the Siegenian the group was in serious decline; by the Emsian, it had become virtually extinct.

One of the first groups of plants to replace the rhyniophytes and rhyniophytoids in the British floras was the zosterophylls (Figure 4.3). Evidence of 'H'- and 'K'-style branching, widely regarded as characteristic of the zosterophylls, is known from the uppermost Silurian, but fertile specimens are not known below the Gedinnian. They rapidly diversified during the Early Devonian and reached their acme in the late Siegenian and early Emsian (Niklas and Banks, 1990); however, soon after they underwent a rapid decline and eventually became extinct in the Late Devonian. The early forms (e.g. Zosterophyllum) had naked axes with sporangia aggregated into terminal spikes. During the Siegenian, however, some developed leaves (e.g. Sawdonia), while in others changes can be seen in the reproductive structures (e.g. Gosslingia, where the sporangia are not clustered into terminal spikes). Although Britain is not the only place where fossils of these plants occur, some of the best preserved examples have been found here and have been the basis of much of what we know about them, especially of their reproductive and vegetative anatomy (e.g. Lele and Walton, 1962a; Edwards, 1969a, b, 1970a, 1972, 1975).

From an evolutionary standpoint, the zosterophylls are important as they were probably closely related to the early lycopsids (Hueber, 1992), which became one of the most important components of land vegetation during the Late Devonian and Carboniferous (Thomas, 1978a, 1992). This phylogenetic model is strongly supported by the sequence of intermediaries Sawdonia—–steroxylon–Drepanophycus that can be clearly demonstrated in the British Devonian fossil record (Figure 4.4). There is the problem of the presence of the lycopsid Baragwanathia in the Silurian of Australia, which pre-dates the earliest known zosterophylls, but this probably just reflects the incompleteness of the fossil record. In Britain, the earliest lycopsids are found in the Siegenian.

Another characteristic plant group of Early Devonian Britain was the trimerophytes (Figure 4.5). The taxa varied considerably in general morphology, and could have naked (Psilophyton dawsonii Banks et al.) or spinose (P. princeps Dawson) axes, although most appear to have been herbaceous. However, they are all characterized by their fructifications, which consisted of loose trusses of sporangia (e.g. Andrews et al., 1977). The group seems to have been relatively short-lived, appearing first in the Siegenian, reaching its acme in the Emsian, and becoming extinct during the Eifelian. However, it was of critical importance for the evolution of vascular plants, since it was probably ancestral to both the ferns and progymnosperms (and thus the gymnosperms).

In addition to the above, the Early Devonian vegetation included a number of enigmatic plants. Among the vascular plants, there is the Barinophytopsida, whose earliest occurrence is in the Siegenian of Britain (Krithodeophyton from Craig-y-Fro Quarry). Although they are clearly vascular, and even show some points of similarity with the zosterophylls and lycopsids, many have distinctive fertile structures with pinnately arranged sporangia, sometimes separated by sterile appendages. In some cases they are heterosporous, with mega- and microspores borne in the same sporangia. The barinophytopsids do not readily fit into any known group of pteridophytes, either living or extinct.

Among the non-vascular land plants, Prototaxites, Nematothallus and Pachytheca continued to be abundant in the Early Devonian, although all but the first became extinct before the mid-Devonian. In addition, Parka decipiens Fleming flourished for a short time, in the Gedinnian. This was a cutinized, encrusting alga, whose thallus was covered in large sporangia. Hemsley (1989, 1990a) has suggested that it might represent an immediate evolutionary position between the algae and the bryophytes.

The mid-Devonian saw a major change in vegetation. Most of the dominant groups of the Early Devonian became extinct or were declining in importance. Only the lycopsids continued to play a major role in land vegetation, particularly in the higher latitudes. Most lycopsids retained a herbaceous habit in the mid-Devonian, although some larger forms (Lepidodendropsis) made their first appearance here, providing a foretaste of some of the arborescent forms that came to dominate much of Carboniferous vegetation.

