Laggan

Highlights

This is a significant site for Lower Carboniferous plant petrifactions, particularly for lycopsids. They include the best known examples of in situ petrified stumps of arborescent lycopsids, and some early examples of herbaceous lycopodiaceans. It has also yielded some of the best known examples of anatomically-preserved strobili of the Archaeocalamitaceae (Sphenopsida).

Introduction

The in situ stumps of fossil trees preserved in Lower Carboniferous volcanogenic deposits at Laggan, on the north-east coast of the Isle of Arran [NR 982 506], were discovered by E.A. Wunsch in 1865 (the early history of work on the site is summarized by Walton, 1935). Despite early contributions by Carruthers (1869) and Binney (1871), significant progress was not made until W.C. Williamson visited the site in 1877, when a considerable quantity of specimens was collected (a colourful account of the visit is given by Williamson, 1896, pp. 169–177). The lycopsid stumps were described by Williamson (1880, 1883, 1895), and later by Walton (1935). Other elements in the assemblage have been described by Calder (1935), Walton (1949b), Fry (1954), Beck (1958), Pant and Walton (1961) and Chaphekar (1963), mainly based on specimens from Williamson's original collection.

Description

Stratigraphy

The most recent detailed accounts of the geology at Laggan appear to be by Tyrrell (1928) and Walton (1935), although no stratigraphical log was given. The fossils occur in a basaltic ash deposit, some distance below the Corrie Limestone, probably in the Upper Sedimentary Group. Their exact chronostratigraphical position is uncertain, but the deposits probably fall in the upper Visean.

Palaeobotany

The following taxa, preserved as calcite petrifactions, have been described to date:

Lycopsida:

Lepidophloios wuenschianus (Williamson)

Walton

Levicaulis arranensis Beck

Paurodendron arranensis Fry

Lycostachys protostelicus Pant and Walton

Lepidostrobus (?) ambiguus Binney

L wuenschianus Binney

L latus Binney

Lepidocarpon sp.

Stigmaria sp.

Equisetopsida:

Protocalamites goeppertii (Solms-Laubach) Bateman

Protocalamostachys arranensis Walton

Lagenostomopsida:

Lyginorachis waltonii Calder

Lyginorachis sp.

Kaloxylon sp.

Interpretation

The most famous of the Laggan fossils are the Lepidophloios wuenschianus stumps (Williamson, 1880, 1883, 1895; Walton, 1935). They are the best known examples of in situ, petrified stumps of arborescent lycopsids; the only other reported specimens are Lepidodendron saalfeldense Solms-Laubach from Germany, which are smaller and not so well preserved (Solms-Laubach, 1896). They are particularly important as the stumps contain pieces of the stele from different parts of the trunk (e.g. (Figure 5.29)). This preservational feature appears to have resulted because most of the outer cortex disintegrated shortly after the death of the plants, and the stelar column then collapsed down into the stump, where it became lithified. From these fossils, Walton (1935) was able to deduce that at the base of the trunk there was only a relatively slender protostele, surrounded by a thick layer of secondary wood. In more distal positions, however, the primary xylem formed a wider, medullated siphonostele, but the surrounding secondary wood became concomitantly narrower. These results had important consequences for subsequent ideas about the developmental growth of the arborescent lycopsids (Andrews and Murdy, 1958; Eggert, 1961). Where pieces of the cortex are still preserved, they show that this tissue had clearly developed bands of secretory cells.

Also found in the stumps are fragments of the distal branches of the tree, which are similar to the adpression species Lepidophloios scoticus Kidston. They can also be compared with the Lepidophloios shoot described from Bearsden by Galtier and Scott (1986b). However, similar shoots are also found associated with Lepidophloios scottii Gordon at Pettycur (see above), which differs from the Laggan fossils in not having secretory cells in the cortex. Evidently, different species of lycopsid could produce shoots of essentially identical form.

