Mass Extinctions, Lagerstatten, the Cambrian Explosion and Hominids’ obsession with Fossils: Part Two

Part Two: Lagerstatten

Lagerstatten are localities which are highly remarkable for for either their diversity or quality of preservation, they are rich with varied, well-preserved fossils, representing a wide variety of life from a particular era. To me, in a way this represents pretty much any and every fossil locality that we can find today. Really, we are astoundingly lucky to have any sort of record and deep insight into things that lived MILLIONS of year ago! Any fossil, ‘normally’ or ‘exceptionally’ preserved or not, is extremely important in the fossil record.

The most important lagerstatten are the Gmund, Solhhofen and Holzmaden deposits.

The Gmund is only a few centimetres thick and metres across and is a typical example of an obrution deposit (smothering). It contains well bedded echinoderm fauna, including crinoids. The fossils are preserved in muds, which is not where they would normally live and is puzzling, but other fossils other than echinoderms are not as well preserved. There are however oyster shells, that may represent many generations of the same species which have been exposed at the surface for a long time due to the overturned shells. They were probably all killed by the same kind of event. The shelly fauna may have become buried alive, which was fatal for the oysters, while other organisms could swim or burrow up and escape. This was also fatal for the echinoderms and hence the preservation of these fossils. Sucks to be them.

The Holzmaden deposit is a stagnation deposit where anoxic conditions were favoured due to changes in the hydrologic regime or mobilisation of brines. The site is much larger than the Gmund extending over large parts of Europe, which is important to keep in mind when comparing any lagerstatten i.e. you can’t compare them. Nektonic forms are dominant (plesiosaurs, ichthyosaurs, ammonites, fish etc.) with horizons of benthic microfauna. This picture is in accord with modern stagnant basins where benthic organisms decrease in size and diversity with decreasing oxygen and large fauna were stranded.

Solnhofen contains Archaeopteryx and medusae formed just before the end of the Jurassic regressive cycle. It extends 90m. In other areas of the unit there are no fossils and if this lagerstatten hadn’t been found the rocks would have probably been called non-fossiliferous. It is supposed that it is a permanently submerged restricted basin and is probably an obrution stagnant deposit (smothering and no oxygen). The most common fossils are crinoids with some reptiles and fish but mainly land animals. The inhabitants of the surrounding area may have become washed into this basin during storm episodes.

Lagerstatten are very different from other exceptional preservation sites like the Burgess shale as these form in many places and lagerstatten in just one. The Burgess shale has now been found in more than one locality which implies that it is more of a ‘time signature’ locality and not a unique physical setting. The Vendian Ediacaran fauna have a very puzzling relationship to the Burgess shale however, because they seem to disappear just as the Burgess is discovered. The fossils are found in sandy storm facies (in geology, a body of rock with particular characteristics) that generally do not preserve soft bodies parts along with turbidite facies. This indicates that the preservation must be an-actualistic in a global sense. Most Precambrian fossil deposits are an-actualistic. There may have been lack of bioturbation to preserve the organisms, and so it is supposed that cyanobacterial mats preserved the Ediacaran fauna and stopped them getting destroyed.

It has been found that where there are lagerstatten, it is likely that the preservation process which allowed them to form was inhibiting to other fossilisation processes. Therefore, in these areas, with the types of environment, or the types of biota encountered in the environment, you would be unlikely to find other biota preserved. So, instead of no fossils at all, at least you have some!

We do know however that few exceptional fossil localities, including lagerstatten, represent accurate pictures of past life at the time. For example, the Burgess shale shows a picture of Cambrian fauna that seemed to lack predators and was considered of little importance until the discovery of exceptionally preserved soft bodied predators. Suddenly we have a completely new perspective on the whole ecosystem with proof Darwinian life had formed by this time. So, the exceptionally preserved fossil on its own cannot give a good impression of life at this time but without it we would have an even greater misconception.

Almost 20% of the entire metazoans for Seposki’s Phanerozoic marine diversity patterns actually come from only three major Palaeozoic lagerstatten! Nearly all of these are soft bodied and have difficult affinities, but their presence and documentation is very important in understanding metazoan evolution. Just imagine what the curve and our current understanding might be like if we did not have this 20% of diversity! It might not shed so much light on aspects such as how much a mass extinction event affected evolution, or the pathway of organisms which do survive them but these factors are important from a palaeontological aspect and also geologically, something I will touch on later.

067-Marine_extinctions-sepkoski

Above is Sepkoski’s Curve, indicating major marine fauna diversity through the Cambrian to Tertiary. The colours indicate the numbers of families belonging to the Cambrian, Palaeozoic and Mesozoic ( or modern). The numbers indicate the ‘big five’ extinction events: Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Palaeogene (the dino killer). These will be discussed in more details in a subsequent post.

Fossils such as found in my mapping area (Beinn an Dubhaich, Isle of Skye; NB Dubhaich is pronounced ‘Ben an Du-vach’ as ‘bh’ in Scottish Gaelic (ga-lik) is a ‘v’) in concretions are sort of ‘conservation traps’ and show in a way some exceptional preservation similarities. They can show a range of fossils within a hand sized sphere which have been mixed up and brought together but do not show in that particular specimen any real indication of the environment they lived in. Even the rock they are composed of is different from the substrate as that can be muddy and the nodule is limestone. So, even ‘normal’ preservation may be not very useful unless taken into account with the surrounding and larger scale view of the geology and time.

In this sense, the identification of the fossil in the first place is the best way to determine the environment and not the fact that the fossil is preserved in that particular environment. If there is precipitation of calcite around a nucleation point such as one dead fauna then it is likely that any other dead fauna pieces around it might get preserved and not the others, which is specific and ‘selective’ fossilisation. But then, so is all fossilisation, but the point is that this does not give a good spatial and temporal relationship of the life and evolution at that location at that time. The same with mainly gryphea fossilised in some beds, indicating either an environment which is only beneficial to those fauna, or it could be selective preservation of these because of a certain factor. This can easily be misinterpreted, and could lead to a low diversity interpretation when it could have been high diversity and just not preserved.

Lagerstatten offer a very different kind of exceptional preservation view. Still, I feel that they are very useful in looking into the past, because with more knowledge on how they can be preserved we get an even more in depth geological view of the land at that time such as the climate and events that occurred. In terms of evolutionary occurrences lagerstatten are not so useful in adding to large scale view of the palaeontological record, but they do show us small exciting glimpses of the past.

Recommended Reading:

Schiffbauer, J D and Laflamme, M C, (2012), Lagerstatten through time: a collection of exceptional preservational pathways from the terminal Neoproterozoic through today, Society for Sedimentary Geology, 27(5), Available online: http://palaios.sepmonline.org/content/27/5/275.extract

McGowan, A J and Smith, A B, (2008), Are global Phanerozoic marine diversity curves truly global? A study of the relationship between regional rock records and global Phanerozoic marine diversity, Palaeobiology,  34(1), Available online: http://www.psjournals.org/doi/abs/10.1666/07019.1

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