Before discussing the specifics of microfossil biology and distribution, it is useful to remind ourselves of the general types of questions we as geologists need to ask ourselves – or the paleontological specialist – when dealing with paleontological data to be used to determine the EOD.
Know Your Fossils
Firstly, it is important that you know the kind(s) of fossils your data source refers to. This may be explicitly stated in the source’s title or within the text. If not, some detective work will be needed. The genus name for the fossil is the most useful here (species names are not so important) and it can be searched for online which is the best way to find out the fossil’s biological affinity (or use the Paleobiology database – see “Resources”). Even if you do not know any detailed information about your fossil, at the very least you should be able to answer the following questions about it. These will help you eliminate some alternatives and narrow down your interpretation. Remember - paleoenvironmental determination is mainly a process of deduction. Some of these questions appear to be rather basic but it can be surprising how a little thoughtfulness about them can provide very useful insights.
This is a fundamental biological difference of course but it is not always easily apparent, especially when it comes to microfossils – for example, diatoms and radiolarian both have siliceous shells approximately the same size, yet one is a plant, the other an animal. Photosynthetic plants and algae (autotrophs) are, of course, dependant on light and inhabit the so-called “photic zone”. They will therefore not (or will rarely) be found alive in darkness. In the geological or paleoenvironmental context darkness means in water depths deeper than about 50-100 metres or in shallower, but turbid waters. Nevertheless, organisms that were once plants and are now dead can of course become part of the fossil record in sediments that were deposited well below the photic zone. Animals (heterotrophs) are not directly dependant on light for energy and have to seek out food from other sources so have, in time, colonised virtually every light and dark environment on earth.
In the context of being plant or animal, we have the added complexity of those organisms that are able to enter into co-operative relationships (willingly or unwillingly!) with others to help one (parasitism) or both (commensualism or symbiosis) survive. A sizable minority of Foraminifera, for example, are good at exploiting this particular strategy by forming symbiotic relationships with certain algae. The algae – through photosynthesis – produce excess nutrients which the foraminiferan then utilises. For those planktonic forams that are symbiotic this allows them to inhabit oceanic photic-zone waters far offshore which are otherwise very low in nutrients. For some benthonic forams it allows them, under the right conditions – to grow a shell up to several centimetres in size – a remarkable achievement for a single-celled animal.
Is your fossil a land or water dweller? A simplistic question perhaps but again, important. Almost all spores and pollen species for example originate from the land (or from fresh water plants or fungi) but because of their extreme resistance to transport damage can be found in significant quantities in sediments laid down hundreds of kilometres from the shoreline and in water depths of up to several thousand metres. Most continental, non-aquatic sediments are generally unfossiliferous although spores and pollen are occasionally recovered.
Organisms that live on or within the subaquatic substrate are perhaps more significantly “in tune” with the sedimentary environment and therefore more responsive to changes in it. In marine terms these organisms are called “Benthic or Benthonic”. Those organisms that live in the water column above the substrate are called “Planktonic” – if they more or less have little or no control over their movements – or “Nektonic” if they have a more active, purposeful way of moving. Planktonic plant organisms in particular (e.g. phytoplankton which includes nannoplankton, diatoms, some dinoflagellates etc.) are usually also found concentrated within the photic-zone of the world’s oceans and seas, i.e. in the upper c.50 metres. Planktonic animal organisms (e.g. planktonic foraminifera, radiolaria, other dinoflagellates etc.) are also concentrated in this region as this is where most of the food is, but they can (and do) exist many hundreds, even thousands of metres deeper.
For Foraminifera – which have representatives that are either planktonic or benthonic – it is important that the minimum information you have about your fossil foram is that if it is “planktonic”, “calcareous benthonic” or “agglutinated benthonic”, as these groupings may significantly aid your subsequent interpretation.
The majority of organisms found in the fossil record are of marine origins. Not only is this because the marine realm is the most organically productive environment on earth but also because the vast majority of sedimentary rocks (as repositories for fossils) are likewise marine in origin.
