Mass Extinctions, Lagerstatten, the Cambrian Explosion and Hominids’ Obsession with Fossils: Part One

So I have been debating what next to blog about and since I have recently been touching on subjects close to my heart at university – discussed in Showing off about Science and 12 Top tips for doing your Geology Mapping Dissertation –  I thought I could briefly touch on some other interesting things I worked on.

The topic of today’s post will be on palaeontology. Because of the vastness of the subject area covered in the heading, I thought it might be good to split it up into several parts in a series. The content will be drawn exclusively from notes leading up to my exam, and only edited a little.

To end the series on a timely note, I will discuss an interesting article in this month’s Geoscientist magazine, of the Geological Society, which looks at fossil collecting through the millennia and provides a very appropriate link to heritage in the process.

Part One: A Broad Introduction to the Fossilisation process and Exceptional Preservation

Understanding the completeness of the fossil record is essential for understanding evolution over long timescales, particularly when comparing biological groups that have different preservation histories. Studies have now veered from the original ‘collecting as much of the record as possible’, and have focused on getting as much data as possible from current fossils and using these for more ‘in depth’ research questions. Now we consider the adequacy of a (sedimentary rock) bed’s suitability to address a specific evolutionary question and the detail that can be acquired from a specific (fossil) specimen.

Consider this new form of questioning in relation to mixing of successive generations of fossils within one bed, and then, the original spatial distribution of a single generation of fossils within one bed. Fossils from one bed might contain very good (exceptionally preserved) fossils but these are highly biased and do not reflect the original morphologic and species distribution of the environment. On the other hand, ‘normally’ preserved specimens through as series of beds might show a complete ‘evolutionary’ series but still be biased by changes in habitat among successive beds. For example, within a single bed there can be changes in the morphological aspects of a species is down to evolution, or just the local environmental change at the time.

Fossils of such specimens as Cambrian Ediacaran fauna are so rarely preserved that it is a great window into past life, however, preservation is usually so far spaced stratigraphically and geographically that there are no good comparisons: fossils are generally environment specific or too ‘wide’ that evolutionary continuums are extremely incomplete. However, well preserved fossils such as common corals (scleractinian), echinoids and molluscs have a high (>50%) preservation rate and therefore provide a great representation of the range of environments and evolutionary processes through time.

Because sedimentation rates are so slow, it is common to find multiple generations of an organism (taxa) within one bed. This can be thought of as ‘time averaging’ of their remains but shows that the organisms were non contemporaneous (didn’t live together) and the phenomenon is pervasive in the fossil record. This is why so many fossils are re-orientated when fossilised and not in their original positions. If the sedimentation rate is so slow that it encompasses environmental change, then remains of more than one habitat can become mixed up. Therefore, a group of fossils from a time averaged deposit is unlikely to be a good ‘census’ model. It is useful to see the range of organisms that were alive over an amount of time out of interest, but not much use in research. Generally this doesn’t happen in areas of high sediment accumulation such as deltas, lagoons and lakes. This is why for seasonal variations etc. only foraminifera are useful, as the deep sea sediments of plankton will deposit continually and record each change. But, in these circumstances there are little high resolution or exceptionally preserved specimens.

This process of time averaging generally also encompasses spatial averaging too, because most population patches migrate and shift over time. Spatial mixing from post-mortem transport does not seem to pose a significant bias for many groups. It is generally easier now to recognise a species that has moved significantly from where it belongs (by using physical characteristics/species comparison). Fossil species assemblages that we don’t think came from a depositional area are more likely to arise from individual behaviour of an organism during its lifetime, or from time averaging of individuals whose environmental boundaries shifted over time.

It cannot be assumed that ‘normal’ samples of the fossil record from one geological time period are comparable to another period or that a species has a constant preservation potential over its entire evolutionary duration. This is down to the numerous plate tectonic movements, changes in climate (causing differences in dissolution rates of skeletons etc.), acquisition of different bio-minerals through time and the evolution of taxa that destroy remains of others (which may affect distributions within different periods). These biases are mainly important when trying to link patterns on large stretches of geological time.

When there is a ‘time gap’ in the sedimentary record (an unconformity) there is likely to be either a layer of no ‘hard part’ fossils, abundant time averaged hard parts or a thin layer of highly damaged hard parts over the discontinuity. This is because in times of no deposition there is less preservation by burial and more abrasion and erosion to erase the fossils completely or ‘mess them up’. Land surfaces are more likely to produce sedimentary gaps that other areas where there is less erosion. Deep sea environments are prone to burial and subduction and therefore we have most of our sea knowledge from shallow sea and lake deposits. In general, ‘normal’ fossils of average preservation and abundance need to be traced over a regional area to track a single habitat and can cover a large chunk of geological time. Increasing the length of time also increases the amount of gaps encountered in the fossil record. Small gaps from tidal rise and fall are easily pieced together, but larger gaps of sea level rise and fall have significantly more uncertainty as they affect a larger area, and coupled with species migration may cause problems for a palaeontologist.

Problems with preservation of fossils can be limited with further understanding of the fossilisation processes. Knowledge of the environments from geological, stratigraphical and chemical data can help determine the climate and habitat that the organism lived and was preserved in. This is then useful for possibilities of further degradation and movement after its death. This also helps when a species may have individually migrated for a purpose, as collective samples may prove. The knowledge of how sediments deposit themselves in different environments and at what time periods is extremely useful and can stop time accumulation biases. The ‘normal’ sites, however incomplete and un-well preserved they may be, can still provide an extremely good idea of the fossil environment and habitat and with increasing data we can change and adapt this knowledge all the time.

Recommended Reading:

Butterfield, N J, (2003), Exceptional Fossil Preservation and the Cambrian Explosion, Integrative and Comparative Biology, 43(1), Available online: http://icb.oxfordjournals.org/content/43/1/166.short

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