Echinoderm Taphonomy and Taphofacies: Using Patterns
of Fossil Preservation to Identify Ancient Environments and New Fossil Bonanzas
Carl Brett (Geology
Department, University of Cincinnati, Cincinnati, Ohio, USA)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
31 March 2000
Crinoids are
the Halimedas
of the Paleozoic. 1014-1015 crinoid individuals are
required to make some Paleozoic encrinite deposits.
Relatively
sizable echinoderm ossicles have the same density as quartz silt or fine sand -
they are easily transported.
Fair-weather
wave base = ~5-10 meters (30’).
Storm
wave base = ~50-100 meters (300’).
Brett
& Seilacher’s Fossil Lagerstatten has a storm taphonomy diagram.
Firmground/hardground
settings are commonly associated with maximum flooding surfaces (MFS) - times
of rapid deepening (transgressive systems tracts).
Obrution
deposits are rich in echinoderms. They tend to occur in later
transgressive systems tracts or in early highstand systems tracts.
Transgressive systems tracts in carbonate and some siliciclastic settings may
show widespread echinoderm packstones or grainstones. Later highstand
systems tracts and lowstand systems tracts generally have fewer echinoderms but
they may yield obrution beds.
Crinoid
logjams - probably perpendicular to current direction. Can see them
between ripple marks sometimes.
Transcript:
We
have this notion of taphonomy - the way in which fossils are preserved, and
what we might learn from them. There is certainly a tendency, up to that
time, in fact there still is among some people, the view of taphonomy, which is
the study of fossil preservation, mainly in a negative way. The fossil
record is robbed of a lot of detail by decay and destructive processes.
We all know that - it’s very biased. It’s not a real sample of the living
assemblages. And so there’s a downside to this field we call taphonomy
that tells us that the fossil record may have only as few as 10% of the species
of even things that had well preservable skeletons. And that’s a discouraging
fact. But, there’s an upside, too. In fact, Dave Meyer and I called
our symposium in 1984 “The Positive Aspects of Taphonomy” because we wanted to
emphasize what we’re seeing, jointly or independently too, to some degree, that
the way in which fossils are preserved can provide a great deal of information
about ancient environments. And this is a very interesting thing. I
did want to give a little credit to some of my former students here [gesturing
to title slide of talk] who helped with this. Heather Moffat, who almost
came here, but not quite. I couldn’t convince her. She had to go
over to Mary Droser out west. And Wendy Taylor was formerly at PRI - you
may know her from there. She’s been at many meetings. Rumor has it
that she might be at Ohio State before too long. Anyway, I’m no longer at
Rochester. But, I do work on this stuff, and I’m going to talk about
echinoderm taphonomy.
Taphofacies
was a term that we coined at the time of that meeting, in 1984 here in
Cincinnati. And it seems to have caught on. But the idea of that
one is characterizing particular environments not based on the sediment or the
species of fossils that are present, rather based on the way the fossils are
preserved. And this odd word [gesturing to the word “lagerstatten”] - of
course this was a slide meant for a meeting, but that strange Germanism is a
term that means “mother-lode”, actually, in German, and refers to bonanza
deposits where you have lots of fossils, and also sometimes extraordinarily
preserved fossils - extraordinary fossil assemblages.
Now,
echinoderms are very special, in looking at taphonomy, because they are so
fragile, as we all know. A field of modern sea lilies, crinoids,
photographed out the window of a sub, in the Bahamas, showing these things as
they are living today in a deep sea environment. And when we think about
crinoids today, they’re relatively in the small numbers. Dave Meyer
probably does more than anyone else to document the life habits and even the
taphonomy of modern crinoids. But, there really are relatively small in
their impact on sediments. Today, the main sediment formers are green
algae, like Halimeda. But, we sometimes call crinoids (in
particular), but other echinoderms too, the Halimedas of the Paleozoic,
because they had a role in that age of time which is similar to that of modern
calcareous algae. They produce enormous amounts of sediment which then
form the great limestone deposits that we see. And everyone’s familiar
with scenes like this, where we see this broken up junk that looks like nuts
and bolts, which is pieces of crinoid stem, pieces of echinoderm plates,
usually broken apart. Now, when we look at vast limestone deposits like
these - these are in Alberta. This was the site of a thesis, I guess
almost completed, by Rich Terry of Cincinnati. He did his work in Alberta
on a Ph.D. project. And, looking at these rocks, which are the Samm and
overlying Rundall Formation - these are very thick, and represent enormous
volumes of carbonate sediment. And, when you get down to it, a great deal
of that sediment is made of things like this - these remarkable plates of
echinoderms. Here are crinoid stem pieces shown in thin section.
