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Project Background

The fossil evidence of terrestrial plants
Fossil evidence suggest that plants evolved in an aquatic environment and first colonised land during the Early Ordovician ~475 million years ago. The Ordovician fossil record consists entirely of spores, which are thought to derive from land plants. This is because these spores have a tetrad morphology that is indicative of alteration of generations and sporophyte meiosis, reproductive processes that are unique to extant land plants.

The earliest spore assemblages were all located at high southern palaeolatitudes. However, by the Early Silurian the fossilised distribution of spore assemblages infer that embryophytes (land plants) had become a global phenomenon.  Unfortunately, land plant megafossils are practically unknown until the Late Silurian (~420 million years ago) probably because pioneering land plants are believed to have lacked the appropriate recalcitrant tissues suitable for preservation. 

The environment of Ordovician Earth
The Ordovician Earth was very different from today, especially in terms of the palaeogeography of the continents. During the Mid-Ordovician, Gondwana (by far the largest continental plate which included South America, Africa, Antarctica, Australia and India) extended from the South Pole to the equator. Whilst Laurentia (~North America) and Siberia were in equatorial positions and Baltica was located in the intermediate latitudes in the Southern hemisphere.

Palaeoclimates are mainly inferred from geological interpretations of the rock strata. Unexpectantly the lithologic record infers that the first embryophytes, that were thought to have colonised the extreme southern palaeolatitudes, thrived in a cold environment as opposed to the tropical climate of the equatorial realm. The Mid-Ordovician embryophytes would have also experienced a long polar winter with months of darkness, as is still experienced at extremely high latitudes today.

GEOCARB, is a geochemical process based model of atmospheric CO2 concentration through geological time constructed by Robert Berner at Yale University. Based on mass balance equations of the long-term carbon cycle of exchange between carbon in the atmosphere and carbon stored in rocks, GEOCARB predicts an unprecedented CO2 level of 8-18 x present atmospheric levels during the Mid-Ordovician.

Before the colonisation of the land by embryophytes, cyanobacteria, algae and fungi are hypothesised to have dominated the land, inhabiting soils analogous to desert crusts. Ordovician soils are expected to have been no more fertile than is normal today, it is important to consider that nutrient availability in Ordovician soils would have been much lower in the oxidative soils of dry land, relative to the aquatic environments in which plants first evolved.

Effect of elevated CO2
It has been proposed that a higher atmospheric CO2 concentration leads to an increase in growth rate by suppressing photorespiration, the energy wasteful alternative pathway for Rubisco when O2 is the substrate rather than CO2. Indeed, a survey of 156 C3 species gave an averaged 41% simulation in biomass production with a doubling of atmospheric CO2 concentration.

Effect of mycorrhizal association
It is estimated that 80% of living plant species form a mycorrhizal association with a fungal partner. Ordovician land plants growing in full sunlight would have had abundant supplies of photosynthentically produced carbon compounds that accumulated as starch and leaked as exudates into the soil. However, these embryophytes are likely have been limited by mineral nutrients because their course rhizomes would have been inefficient at acquiring nutrients. Fungi, which had occupied soils for much longer than plants, would already have evolved efficient means of foraging for mineral nutrients. It is the capacity of some extant bryophytes to form mycorrhizal interactions that suggests the possibility of an Ordovician origin of the symbiosis: the most basal group of symbiotic fungi and they are the only group associated with the basal group of embryophytes.

Hypotheses
H1:
Evidence indicates the terrestrialisation of early land plants began with their colonisation of the high latitude continental landmasses. It is hypothesised that the high CO2 Ordovician atmosphere allowed these photosynthetic pioneers to cope with extreme high latitude photoperiods – long periods of continuous darkness in the polar winter followed by long periods of daylight in the polar summer – by promoting growth and reproductive success via CO2 fertilisation.

H2: Early land plants may have been unable to colonise the lower latitude landmasses because the warmer climates induced high respiration rates that were not sufficiently compensated for by higher rates of photosynthesis.

H3: Evidence indicates early plants were mycorrhizal, it is therefore hypothesised that making mycorrhizal connections improved nutrient supply to the plants at a time when mineral nutrients in the soil were extremely limited.

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Claire Humphreys 2007