However, some fossil groups can transcend the marine-freshwater barrier and, for us, such groups include bivalves, ostracods and diatoms as well as the more obvious groups like fish and insects. For example, studies of lacustrine (lake-dwelling) ostracod species are very useful in both biozonation and paleoenvironmental interpretation in places like the Caspian, SE Asia or South America/West Africa.
Ecological zones with intermediate, or changing salinities such as mangrove belts, hypersaline (very salty) lagoons or brackish/hyposaline (low salinities where fresh and marine waters mix) marshes or estuaries cause special problems for organisms and tend to be inhabited by highly specialised animals and plants which, for this very reason, can be extremely useful paleoenvironmental indicators. Relative measurements of the apparent width of the mangrove belt can, for instance, be used to estimate high or low stand conditions.
Very, very few animals can exist without oxygen. While the vast majority of organisms prefer life with “normal” oxygen levels (either as free gas in air or as dissolved oxygen in water) some forms are capable of, or even prefer to, live under conditions of reduced oxygen levels – called “dysaerobic” conditions. For all practical purposes this is a feature of subaquatic animals and plants only rather than those that breathe oxygen in its gaseous form.
An important group that can do this, from our point of view at least, are the agglutinated foraminifera. This group can and does, of course, live quite happily under normally oxygenated conditions anyway but they can also tolerate lower than normal levels of dissolved oxygen. Also a small subgroup within the calcareous benthonic foraminifera, who construct their shells with Aragonite – a less common polymorph of calcium carbonate which is predominantly Calcite – can also tolerate low oxygen conditions as well.
The identification of low-oxygen paleoenvironments is important from a petroleum geology perspective as such environments are highly conducive to the deposition of source rocks.
Paleoclimatic information can be quite hard to come by unless good quality, quantitative data are available. Isotope measurements from the shells of (among others) planktonic foraminifera can be used to estimate oceanic paleotemperatures and also the proportion of one particular planktonic foram species relative to another can also indicate relative periods of warm and cool. The presence of so-called “larger (benthonic) foraminifera” – often associated with reef or carbonate-bank development – is a good indicator for warm, shallow waters.
Comparing relative trends in different groups of palynomorphs (particularly spores and pollen) can often yield good paleoclimatic information. The presence of charred (i.e. burnt) grass cuticle, even in deep offshore marine borehole samples, are the remnants of contemporaneous grassland fires and indicate hot and dry climates in the hinterland, whereas increases in montane pollen species indicates colder conditions. The presence of abundant freshwater algae can suggest periods of higher than normal rainfall.
A rather strange question to ask (“all fossils are dead aren’t they???”) but the answer can yield important paleoenvironmental information and one that is particularly relevant to the conundrum posed at the beginning of this section as to whether a fossil is “supposed to” be in a sample or not. Strictly speaking, the question should more correctly be “was the organism in question alive when buried or was it already dead?” In most cases the organism was probably already dead but in other instances this may not be the case.
Again a human analogy provides a good, if rather creepy, example here. Human societies that bury their dead bring the bodies to a specific location – cemeteries or mausoleums – where, if you like, the remains enter the fossil record. Those buried humans did not live at that particular locality – they have been brought there by external mechanisms. On the other hand, those well-known victims of the Pompeii volcanic eruption of Mount Vesuvius in the first century CE were buried by ash and became part of the fossil record in the places they actually lived at the time of death. Their bodies were not relocated afterwards – palaeontologists call this “post-mortem transport”. The first situation is referred to as a “death assemblage”(the people died first and were then transported to their final resting place), the latter is called a “life assemblage” (where the people were buried while in their life positions). The same principles apply to all fossils.
The fossil record contains a surprisingly high degree of post-mortem transport, from the simple sinking of dead bodies which in life inhabited surface oceanic waters but become part of the fossil record in deep water sediments, to organisms that lived and died in shallow waters, but are brought down into deep water sediments by erosion/shelf bypass mechanisms, turbidites etc.
In many occurrences where fossils of similar size or type are found in concentrations this often means the hard parts were transported to an accumulation site after death and therefore may be preserved in environmental settings alien to their normal mode of life.