Synopsis

My PhD was based around the study of the classic 'coal swamp' floras (see below) which are commonly featured as museum dioramas across the world, and which were responsible for the coal deposits which are still crucial in fulfilling our fossil fuel needs to this day.

Late Palaeozoic times, around 320 to 250 million years ago (latter Carboniferous to the end of the Permian periods), saw the rise and fall of these complex and extensive forests, which at their peak covered nearly 2.5 million square kilometres across the equator, and were influential on the climate of the entire planet.

During this time the continents were amalgamating into one mass, known as Pangaea, and present day Europe and North America were indistinguishable, leading to the commonly used name 'Euramerica' for the region. It was here that the coal swamps flourished for around 20 million years, to the end of the Carboniferous period.

Simultaneously however, the forests were developing on continental blocks which would become present day China (often termed 'Cathaysia' at this time), and this area of the ancient world went on to support these forests for another 50 million years, to the end of the Permian.

My PhD investigated the link between these two ancient regions, and concluded that the forests found in each area were essentially the same, having undergone intermittent connection across their overlapping time ranges, effectively 'reseeding' themselves until loss of habitat and climate change precipitated the localised extinction of the swamps in Euramerica around the end of the Carboniferous. This shows the remarkable resilience of these ecosystems, being able to migrate for thousands of kilometres under severe tectonic and environmental stresses. After their extirpation in Euramerica, the forests continued to thrive on initially the North China block, and later the South China block, preserving the established template and laying down many more millions of tonnes of peat which would eventually become huge coal reserves.

My dataset was primarily a self collated database, created from over 200,000 records gleaned from museum collections and monographs around the world. I analysed this using multivariate statistics in  order to compare and contrast taxonomic data at species and genus levels, and from different geographic areas and time periods. I augmented this with traditional examination of selected fossil taxa, both in hand specimen and on a cellular level through plant cuticle study. Alongside this I built up a photo library of over 4000 specimens for reference.

 

A late Palaeozoic 'coal swamp'. Gastaldo et al., 1996.

Huge opencast mine face, southern Illinois.

To the left is a reconstruction of a wetland plant community ('coal swamp') as it was in life, around 300 million years ago. To the right is how this hugely diverse and dynamic system is preserved in the rock record today, as a cyclic series of coarser and finer grained sediments with coal seams interspersed through the sequence. Coal was formed over millions of years, under huge pressures and heat, from peat, which was itself formed from huge thicknesses of rotting plant matter. This would have accumulated in calm, deltaic, lagoonal settings, close to the sea, and so any accompanying sediment would have been very fine as it settled out of suspension. When water levels periodically rose relative to the land, these swampy areas were flooded and inundated with coarser sands, which killed the plants and began the process of burial of the peat, the first step on the long journey to becoming coal. As water levels receded, or as the plants recolonised on slightly more elevated land, peat formation would have resumed, and the cycle began again. Each cycle would have lasted in the the order of tens of thousands of years or more.