Thursday, 25 September 2014

How changes in Plant ecology shed light on the End Cretaceous Extinction Event.

One of the two main theories that seeks to explain the extinction event at the end of the Cretaceous postulates that a large bolide (extra-terrestrial object such as a comet or asteroid) smashed into the Yucatan Peninsula in Mexico close to the modern town of Chicxulub, resulting in a devastating explosion and long term climate change. Such an event would have led to a ‘nuclear winter’ as large amounts of material thrown up into the atmosphere significantly reduced the amount of sunlight reaching the planets surface, leading to a breakdown in food chains and causing the mass extinction event.

Such an event would have a profound impact on plant survival strategies, as darker skies would lead to a much shorter growing season, favouring deciduous plants over evergreens. It is difficult to tell directly from the fossil record whether a plant was deciduous or evergreen in nature, though deciduous plants on the whole have lower leaf mass per area (high leaf mass requires higher carbon investment) and higher vein densities (which aids carbon assimilation), so comparison of these two measures across an ecosystem should give some indication as to whether it was dominated by deciduous or evergreen forms.

In a paper published in the journal PLoS Biology on 16 September 2014, a team of scientists led by Benjamin Blonder of the Department of Ecologyand Evolutionary Biology at the University of Arizona and the Rocky MountainBiological Laboratory in Gothic, Colorado, describe the results of an analysis of fossil leaves from the Hell Creek and Fort Union formations in southwestern North Dakota, which cover a 2.2 million year section across the Cretaceous/Palaeocene boundary (the last 1.4 million years of the Cretaceous and first 800 000 years of the Palaeocene).

Blonder et al. found that the leaf mass per area of leaves in the study dropped by an average of 6 grams of dry leaf mass per meter squared across the Cretaceous/Palaeocene boundary, while vein density rose by 1.1 mm of vein per mm2 of leaf area across the boundary. These are small values across the total range of variability found in modern plants, but consistent with a change in ecosystem from (for example) tropical rainforest to tropical deciduous forest.

Visual representations of trait changes across the Cretaceous/Palaeocene Boundary. Top row, increase in vein density as seen in (A), ‘‘Dryophyllum’’subfalcatum,230.7 m stratigraphic depth, vein density = 2.5 mm-1 and (B) unknownnonmonocot (morphospecies FU87), 1.275 m depth, VD = 5.3 mm-1.Bottom row, decreases in leaf mass per area as seen through decreasing petiole width forsimilar leaf area in (C) ‘‘Ficus’’planicostata, 23.6 m depth, leaf mass per area = 136 g m-2and (D) ‘‘Populus’’nebrascensis, 7.2 m depth, leaf mass per area = 48 g m-2. Scale bars,(A and B) 500 mm and (C and D), 5 mm. Blonder et al. (2014).

These variables have been shown to change across the Cretaceous/Palaeocene boundary before, but over longer periods and over wider areas. By producing a study of a much shorter time-span over a limited area, Blonder et al. hope to provide evidence for a much faster change in ecosystem than was previously possible. They note that a drop in leaf mass per area and rise in vein density could also be caused by a drop in atmospheric carbon dioxide, which is a probable symptom of the other main theoretical cause of the End Cretaceous Extinction Event, the extensive volcanism associated with the formation of the Deccan Traps flood basalts in India. However such volcanism would be more likely to cause a long-term drop in atmospheric carbon dioxide, leading to a gradual change in leaf mass per area and vein density, where as the study supports an abrupt change in variables, harder to explain though a change in atmospheric chemistry, unless this was more sudden than could be explained by our current understanding of flood volcanism.
See also…

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