The failure of fossil records

[CaptainPanic]

I always wonder how we can form a picture of an ecosystem from fossil records?

All a fossil can tell us is that the animal lived in a certain age... but it cannot tell us what else lived (unless we see bite marks or find prey in its gut or something).

I'd like to have a little discussion, and I'm curious what you have to say. Here's a few of my issues (which may not necessarily be true):

 

1. Larger animals fossilize easier (or, rot away slower). Digging into quite recent layers of soil, how often do we find the smaller animals, despite the fact that we know smaller animals are more common in almost every ecosystem on earth at the moment? Has there ever been a study searching for animals which are fossilizing right now - and whether that record forming right now corresponds to the actual ecosystem?

 

2. We can see from modern ecosystems that most animals do not fossilize. In hot moist climates, everything rots away before it has a chance to fossilize, while in dry or cold climates animals have a much larger chance to be buried before rotting away. In the tropics, there's a lot of rotting, and relatively little soil movement, but in a desert, the soil moves a lot but not much rotting is going on.

 

3. We can date a fossil back to a certain number of million years ago, give or take a few million. But we know from more modern times that animals can (but don't always) evolve really fast. Also, populations can increase and decrease. In the last 10,000 years, the flora and fauna changed quite a bit on earth. Theoretically, it only has to take a 10 years for a population to double, in the right circumstances. Smaller animals can achieve that in a single year.

So, what we conclude now was living together, may well have been successors and even competitors rather than animals sharing an ecosystem.

 

4. Given the large - possibly climate induced - fluctuations in ecosystems, isn't it likely to assume such fluctuations also existed a long time ago? And animals that are thought to have lived in a single area might never have co-existed?

 

5. Animals we think live in a same area might both be migratory? One might be a summer visitor, the other a winter visitor?

 

6. Evolution might be a lot quicker than we think. In the right circumstances (a drastically changing environment), animals can evolve quickly. Even elephants only needed a few decades or a century to evolve smaller and less frequent tusks.

 

To take a random example: we seem to think that triceratops and t-rex lived together the same age - possibly in the same area (don't know - didn't check).

But what if both animals only thrived in short boon-times when life was perfect for them? Maybe the triceratops population was huge during only 5,000 years when some conditions changed locally - maybe a river changed its course or a predator went extinct due to a disease, breaking the predator-prey link. And maybe this huge population growth only happened twice or three times, causing us to think it was actually common during all 3 million years that carbon dating tells us this animal existed.

[Tres Juicy]

Good questions...

 

I can't help you I'm afraid. But I'll stick around for someone who can...

[Ophiolite]

They are good questions for vetebrate palaeontologists, perhaps, but for the majority of palaeontologists they tend to the irrelevant and wrong. I am somewhat mystified by the obsession the layperson has with dinosaurs. No! Not mystified, disgruntled. The majority of palaeontologists have no difficulty undetaking ecological studies and have been doing so for many decades. There are well established techniques for establishing whether we are dealing with an assembly of animals that lived together or were brought together after death. Even fifty years ago researchers could identify diachronous horizons and assess how the community had changed over time in parallel with the changing geography. (If you are interested, I think you could reference the work of Hallam in the UK, circa late 1960s.) I am confident techniques today are much more refined and discerning.

 

As to your example of Triceratops and T-Rex, frankly I have no idea as to the extent of their temporal or geographic overlap. However, species or genera known by more than one or two specimens are seen to live over an extended period of time. Overlap would be relatively easy to identify with confidence. Very many things about our techniques would need to wrong for your thesis to be valid.

 

Around three or four years ago it occured to me that if global extinctions, similar to what we are currently seeing did occur in a short time frame, then they might be difficult to spot in the fossil record. However, despite reasonably serious literature searching I could find nothing to support the notion. Your ideas seems quite similar, it just doesn't seem supported by the evidence.

[CaptainPanic]

They are good questions for vetebrate palaeontologists, perhaps, but for the majority of palaeontologists they tend to the irrelevant and wrong.

What's the difference between the two that my questions make sense to one but not to the other?

 

And aren't the majority of paleontologists also vetebrate paleontologists? (Wiki tells me that vetebrate means backbone, which includes fish, reptiles, amphibians and mammals). I know they aren't the largest group of animals in terms of number of species, but they included for a long time the largest animals alive.

 

Anyway, I don't really get your point (which says more about my knowledge about the topic than about your point, probably).