Just small bits and pieces, broken up, sometimes better preserved, but in many
cases, just disarticulated into pieces. When you think of some of these
limestones, like this, the Mississippian of Alberta, or the famous Burlington
Limestone of the midwest, they extend for hundreds of square miles and are
sometimes a hundred or more feet thick, you are really talking about an
enormous volume of skeletal material. And again, they have made some
estimates some time ago about how many modern crinoids it would take to fill a
cubic decimeter, and I will probably will err if I give that number. What
is it - do you remember, offhand? About a hundred-some? Something
on that order. And when you compute out the total volume that is in some
of these rocks, these little bits and pieces of skeletal material, you are
talking about numbers of individual crinoids that we can only express in
scientific notation: 1014 or 1015 - that’s “1” with 15
zeros behind it - we don’t have a name for that - it’s well beyond
trillion. So, there’s a lot of crinoids in this - tremendous numbers
going into making some of these deposits. And they are special because,
like all echinoderm plates, they’re made of single crystalline calcite, and
under a microscope, we can see that each plate of a crinoid or of an echinoid
or a starfish has a single crystal. And those crystals, because of their
relative large size, are quite resistant to dissolution. Even some
marbles - limestones that have undergone metamorphism - will still show
occasional echinoderm fossils in them. They’re one of the last things to
be destroyed in the destructive processes of metamorphism. And, because
of that, they tend to accumulate in vast numbers on the sea bottom, or they did
in the past.
Now,
I want to talk a little bit about taphonomic processes. What we’ve
learned from modern experimental studies, of which there have been quite a few,
and again, I think that one could say: a lot of that goes back to a famous
abstract by Dave Meyer in 1972 (I think) which was one of the first
experimental taphonomy studies with echinoderms, in which he looked at the
decay of modern crinoids and brittlestars and found out a stunning fact that
was really quite significant when we’re seeing the fossil record, and that is
that these things decay very rapidly. Virtually all echinoderms have a
skeleton made of multiple elements. These disarticulate quite rapidly,
and we can say, depending on the kind of echinoderm, that days to a few months
in a normal marine environment. The carcasses will resist
disarticulation, or breaking up into individual pieces, for a while, even with
some transport, as long as it happens just a few hours after they die.
For some echinoderms, people have taken them and put them in tumbling barrels
of the sort that jewelers use to make polished stone, and let them roll.
And they can roll for a while before they start to shred. But, let them
decay for just a few hours, and all it takes is a slight swirl of the water,
and the pieces all fall apart. So, they hang together, but for only a
short time. And that means they can actually be transported as long as
you find some agent to transport them that can act very quickly after they’re
dead. And, by the same token, most echinoderms, if they’re found
relatively complete, probably have not been transported very far at all.
The time required for complete disarticulation, that is breaking down into
individual pieces, varies. It’s increased at low temperatures - it takes
longer. There have been some nice experimental studies by Susas Kidwell
and Tom Baumiller and other people working with modern sea urchins and others,
and they found this. They found that high salinity retards decay also
because most bacteria don’t do well in high salinity. And they find that
anoxia, or non-oxygenated environments, also contribute, because they also somewhat
retard decay, but not too much, as we’ll see in the very next slide.
Anaerobic decay of tissues, that is, that [decay] occurs without oxygen, is
almost as fast, but not quite, as aerobic decay, or in an oxygenated
environment. But, anoxia does exclude scavenging animals, and scavengers
have a major role in knocking skeletons apart.
Echinoderm
ossicles, or their skeletal pieces, are very low density. This is also
interesting. And so, individual pieces are easily transported. A
big chunk of echinoderm plate or a big stem piece of a crinoid may be
equivalent in its density to a very small grain of quartz. So a current
that will pull only quartz silt or very fine sand may still move some pretty
sizable pieces of echinoderms. And because they are low density, it may
be difficult for them to become abraded. But, this does happen.
Dave showed this. Other people have shown it. It might especially
be true if they’ve undergone just a little bit of burial and some minerals
fills in their holes. If I didn’t say so, echinoderm plates are not only
single crystals, they are full of holes, they are a lattice work full of
holes. And if those holes get filled in, they are more easily
abraded. And this would happen, and probably did happen, in cases where the
skeletal remains were buried once, then dug up. If we look at
Cincinnatian rocks, we can see evidence of skeletons being buried, exhumed,
buried again, exhumed, and tumbled over and over. And this, of course,
can lead to some very degraded looking skeletal pieces.