DiMichele (1979) used L wuenschianus as the 'type' of one of the two groups of Lepidophloios species that he recognized, characterized by features of periderm structure, leaf cushion anatomy, and lateral branch architecture. Other members of the group include L. scottti Gordon from Pettycur (see above) and L. johnsonii from the basal Upper Carboniferous of North America, but L. wuenschianus is by far the best known. The differences between the two groups of Lepidophloios may reflect palaeoenvironmental differences, the L. wuenschianus group occupying more open habitats, whereas the L. harcourtii group were forest dwellers.

It has been argued that Lepidophloios bore Lepidocarpon strobili (e.g. Phillips, 1979). It is not surprising, therefore, that Walton (1935) recorded Lepidocarpon in the Laggan stumps, although no description was given.

Binney (1871) described three species of Lepidostrobus from Laggan, but they are all incomplete and in need of renewed investigation.

Two other types of lycopsid in the Laggan assemblage were herbaceous, and are both unique to this locality. The stems known as Levicaulis arranensis Beck (1958) were less than 40 mm in diameter, with a terete protostele, and apparently without ligules. An associated strobilus known as Lycostachys protostelicus Pant and Walton (1961) has a central axis with an almost identical structure and is assumed to have belonged to the same plant. The preserved part of the strobilus was exclusively microsporangiate, although some megaspores were found in association. If this association is merely coincidental and the cone was homosporous, as suggested by Pant and Walton, then the affinities of this plant probably lie with the Lycopodiaceae.

A second type of herbaceous lycopsid is represented by the stems Paurodendron arranensis Fry (1954). Unlike Levicaulis, Paurodendron is ligulate and thus more similar to Oxroadia from Oxroad Bay (p. 131), except for details of the vascular anatomy. It was placed in synonymy with Selaginella fraipontii (Leclercq) Schlanker and Leisman from the Upper Carboniferous of North America by Schlanker and Leisman (1969). However, in view of their stratigraphical separation, and the fact that details of the strobili are known from S. fraipontii, but not the Laggan plant, it seems wiser to keep them separate. Bateman (1988) has argued that Paurodendron and Oxroadia represent a distinctive order of lycopsids, which is probably a sister group of the Lepidocarpales, although he has subsequently recanted this view (Bateman et al, 1992).

Equisetopsida

The Laggan equisetopsid stems, known as Protocalamites goeppertii (Walton, 1949b) differ from Protocalamites pettycurensis from Pettycur in having more primary vascular strands, and probably representing larger plants (Bateman, 1991). They are also associated with larger strobill, known as Protocalamostachys arranensis Walton (1949b). Historically, the latter are of interest because they were the first strobili of these primitive equisetopsids to be discovered petrified, which helped clarify the distinctive characters of the Archaeocalamitaceae (i.e. the sporangia were borne on peltate sporangiophores, and there were few intervening sterile bracts).

Pteridosperms

There has been little work on the pteridosperms at Laggan. Calder (1935) described a distinctive lagenostomalean rachis as Lyginorachis waltonii Calder, whilst Walton (1935) recorded Kaloxylon, which usually refers to lagenostomalean rooting structures.

General remarks

Laggan is another of the internationally important Lower Carboniferous petrifaction sites in Britain, which includes a particularly significant lycopsid component. Some of the species have been described from elsewhere, such as Lepidophloios wuenschianus from Dalmeny (Seward and Hill, 1900), Protocalamites goeppertii and Protocalamostachys arranensis from Loch Humphrey Burn, and Lyginorachis waltonii from Oxroad Bay and Loch Humphrey Burn. Nevertheless, the balance of taxa at Laggan remains unique, as well as being the only known locality for two herbaceous lycopsids.

Conclusion

Laggan is one of a series of important sites that show Lower Carboniferous rocks in southern Scotland, and which yield plant fossils with their anatomy preserved; they are probably about 340 million years old. This particular site is especially important for its club-mosses, which include both trees (Lepidophloios) and small, herbaceous forms (Levicaulis and Paurodendron). It has also yielded important specimens of horsetails, including a number of reproductive cones, that have been important for understanding the evolutionary history of this group of plants.

References