The characteristic rhyniophyte/zosterophyll/trimerophyte complex of the Early Devonian was gradually replaced in mid-Devonian times by early, fern-like plants and progymnosperms (Figure 4.1). Both groups probably evolved from the trimerophytes. The fern-like plants include representatives of the Cladoxylales, Iridopteridales and Coenopteridales, and can be recognized to be allied with the ferns mainly on the basis of their stem anatomies (Scheckler, 1974). However, they were quite different in outward appearance from recent ferns, most noticeably in not having laminate fronds. The foliage instead consisted of three-dimensionally branching systems of terete axes, which would often bear terminal trusses of sporangia. All known forms were herbaceous, although some could be up to one metre or more high (e.g. Pseudosporochnus, (Figure 4.6)).

The earliest and most primitive progymno-sperms are found in Britain (Protopteridium, (Figure 4.7)). Like their presumed ancestors, the trimerophytes, these early progymnosperms did not have fully laminate foliage, and are distinguished mainly by the presence of secondary wood in the stems, and the greater complexity of the proto-xylem (Beck, 1976). The first of these features is of great significance, as it allowed the evolution of the arboreal habit and thus of forest habitats. The size of these earliest progymnosperms is still a matter of conjecture, although they were probably at least several metres high.

The Late Devonian saw the extinction of the typical Early Devonian forms, while the lycopsids, progymnosperms and fern-like plants continued to diversify. The arborescent habit became fully developed, with trees growing to over 20 metres high (Beck, 1962).

Equisetes, in the form of the Bowmanitales and Pseudoborniales, also made their first appearances in the Late Devonian (Figure 4.1). It was once thought that equisetes first appeared in the mid-Devonian, represented by fossils such as Hyenia. However, the latter are now generally thought to belong to the cladoxylaleans.

The earliest British examples of seed plants are found in the Famennian of Devon (Arber and Goode, 1915), and are only just pre-dated by examples from North America (Rothwell et al., 1989). Little is still known about the Devonian gymnosperms, although they seem mostly to have been herbaceous and probably belonged to the lagenostomalean pteridosperms. The development of seeds was of critical importance for the evolution of terrestrial vegetation, as it finally freed plants from the constraint of requiring moist conditions to facilitate fertilization. From the end of the Devonian, plants became adapted to take advantage of the so-called 'upland' or extra-basinal habitats. Thereafter, the fossil record provides an increasingly selective record of the Earth's total vegetation.

Devonian plant fossils in Britain

The record of Devonian plant fossils in this country is most complete in the Lower Devonian, with an extensive range of sites from the Gedinnian to the Emsian in Wales, the Welsh Borders (Figure 4.8) and in Scotland. From these, it is possible to document the early phases in the Devonian radiation of land vegetation, mentioned at the beginning of this chapter. Gedinnian plant fossils are found widely in the Arbuthnott Group of Tayside, Scotland, although there are also a number of sites in the Ditton Group of Shropshire and the St Maughan's Group of South Wales (Edwards, 1990). Siegenian plant fossils are best represented in the Senni Beds and its equivalents in South Wales and the Welsh Borders (Croft and Lang, 1942). There is also the world-famous Rhynie Chert in the Grampians of Scotland, which is now thought to be Siegenian in age. The Strathmore Group of central Scotland, provides the most abundant Emsian plant fossils in this country (Henderson, 1932; Rayner, 1983, 1984).

Middle Devonian plant fossils are mainly found in the fossil fish beds of northern Scotland, ranging from Cromarty north to the Shetlands. There are also records of poorly-preserved fossils from Cornwall (Fox, 1900, 1901, 1904; Lang, 1929; Hendriks, 1935, 1966; Hendriks et al., 1971).

For reasons which are still far from clear, Upper Devonian plant fossil occurrences are rare throughout the world, and Britain is no exception to this. The best British examples are found in the Baggy Formation and its lateral equivalents in north Devon (Arber and Goode, 1915). In Ireland, the Kiltorcan assemblage is of about the same age, and has yielded well-preserved examples of Archaeopteris and Cyclostigma (Chaloner, 1968). Comparable assemblages have also been found in Scotland and the Forest of Dean (Miller, 1857; Crookall, 1939; Long, 1973). However, the Forest of Dean site no longer yields fossils and, despite an extensive search being made as part of the Geological Conservation Review, the Scottish site could not be located. A reference to plant fossils of this age from southern Scotland by Smith (1862) remains to be properly documented.

From this range of Devonian palaeobotanical sites in Britain, ten have been selected as GCR sites. The stratigraphical positions of these sites are shown in (Figure 4.9).

References