One
thing I want to talk about briefly is 3 different styles of echinoderms, in
terms of their tendency to break down. This is a generality. As a
rough approximation, I divided them into 3 broad categories. Type I
echinoderms, in terms of decay, are ones that are so fragile, like brittlestars
and starfish, that they virtually cannot hold together more than a few days, at
the most, and have any articulation at all. So, in a very short period of
time, these will be degraded into a pile of plates. And this means
something very important. If we see these in the fossil record, it’s an
all-or-nothing sort of deal. Either they were snuffed out almost alive,
or indeed sometimes alive, or they’re not preserved at all, usually.
They’re probably there in the sediment, and those that are very clever may be
able to pick little individual pieces and say “Yes, these were parts of
starfish”. But, in general, you won’t see them unless you see the results
of a very bad day. The Type II echinoderms are things like some crinoids
and certain sea urchins that have some parts to the skeleton that were more
resistant to decay than others. And these will yield a variety of
different kinds of pieces, depending on how long they’ve had to be around on
the sea bottom and subject to decay, scavenging, and so on. If they’re
buried instantly, of course, then you may get this kind of preservation.
Otherwise, you’ll see this kind of thing - crowns, or partial crowns parts of
the arms gone, in the case of the crinoid; sea urchin - most of its spines
gone. And, after a period of a few weeks to a year or so, these are just
rough sort of estimates, but they’re the right ballpark, you’ll see now broken
parts, maybe a proximal calyx, pieces of the corona, or test of the echinoid.
And then again, eventually, within a pretty short time geologically, but longer
than this kind [gesturing to Type I echinoderms]. And the third kind they
talk about is those that have quite a bit of their skeleton, or quite a bit of
their body encapsulated in a skeleton that is rigid, and it may have really
interlocking, zipper-like affairs at the edges of the plate so that they don’t
come apart very easily. So, like sea biscuits, heart urchins, and sand
dollars, and the calices of blastoids that may even suffer abrasion and some
physical breakage without disarticulating. And that’s an unusual
thing. Those actually probably tell us less, because it’s hard to judge
with these whether it’s been a few days or a few years, than it is in a case
like these middle ones [Type II echinoderms] which are really quite sensitive,
I think, to telling you the timescale of the accumulation of the
sediment. Was it a day, a few weeks, less than a year or so? And, I
think just to illustrate once again, Type I echinoderms - when we see them,
it’s a grand day for us, it’s a very bad day for them. And this is a
brittlestar out of the famous starfish beds in the Jurassic of England, which
must have been buried very, very quickly. It is perfectly intact.
It may have been caught up and buried alive by a surge of storm sand.
Here’s the opposite extreme: here’s an echinoderm, Eucalyptocrinites,
that you’ve seen out of the Waldron Shale nearby here, where the calyx is so
tough, that it would last for periods long enough to get encrusted by bryozoans
as it sat as a little island of hard substrate on the sea bottom and served as
a place for little animals to settle (larvae), and grow up even as colonies
before that fell apart into plates. So, here we are talking more about the
Type III situations. We have to be aware of the kind of echinoderms that
you’re dealing with in order to use these as some sort of indicator of
conditions.
A
notion that, again we thought about, Dave Meyer proposed also a scale like this
in a paper written with Rich Terry, is the notion of taphonomic grade.
And, that is when you’re looking at particular species of fossils, such as Eucalyptocrinites
here, and see different stages that are telling us different things about how
they were buried. Now, Type A is just totally arbitrary in terms of how
you call them. But, the perfect articulation of the entire skeleton,
something like that, even with appendages, delicate features like pinnules on
arms still, little side branches, ........... nearly complete specimens like
this. Colin Sumrall picked up one of these about the size of a football
[referring to a Waldron Shale Eucalyptocrinites calyx] last summer and
made me ill on my birthday. Anyway, it was a wonderful gift on my
birthday. And there is this beautiful thing. But, the arms are
missing here, and only the partition plates in this case remain. This one
wasn’t buried as fast as this. It looks like we may have these little
rhynchonellid brachiopods that tucked themselves in there and grew. And,
then again, the next sort of skeletal grade for this might be just a calyx
without these structures, intact tougher modules, or pieces, along these pieces
of stem, and so forth. But, you know ultimately, they’re going to go into
things like this. These are also from that ...... sort of thing. A
holdfast still anchored in place might be one of the last things to break
apart. Or just skeletal sand, you can even see a piece of Eucalyptocrinites
- here’s one of those partition plates, kind of abraded-looking. And so
you’re getting these different grades indicative of the conditions under which
these sediments accumulated. And, they’re really pretty sensitive
indicators this way. People have tried to make scales like this, for
echinoids, based on the percentage of spines and whether the top and bottom
portions like here, which are pretty fragile, are still in place; whether
they’ve tried to break apart and so on. So, we look at these different
categories and try to quantify them.
