During the Carboniferous Scotland lay on the equator and therefore had a tropical climate much the same as in central Africa today. Global sea levels at this time oscillated in response to growth or shrinkage of the polar ice sheets and in the warmer intervals shallow seas would encroach across the lowlands of the Midland valley.

  
The early part of the Carboniferous was marked by vast outpourings pf lava that spewed out across the Midland valley to create, for example the Campsie Fells and the Gargunnock Hills. More volcanic activity also formed Arthur’s Seat and Salisbury Crags in Edinburgh and also the nearby Bathgate hills.

Apart from occasional marine inundations the Bathgate hills generally stood higher than the sea level. The local volcanoes were not thought to be large perhaps only one to two hundred meters high. Small lakes may occasionally form within volcanic terranes. Rainwater may collect in craters or calderas or as the result of a river becoming dammed. It was in one of these “palaeo-lakes” surrounded by quiescent volcanoes that a globally unique habitat of plants and animals were preserved as fossils.

At East Kirkton Quarry Stan Wood, a private fossil collector, discovered a unique collection of fossils including the rare fossil called “Lizzie” or Westlothiana lizziae, Lizzie is the fossilised remains of the oldest known reptile to be found anywhere in the world. It was previously thought that reptiles did not evolve until much later in the Carboniferous period. All lakes are ephemeral and this lake may have only existed for a few thousand years. Its size and shape are unknown but it appears to have been shallow and subject to occasional drying out. The deposits that did accumulate within it to a depth of 11 metres were of layers of oily mud and basaltic ash interstratified with fresh-water limestones and siliceous (chert) horizons. The mud and volcanic debris represent material washed into the lake while the limestones and cherts were precipitated directly from the lake waters. It is thought that very little lived in the lake and that most of the fossil plants and animals were washed into the lake after their deaths. Hot springs also fed into the lake making it periodically too hot or too chemically polluted for all but the most specialised life forms. However from the study of the flora and fauna from East Kirkton a vivid picture of the area around the lake and in the Carboniferous forests can be visualised.

The perfect preservation of so many plants and animal fossils at east Kirkton was die to a mineralization process attributable to the percolation of warm solutions through the lake mud and sediments. These hydrothermal solutions were the result of the immediately preceding volcanic activity. In that sense it is similar to the processes at work as described earlier at Rhynie.

Therefore around the volcanoes in the Midland valley were tropical swamps made up of trees and ferns that are now long extinct. The main forest tree, with trunks up to 50 cms across, were ancestral forms of modern tree-ferns and conifers (pteridosperms and gymnosperms). Many of the coal horizons in the sediments are composed of a type called vitrain, which is fossilised wood ash (signifying forest fires). Forest fires were clearly a common occurrence since the oxygen content of the Carboniferous atmosphere was higher than it is today. Some have suggested that the oxygen content may have been as high as 40% as compared to the modern level of about 21%. The fossils found at this locality indicated that the surrounding forest supported a wide and prolific fauna. Among the invertebrates were myriapods (relatives of millipedes and Centipedes) arachnids (mites and harvestmen, true spiders having not yet evolved) and members of the extinct group of arachnid-like creatures called eurypterids. Flying insects had not yet evolved so the forest canopy would have been rather silent as well as devoid of colour as flowering plants had also not yet evolved. The remains of scorpions up to 70 cm long were also found and were probably one of the principal predators around the lake.

Another consequence of the high oxygen content of the atmosphere was that some of these early insects could grow very large. An example of this from the Carboniferous can be found near Laggan on the Clyde island of Arran. Here preserved in a sandstone outcrop overlying a worked out coal seam is the track of a very large ancient Myriapod, Arthropleura. From the track, that extends for about 5 meters and spaced about 0.35 cm apart, it can be estimated that Arthropleura must have been nearly 2 meters in length. It had 23 pairs of legs, was flat bodied and would have lived on the forest floor eating leaf litter.

Myriapod tracks preserved in sandstone from near Laggan on the island of Arran in the Firth of Clyde. Photo taken 1st September 1994. Length of hammer head is 170 mm.