 

I am somewhat mystified by the obsession the layperson has with dinosaurs. No! Not mystified, disgruntled.

What? Did I do something wrong?

The majority of people interested in engineering (which is my field of work) like fast cars, jet fighters or sci-fi. I don't complain, even through the majority of engineers do far more relevant things for our society.

 

Dinosaurs are cool because they are the jet fighters of paleontology. In pop culture they are 15 meter tall chickens with teeth and a morning temper (which is probably wrong). They were just an example. Let's forget about the dinosaurs.

 

The majority of palaeontologists have no difficulty undetaking ecological studies and have been doing so for many decades. There are well established techniques for establishing whether we are dealing with an assembly of animals that lived together or were brought together after death. Even fifty years ago researchers could identify diachronous horizons and assess how the community had changed over time in parallel with the changing geography. (If you are interested, I think you could reference the work of Hallam in the UK, circa late 1960s.) I am confident techniques today are much more refined and discerning.

 

As to your example of Triceratops and T-Rex, frankly I have no idea as to the extent of their temporal or geographic overlap. However, species or genera known by more than one or two specimens are seen to live over an extended period of time. Overlap would be relatively easy to identify with confidence. Very many things about our techniques would need to wrong for your thesis to be valid.

If I understand the meaning of a diachronous horizon (google doesn't give a simple definition), then it is related to the geograpical location? I am not talking about the geographical vicinity of fossil findings. I am talking about how experts can learn about factors that would severely change a population and eco-system (if any).

 

The examples of triceratops and t-rex were just an example. I should have written "random animal A" and "random animal B", both of which fossils have been found in the same period of several million years. I already said: we forget about the dinos.

 

But one of my points was that you cannot distinguish between different millenia that occurred several million years ago (or can we?). How many clearly distinguishable layers are there?

So, how do we know that the ecology in the past did not go through severe changes of eco systems, which would mean that two species found at one site, in the same layer of soil did not live in the same ecosystem?

 

I know that without a good reason to assume that such drastic changes in an ecosystem occurred, there is no reason to assume it did... except that as we look at increasingly recent periods (which also become shorter in duration) we can also distinguish more details and changes. Isn't it reasonable to assume that there have always been a lot of changes?

 

I will refrain from giving any specific examples of such changes or how it might be affecting an ecosystem because I do not want to be attacked on my choice of example, but on the general idea.

 

Around three or four years ago it occured to me that if global extinctions, similar to what we are currently seeing did occur in a short time frame, then they might be difficult to spot in the fossil record. However, despite reasonably serious literature searching I could find nothing to support the notion. Your ideas seems quite similar, it just doesn't seem supported by the evidence.

Is it not supported by the evidence because there's evidence against it, or because there is nothing to support it?

 

p.s. Thanks for having the patience to go through this.

[Edtharan]

There are some things we can determine about an ecosystem based on an incomplete fossil record.

 

For example:

If we find a large predator, then we know there must be a prey species that it eats. Not only that, we can determine from the physiology of the predator (well what we can determine that is) how much prey it needs to survive.

 

This then allows us to work out things like birth rates of the prey, size of the prey species, birth rates of the predator (and thus whether the predator had a predator), and so on (eg: that parasites exist, scavengers exist, etc).

 

This is because no animal exists in isolation, and the relationships between them and the existence of one niche means that it create other niches too.

 

Sure, we can not know for certain that it is exactly that way, and we can not know every niche that existed, but there is much we can determine about an ecosystem from just a few fossils.

[questionposter]

2. We can see from modern ecosystems that most animals do not fossilize. In hot moist climates, everything rots away before it has a chance to fossilize, while in dry or cold climates animals have a much larger chance to be buried before rotting away. In the tropics, there's a lot of rotting, and relatively little soil movement, but in a desert, the soil moves a lot but not much rotting is going on.

In the older eras there were also localized desert pockets scattered about even in tropical regions, like in Africa, and in tropical climates I guess things did decay faster, but there is also a lot of soil erosion, so fossilization could have been accelerated in some areas due to the constant depositing of soil and sediments carried by streams.

 

3. We can date a fossil back to a certain number of million years ago, give or take a few million. But we know from more modern times that animals can (but don't always) evolve really fast. Also, populations can increase and decrease. In the last 10,000 years, the flora and fauna changed quite a bit on earth. Theoretically, it only has to take a 10 years for a population to double, in the right circumstances. Smaller animals can achieve that in a single year.

