Life on land prior to the origin of land plants: fossil evidence and palaeoenvironmental significance

Approximately 540 million years ago there was a dramatic diversification of multicellular life in the oceans. The so-called 'Cambrian explosion' witnessed the appearance and diversification in the oceans of abundant, diverse and disparate multicellular animals, fungi and algae. Rather surprisingly, however, whilst the oceans teemed with complex life, the land surface is considered to have been essentially barren. It wasn't until the Mid Ordovician, some 75 million years later, that the first evidence for land plants appears. Why was there such an extensive lag before the invasion of the land by complex multicellular organisms (plants) and what, if anything, lived on the land prior to the Mid Ordovician? The aim of this project is to study newly discovered fossils from ancient continental deposits that suggest that the land surface harboured a simple vegetation of algae before the land plants evolved. It has long been suspected that prior to the origin of land plants the continents may have harboured a simple biota often referred to, rather disparagingly, as an 'algal scum'. It has been postulated that this biota consisted of simple photosynthesizing organism such as bacteria and algae, and possibly also decomposers such as fungi. This biota probably inhabited a variety of environments and exploiting a variety of lifestyles including (i) free living in bodies of freshwater (lakes, rivers etc.); (ii) as algal mats on periodically inundated areas (such as estuary mud flats); (iii) within and on the surface of rudimentary soils as biological soil crusts; (iv) within rocks (as endoliths). Sadly this is mostly speculation. There is very little firm evidence for such terrestrial life, barring extremely rare but tantalizing reports of simple fossils and faint geochemical signals. The newly discovered fossil material consists of simple, spherical, sac-like structures (sphaeromorphs), tubes and filaments, and various other organic microfossils. The microfossils represent organisms that inhabited aquatic environments, but also those inhabiting the surrounding land surface (soil crusts etc.) that were washed into the aquatic environments. They clearly indicate the presence of early terrestrial life. But the challenge is understanding what types of organism they represent? The plan is to compare them with living material believed to represent counterparts of the ancient terrestrial ecosystems. Today land plants inhabit all of the Earth's surface except for the coldest polar regions and high peaks and the driest deserts. These inhospitable environments are inhabited by algal mats, biological soil crusts and endolithic communities believed to represent relict communities from before the origin of the land plants. Life from these communities will be collected, from the hot deserts of North America and Australia and the cold deserts of Iceland. It will be experimentally turned into fossil material that might be expected to survive in the fossil record. Then the living and fossil material will be compared to determine what the fossils actually represent. Analysis of fossils recovered from ancient terrestrial deposits will greatly illuminate our understanding of the nature of pre-land plant terrestrial biotas. Clearly an 'algal scum' inhabited the land surface for a very long time. This has immense evolutionary and palaeoecological significance, in addition to enormous implications for understanding the nature of the environment and global change prior to the origin of land plants The current terrestrial vegetation constitutes some 90% of the living biomass of the planet and has a major impact on the environment (affecting climate, atmospheric composition and pattern of sedimentation through carbon sequestration, soil formation, weathering and so on). Just how significant was the oft neglected pre-land plant terrestrial biota and how much did it influence the environment through early terrestrial-based biogeochemical cycles?