Another
thing I want to talk about is the notion of background versus event signatures
in our sedimentary rocks. We get 2 different kinds of taphonomic, or
preservational, signatures coming out of sediments and rocks. And one of
them is what I call background, and that refers to preservational features of
the skeleton develop over long periods of time during normal day-to-day
conditions - things fall apart more or less in place. If it’s a high
energy environment with a lots of waves and current, they may be broken or they
may be abraded. In the low energy environment, they won’t be broken or
abraded, but they may be pitted by acids on the sea bottom. So, one can
look at the way in which even little skeletal particles are preserved, like
these pieces of crinoid, to get some clues as to the environment - was it very
energetic, or rough, water, or was it very quiet and so on. And, the
features that we look at there are, of course, the degree of disarticulation -
re.......tation, that’s the way you say it in western New York ....... abrasion,
or worn, bioerosion ................ [end of tape] ........ every day, 99% of
the time. But it so turns out that our fossil record is not really always
one of normal, background conditions. In fact, very many times, if you
look really closely, we find that there is some signature of a big event.
A big event will come along and make a difference. And these are events
which would have lasted a period of only perhaps hours or a few days, and they
may be very rare. A storm which only happens once a century or once a
millenium may take all the sediment which has accumulated there the past
several decades or centuries and stir it all up and slap it back down, and in
this way, preserve the frozen moment of terror, as Eric Ager once said.
And Dave Meyer has also quoted this. He said the geologic record is like
the life of a soldier with months of boredom and hours of terror. That’s
great. And it’s the hours of terror that really leave a major imprint in
some beds. And here, from the Silurian of New York, crinoids buried with
their crowns outstretched, even to the extent that we’re capturing even the
extent of tiering, or different levels in the sediment. What we’ve
actually found in some cases is you find a buried seabottom - short-stemmed crinoids
down almost on the bottom, with the whole thing intact, long-stemmed crinoids
sort of zig-zag sometimes, or telescope up through a whole layer of nearly
barren mud. And what that’s telling you is that crinoid fell apart as the
mud was accumulating. Because it was long-stemmed, pieces of it are
preserved way up higher in the sediment than these because it grew at a higher
level. And here, even more interesting things, we found sometimes, when
we would split the barren mud above a burial layer, at least in the Rochester
Shale, which is something we spent a lot of time looking at. What we
found - brittlestars in there, and in very odd positions. Why are they
there? We suspect that what they represent is failed refugees that tried
to escape burial but didn’t make it. So they’re found in various odd
positions up in the sediment and they apparently were preserved, perhaps trying
to struggle their way out, but didn’t go all the way. As were the
enrolled trilobites, another very interesting story, but I’m talking about
echinoderms tonight.
Now,
thinking about events, it’s important to think a little bit about what happens
during an event. And the most typical kind of event, as you probably
know, that influences shallow marine sediments, and you see it all over the
place around here, is the influence of oversized storms - giant hurricanes and
other storms that rip through. And I want to say just a little bit about
what happens during one of these big storms. This is a very, very
schematic cartoon, trying to show some of those things. During a very big
storm, several things happen. First, in many cases, water is pushed
onshore, pushed onshore by the force of onshore-blowing winds and by pressure
gradients. And this water piles up, and especially if it’s coupled with
tides, it may surge way up onto the coast. A second thing that happens
during storms is that the level at which waves work the bottom is lowered very,
very much from normal conditions, where maybe only about 10 meters, about 30
feet, down where waves have their most, their deepest impact. But during
a storm, this depth may go down to 50 or maybe even 100 meters of water, maybe
down 300 feet in some cases, where the waves are at least slightly influencing
the bottom. Those waves, in shallow water, pick up material and lift the
finer-grained sediment up and out and leave behind the coarser, heavier stuff,
such as pieces of big skeletons of animals. But, what also happens in a
storm is that all this water that has been stirred up onto shore must go
somewhere - it eventually has to recirculate back down into deep water because
there’s a pressure difference here. The pressure pushes down this water
and it squirts it back down into the deeper water, forming what we call a
gradient current. Now the gradient current starts out as a rip
current. But, they are very strong - they occur during the late phases of
a storm, and what happens is, all the fine-grained sediment, all the
fine-grained sand, silt, and especially mud, which are churned up in the shallow
water, get caught up in these gradient currents and then they are drifted and
carried down into deeper water, even areas which are not normally ever affected
by storm waves. So, there are several effects that happen. As a
result, the kind of sediment packages that we get differ depending on whether
you’re in shallow water or deep. In the very shallow water, storm after
storm hits that bottom and picks up the debris, it slaps it back down, and it
may in fact pick up debris from older storm beds and rework the whole mess and
let it all fall back down. As you go farther out, though, what we see in
the beds is beds which are graded, which have coarse debris at the bottom, but
then finer silt and mud going upward. And when you get way out, maybe in
50 or 100 meters of water, you are in a place where the actual influence of the
waves stops, but the layers of silt and mud which were picked up back here
entrained and carried out and slapped back down fairly quickly. And it’s
here, just here, where the best preservation occurs. Of course, you can
also get beautiful preservation up here if you just so happen to catch your
echinoderms in one of these burial layers and it never happens to be picked up
and worked over by any more storms. That’s going to be rare, because the
next storm is going to come through and it is likely going to pick up and toss
everything around. So, there’s a spectrum of different kinds of deposits
that we can relate to storms. And we carry this also to the way in which
echinoderms are preserved.