So, what we conclude now was living together, may well have been successors and even competitors rather than animals sharing an ecosystem.

The rate of evolution is usually a very long time, a large shift within a species doesn't really happen in only 10 thousand years without some kind of major catastrophe, otherwise I don't really get what your saying.

 

4. Given the large - possibly climate induced - fluctuations in ecosystems, isn't it likely to assume such fluctuations also existed a long time ago? And animals that are thought to have lived in a single area might never have co-existed?

Of course fluctuation in climate happened in the past, the only difference is before they happened more slowly and right now they are happening a lot faster than they ever did before.

 

 

 

6. Evolution might be a lot quicker than we think. In the right circumstances (a drastically changing environment), animals can evolve quickly. Even elephants only needed a few decades or a century to evolve smaller and less frequent tusks.

Smaller changes are more likely to happen quicker.

 

To take a random example: we seem to think that triceratops and t-rex lived together the same age - possibly in the same area (don't know - didn't check).

But what if both animals only thrived in short boon-times when life was perfect for them? Maybe the triceratops population was huge during only 5,000 years when some conditions changed locally - maybe a river changed its course or a predator went extinct due to a disease, breaking the predator-prey link. And maybe this huge population growth only happened twice or three times, causing us to think it was actually common during all 3 million years that carbon dating tells us this animal existed.

What your saying is possible but it's not supported by any evidence, so it's not very likely.

Edited January 23, 2012 by questionposter

[CaptainPanic]

The rate of evolution is usually a very long time, a large shift within a species doesn't really happen in only 10 thousand years without some kind of major catastrophe, otherwise I don't really get what your saying.

But there may be catastropes that will not show in fossil records. Diseases, changes in predator behavior, climate changes (see below) can all cause an accelerated evolution. And one change might trigger another change too.

 

For example, a certain behavior can cause big changes in a prey animal. Dolphins and orcas are known to invent new hunting methods. That will affect the prey soon enough as the most successful prey animals will have offspring.

 

Of course fluctuation in climate happened in the past, the only difference is before they happened more slowly and right now they are happening a lot faster than they ever did before.

Of course, I wasn't talking about the global warming of this modern time. I talked about the ice ages and interglacial periods though. (I'll try to be more specific next time, to avoid confusion).

 

We have evidence that the temperature on earth fluctuated over the last 400,000 years with a maximum peak-to-peak amplitude of 10 degrees Celsius. The differences in temperature happened in as little as 5000 years, and caused mass migrations, and possibly extinctions, population booms.

 

For example, we might think a certain animal lived in a vast area, but it actually might have migrated due to a changing climate while it lived in a much more narrow climate band.

[Ophiolite]

What's the difference between the two (vertebrate and invertebrate paleontologists) that my questions make sense to one but not to the other?

Invertebrate fossils are abundant. Some rocks are 99% composed of fossil invertebrates. Typically complete invertebrate specimens are found - this is much less common for vertebrates and there are many orders of magnitude fewer fossils. It is quite common to find invertebrate fossils in the spatial relationships they were in in life - this gives great insight to their ecology. This is rare for vertebrates.

 

 

And aren't the majority of paleontologists also vetebrate paleontologists?I know they aren't the largest group of animals in terms of number of species, but they included for a long time the largest animals alive.

I would be surprised if this were true. The prime means, after the Law of Superposition, of relative dating of sediments lies in its invertebrate fossil content. So apart from the intrinsic interest of each fossil family there is the need to understand these for stratigraphic purposes. Further, using the ecological insights from invertebrate study, coupled with detailed sedimentological information we can more readily define ancient environments.

 

As a rather informal and potentially inaccurate assessment of the number of vertebrate versus invertebrate palaeontologists one could google both terms in, say, Bing and Google scholar. This shows a preponderance in favour of vertebrate, which seems to support your assessment. However, I think if we searched by phyla, we might find a swing in the other direction. Certainly my university had numerous inveretebrate paleontolgists and zero vertebrate paleontologists, but anecdotal evidence isn't of much value.

 

The majority of people interested in engineering (which is my field of work) like fast cars, jet fighters or sci-fi. I don't complain, even through the majority of engineers do far more relevant things for our society.

Exactly. The general public are unaware that in the last two decades the average length of a drill bit run in the oil and gas industry has increased by an order of magnitude, yet without that change gas would be even more expensive than it is. (This is the field I am now in.) The same applies in palaeontology. It is invertebrate work where much of the really important stuff that seems mundane ot outsiders happens.

 

If I understand the meaning of a diachronous horizon (google doesn't give a simple definition), then it is related to the geograpical location? I am not talking about the geographical vicinity of fossil findings. I am talking about how experts can learn about factors that would severely change a population and eco-system (if any).

In the early days of geology stratigraphers made the simple assumption that a given rock bed of a particualr lithology, extending far geographically, was the same age wherever it was found. Later it was realised that since environments of deposition migrated that a particualr bed could represent different times. Such a bed is said to be diachronous. The work I was referring to, which I am recalling dimly from over four decades ago, was able to identify this diachronicity and subtle change sin ecology from the mix of fossils found in the bed.

 

But one of my points was that you cannot distinguish between different millenia that occurred several million years ago (or can we?). How many clearly distinguishable layers are there?

So, how do we know that the ecology in the past did not go through severe changes of eco systems, which would mean that two species found at one site, in the same layer of soil did not live in the same ecosystem?

In a given locality we can readily distinguish hundreds of layers that represent seasonal, or longer time intervals. There are typically marker beds that allow correlation from locality to locality. (Ash beds are a very good in this regard.) The lithology itself, through structures, texture, geochemistry and mineralogy tell us about the environment so we can identify abnormal conditions. Has this been done for every location and every age? No. But it has been done in more than enough cases to be as confident as one can be that we are not missing anything globally along the lines you have proposed.

 

 

 

Is it not supported by the evidence because there's evidence against it, or because there is nothing to support it?

I would say it is both. There is no evidence for it and in those instances where I have looked (and I concede it is not an in depth look) I find evidence that it is not the case. I would like to be able to summarise that evidence here, but it is not clear cut. I was looking only to satisfy myself, either way, not to prepare to convince others. However, if you care to suggest some specific examples I think I could find the data to demonstrate why the idea won't work.

[CaptainPanic]

Lacking any better way to express my surprise, I will say: Holy crap

That was a long, and very educational post. Thanks, Ophiolite.

 

I didn't know there was that much information available. Pardon me for being so ignorant in this field, but I really thought that the majority of paleontologists were digging for dino bones.

 

I knew that microorganisms had formed rocks, and I knew that this happened and happens (it still goes on) on a global scale. But I didn't know this was still visible to us in those rocks. I thought those rocks were, well, rocky at a molecular level. When you take some piece of rock, what can you see? You said that the fossils are mostly complete...

 

And what about my other comment in the later post? In the last 400,000 years, we have had quite severe fluctuations in the earth's temperature, which may have moved entire ecosystems by thousands of kilometers north/south and probably kicked numerous animals into extinction? Is there evidence of that in the past too? (I'm not in a rush, even though my 1st sentence of this last paragraph might sound impatient).

[Santalum]

I always wonder how we can form a picture of an ecosystem from fossil records?

All a fossil can tell us is that the animal lived in a certain age... but it cannot tell us what else lived (unless we see bite marks or find prey in its gut or something).

I'd like to have a little discussion, and I'm curious what you have to say. Here's a few of my issues (which may not necessarily be true):

 

1. Larger animals fossilize easier (or, rot away slower). Digging into quite recent layers of soil, how often do we find the smaller animals, despite the fact that we know smaller animals are more common in almost every ecosystem on earth at the moment? Has there ever been a study searching for animals which are fossilizing right now - and whether that record forming right now corresponds to the actual ecosystem?

 

2. We can see from modern ecosystems that most animals do not fossilize. In hot moist climates, everything rots away before it has a chance to fossilize, while in dry or cold climates animals have a much larger chance to be buried before rotting away. In the tropics, there's a lot of rotting, and relatively little soil movement, but in a desert, the soil moves a lot but not much rotting is going on.

 

3. We can date a fossil back to a certain number of million years ago, give or take a few million. But we know from more modern times that animals can (but don't always) evolve really fast. Also, populations can increase and decrease. In the last 10,000 years, the flora and fauna changed quite a bit on earth. Theoretically, it only has to take a 10 years for a population to double, in the right circumstances. Smaller animals can achieve that in a single year.

So, what we conclude now was living together, may well have been successors and even competitors rather than animals sharing an ecosystem.

 

4. Given the large - possibly climate induced - fluctuations in ecosystems, isn't it likely to assume such fluctuations also existed a long time ago? And animals that are thought to have lived in a single area might never have co-existed?

 

5. Animals we think live in a same area might both be migratory? One might be a summer visitor, the other a winter visitor?

 

6. Evolution might be a lot quicker than we think. In the right circumstances (a drastically changing environment), animals can evolve quickly. Even elephants only needed a few decades or a century to evolve smaller and less frequent tusks.

 

To take a random example: we seem to think that triceratops and t-rex lived together the same age - possibly in the same area (don't know - didn't check).

But what if both animals only thrived in short boon-times when life was perfect for them? Maybe the triceratops population was huge during only 5,000 years when some conditions changed locally - maybe a river changed its course or a predator went extinct due to a disease, breaking the predator-prey link. And maybe this huge population growth only happened twice or three times, causing us to think it was actually common during all 3 million years that carbon dating tells us this animal existed.

 

1) The older the fossils the less variety that is generally recovered and the broader the brush strokes of their reconstruction of paleo-ecosystems.

 

2) One thing you need to remember is that pollen grains preserve quite well and last for a very long time. So they can reconstruct a fair amount of details about the plant communities that existed at the time. And from that you can extrapolate to the animals present at the time based upon precedences in modern ecosystems.

 

3) Remember that the younger the fossil deposit the more biodiversity is preserved and the better that it is preserved. Consider the megafauna deposits at Riversleigh in QLD Australia for example. And extraordinary amount of biodiversity has been preserved in these and is still being recovered. Also given that the megafauna is, in evolutionary terms, very close to current fauna a very large amount of detail about past ecosystems can be accurately reconstructed.

[D H]

And what about my other comment in the later post? In the last 400,000 years, we have had quite severe fluctuations in the earth's temperature, which may have moved entire ecosystems by thousands of kilometers north/south and probably kicked numerous animals into extinction? Is there evidence of that in the past too?

There's plenty of evidence, arguably better than fossil evidence.

 

The title of this thread, "The failure of fossil records", is a bit disingenuous. Perhaps I'm being over sensitive, but that is a phrase straight out of the creationist playbook. Creationists argue (underhandedly, IMO) that anything but a complete record is an abject failure. Biologists from Darwin see it as remarkable that the fossil record can tell us anything. A complete record, as others have already pointed out, is not needed to tell us quite a bit about how things were and what lived a long, long time ago.

 

 

To answer the question raised in this post, ice cores from the Antarctic, Greenland, and mountain glaciers all over the world tell us a lot, an awful lot, about the climate of the recent past, recent meaning the last million years or so. The seasonal changes mean that those ice cores are banded, year by year, just like tree rings. They are time machines that let us look year by year into the ancient past. The variations in thickness, variations in amounts of soot and other aerosols, variations in chemical makeup, and variations in isotopes tell us quite a bit about snowfall, volcanic activity, average temperature, continental erosion, biological load, etc. Those ice cores are a treasure trove.

[swansont]

To take a random example: we seem to think that triceratops and t-rex lived together the same age - possibly in the same area (don't know - didn't check).

But what if both animals only thrived in short boon-times when life was perfect for them? Maybe the triceratops population was huge during only 5,000 years when some conditions changed locally - maybe a river changed its course or a predator went extinct due to a disease, breaking the predator-prey link. And maybe this huge population growth only happened twice or three times, causing us to think it was actually common during all 3 million years that carbon dating tells us this animal existed.

 

The argument is in the statistics. Fossilization is rare, so it is unlikely that you will have samples if the population is small, and even more unlikely you will have multiple samples. With Triceratops you have dozens, spread out geographically and temporally.

 

BWT, it would not be carbon dating.

[CaptainPanic]

[...]The title of this thread, "The failure of fossil records", is a bit disingenuous. Perhaps I'm being over sensitive, but that is a phrase straight out of the creationist playbook.

Oops... didn't mean to bring creationism into this thread. But I admit it was deliberately provocative.

Please accept my apologies for choosing exactly the wrong words.

 

To answer the question raised in this post, ice cores from the Antarctic, Greenland, and mountain glaciers all over the world tell us a lot, an awful lot, about the climate of the recent past, recent meaning the last million years or so. The seasonal changes mean that those ice cores are banded, year by year, just like tree rings. They are time machines that let us look year by year into the ancient past. The variations in thickness, variations in amounts of soot and other aerosols, variations in chemical makeup, and variations in isotopes tell us quite a bit about snowfall, volcanic activity, average temperature, continental erosion, biological load, etc. Those ice cores are a treasure trove.

My point, of course, was not that any data we gather is incorrect. I wanted to talk about the interpretation, interpolation and possibly extrapolation of these data points into an ecosystem.

 

I just thought (note: past tense) that the picture we're painting for ourselves of the ecosystems of millions of years ago is very much simplified, perhaps even over-simplified. I guess that's inevitable. But that may be more the case for laymen like myself. The picture for experts is much more detailed than I thought possible.

 

The ice cores are (I think?) one of the main inputs for the graphs I linked to earlier, showing the earth's temperature over a period of 400,000 years.

 

The argument is in the statistics. Fossilization is rare, so it is unlikely that you will have samples if the population is small, and even more unlikely you will have multiple samples. With Triceratops you have dozens, spread out geographically and temporally.

 

BWT, it would not be carbon dating.

What I meant, in statistical terms, is that I thought it is difficult to determine whether a certain data point is an outlier if you only have a couple of data points. An outlier could be a fossil of a species which had all but died out by the time the particular individual lived... meaning we think there were millions of them, when there was actually only one.

 

But before I throw more oil on the fire of this thread, I wish to point to Ophiolite's post (and others too, but I think this is a particular good one) which showed me that there are indeed additional arguments to check whether a datapoint is an outlier or not. We can gather additional information about the ecosystem - and if that is all relatively constant, there is an additional argument that assume that the population of the fossil is also relatively constant.

[Ophiolite]

I knew that microorganisms had formed rocks, and I knew that this happened and happens (it still goes on) on a global scale. But I didn't know this was still visible to us in those rocks. I thought those rocks were, well, rocky at a molecular level. When you take some piece of rock, what can you see? You said that the fossils are mostly complete... .

I wasn't talking of microorganisms. I'm speaking of biogenic limestones where the bulk of the rock consists of the skeletal remains of macro-fossils. (And microfossils also.) However, microfossils are an important though minor constituent of many rock types also. And in other rocks there can still be layers where there are many fossils within a single square metre and a few centimetres thickness. Fossils are numerous.

 

I've got a sample of Carboniferous limestone sitting on my desk at the moment, filled with complete brachiopod specimens and a host of fragmented material. If I were to extract these from this and other samples I could build up a detailed picture of the biodiveristy at this horizon. I could compare that biodiversity over time (going up and down the stratigraphic sequence) and space (going to other outcrops). Looking at isotope ratios in the shells we could estimate the temperature of the water during growth and also note geographic and temporal variations of this and how they affect the consequent mix of species.

 

From statistical dimensional studies of particular species we could determine average life span of organisms and note changes of this over time and space, relating these to changes in environment. We could identify the impact of the introduction of a new predator to the mix. And so on. This is not a field I was especially interested in, being a hard rock man, so I am giving a very superficial view of the type of ecological studies that can and are being conducted via the fossil record. Note also, that palaeoecology pays major respect to the Law of Uniformity, the present is the key to the past. Thus the palaeontologists make extensive use of comparison with modern ecosystems.

 

The detail and the surety of the detail is just an order of magnitude greater, at least, than what you surmised in the opening post.

 

 

I have to throw in an anecdote here that illustrates the awareness palaeontologists have of the risks inherent in some of the statisical studies I have hinted at. Our Head of Department, a world expert on the Carboniferous, was acting as external examiner for a student's Master's thesis. The student believed he could determine the specific age within the Carboniferous on the basis of the relative proportion of three common plant species. He had his samples laid out for inspection. The Professor looked at one tray and asked what age it was. The student replied. The professor picked up one of the specimens, broke it into four, replaced the pieces in the tray and asked 'What age is it now?'

[CaptainPanic]

Thanks Ophiolite. Just a quick reply to say I'm reading and learning.

[Halucigenia]

I always wonder how we can form a picture of an ecosystem from fossil records?

It's not just the fossil records, it's also the rock record itself. Even where the rocks do not contain abundant fossils we can distinguish the various environmnetal conditions that the rocks were laid down under using the technique from sedimentary geology known as facies interpretation.

 

When it is stated that river channels for example can be observed in the sedimentary strata, what it means is that the observed sedimentary sequences can be directly correlated with what we can observe in modern day landscapes where fluvial deposits are currently being laid down. Where we see flood plains and river channels we see specific sedimentary facies (a distinctive sequence of rock that forms under certain environmental conditions), which reflects a particular process or environment which can be directly compared with the sedimentary structures that can be observed within ancient rocks. Furthermore, these sedimentary features in one particular area can be traced through the strata through contiguous layers to adjacent areas that grade to different facies, for example flood plain to stable forest to river delta or estuarine to marine deposits showing that these environments existed at the same time in different areas i.e. they are contiguous in time.

 

Over and above that, we can see sedimentary facies changing with time in the same area grading from aeolian, lacustrine, fluvial, shore to deep marine facies and back again in cyclical sequences as we look at the sedimentary sequences in the stratigraphical column.

For example cyclic sequences of the lower Carboniferous show us that facies interpretation based on sedimentary structures (and also the ecology of the fossils) indicate marine regression and transgression over the sequence above the Devonian in the area of South west Scotland (where I live). The main sequence in this area above the Devonian is of calciferous sandstone formations changing to limestone formations above, showing the transition of terrestrial sedimentation in the Devonian to increasingly marine sedimentation of the Carboniferous. The calciferous sandstone formations and limestone formations alternate with terrestrial sediments and this shows a large scale cyclic sequence. Each marine / non marine cycle corresponding approximately to the ~400,000 year Milankovitch cycles.

The site that I studied specifically is itself above the calciferous sandstones in the geological sequence, and transitions from terrestrial to marine conditions are shown by coal beds (evidence of forests, root trace fossils were also in evidence indicating terrestrial flora) grading through shallow water shales to limestones and deeper water shales and back again to coal deposits.

My notes of this particular study indicate that the jellyfish fossils (yes, jellyfish, collections of which are in the Hunterian museum of Glasgow) that I found were found in the shale beds within the larger limestone formations in the north of the quarry where I was working. These shale beds alternate with limestone beds corresponding to the shorter ~20,000 year Milankovitch cycles. These shales are interpreted as a deep water facies at the height of each marine transgression. As well as the jellyfish the shales also contain worm tubes and Gastropods of the Genus Strapharollus indicating a deep water facies. These beds, whether marine or terrestrial, can be traced throughout the area and are not localised inliers as and must have been part of a widespread and long lasting marine transgression regression cyclical sequence.

 

It might interest you to know that we can tell that in the area of SW Scotland within the Lower Carboniferous, which was then a sheltered quiet basin between the continent of Laurassia and an island, that at least 5 sequences of the 400,000 Milankovich cycle can be found and within each of these 20 units of the 20,000 year cycles can be found.

 

For those not in the know this indicates periods of climate change cycling between warming at the height of the marine transgressions (sea level rise) and cooler periods when the waters receded and the areas returned to terrestrial depositional environments time and time again over a period of 2 million years. And we don't even need to date the rocks themselves to know how long this all took, we know by the fact that these sequences tie in with the Milankovitch cycles perfectly.

 

The quarry that I was working in showed one particular transgression sequence quite clearly beginning with seat earth - pale grey to white clay stone with some plant material and root trace fossils, through coals and shales to fine grained mudstones - again with plant remains. Above these were nodular black shales with lingula and fish fossils indicating a still lagoonal depositional environment grading into shallow marine black shales with brachiopod fossils etc. and above this the Dockra limestone containing many crinoids etc. Above this in another part of the quarry were the deep marine facies within which the impressions of jellyfish were found along with deep water gastropods and worm tubes etc.

 

It may not be as exiting as digging for dinosaurs, but it certainly fascinates me that we can get such an accurate picture of the palaeoenvironment from just studying the rocks themselves, the fossils and their associations.

 

In short, facies interpretation can be used to re-construct the palaeoenvironment by interpreting the sedimentary strata by recognising which environments that they would have been laid down in as we can see these facies forming in today's sedimentary deposits and can trace these facies across contiguous ancient strata and through the stratigraphical column in sequences that confirm both the geographical and temporal extent of these environments.

 

Sorry to wade in late to this thread, but I thought that this important part of geology should be mentioned.

I must try and look in here more often.

[Ophiolite]

You mentioned the Hunterian. Are you a graduate or post-graduate at Glasgow? I studied there in the late Eocene!

[Halucigenia]

You mentioned the Hunterian. Are you a graduate or post-graduate at Glasgow? I studied there in the late Eocene!

No, I just did a course there in what now feels like the in the early Eocene.