OK,
..................... later on too much, again, this is a concept we came up
with, the notion that one should look at sedimentary rocks and look at the way
in which any fossils, and here I’m emphasizing echinoderms, are preserved - the
taphonomic features. We can sleuth out background and event signatures -
those features that happened over long periods of regular, normal conditions,
and those that happened during the extreme events. We can typify
particular sedimentary environments by the way in which [fossils] are
preserved. This is a diagram actually from Bill Ausich, work on
taphofacies of echinoderms. All I want to point out here - the left
triangle shows the degree of environmental destructiveness, I guess you would say
- the blacker it is, the worse the day it was, the worse the conditions you get
for preservation - high energy, lots of reworking, actually you can see what
they are: degree of reworking, frequency of major scouring or erosional
events. And as you go down a slope from shallow to deep, the effect is
going to get less and less, as show by this being less and less black, the
triangle. The other upside-down triangle shows conditions of echinoderm
preservation, in which the blacker it is in this case, the better the preservation.
And you can kind of see that it goes the opposite way, that you see more whole
calyxes, you see more with arms on them as you go down in here, up to a point,
it improves. But, then it gets worse again as get into the really deep conditions,
because you get too far away from the source of the burial mud and so on.
OK,
I want to very briefly talk about a kind of spectrum of different styles of
preservation - I’ll go through these fairly quickly. I want to talk about
shallow to deep gradients in limestone depositional areas - what we call
carbonate - shallow to deep, and the way in which preservation conditions
affect echinoderm burial. And then I want to talk about this for
siliciclastics, that means silts, sands, muds that are derived from erosion off
the land.
The
carbonates, the limestones, are formed from usually the skeletons of animals in
place. Now, a common model that we use shows limestone deposition on a
very gently dipping slope or ramp, and usually this is set up in such a way that
we get a high energy zone somewhat offshore, a shallow, restricted lagoon
inboard. In many cases, you may get a zone of reefs or bioherms in
here. And then, as you get farther out, you get the backflow of lime
debris which is carried from where it’s produced. Many crinoids live in
this area, an area which is buffeted by waves, and may be a good site for
production. Now I want to talk about, I’m not going to spend much time on
the lagoon - but we’ll talk about the crinoid banks going down into deeper
water marlstone. And again, this is a sort of a high energy to low energy
gradient. Now, first I want to talk about the kinds of preservation in
what we call skeletal shoals, where we have real rough water conditions, where
what is generally happening is you get massive limestones - don’t worry about
all the details. You may get them showing ripple marks, scour features,
and, of course, most of the fossils are disarticulated into fragments.
OK, these are what these kind of look like - I’m trying to think of a good
local analogue - but some of the Silurian limestones like what Wylie found
around here like this, and of course, parts of the Lexington Limestone look
like this in the Ordovician. And the general product that we get out of
this, at least for most of the echinoderms, is pretty poor in most cases:
broken up and disarticulated into abraded pieces because this stuff is worked
back and forth on a day-to-day basis. Occasionally you’ll get a partial
calyx, or maybe even better - that happens to represent something that was
buried quickly and just stays there and wasn’t dug back up again. But, on
the whole, abraded, very broken apart skeletal material characterizes this
area. Now in some cases when we get just a little deeper, we may see the
buildup of a small reef - this is a bioherm, or small reef as you might see and
do see out in the Silurian nearby here. This happens to be in Niagara
Gorge. And you can see other interesting things - this bioherm is going
upward from this massive limestone into this marly, shaley material and it’s
growing upward as the water is getting deeper. Now, these are very
interesting situations. These were probably the most optimal places for
Paleozoic echinoderms, especially crinoids, to live. And, when we look at
them, we find an interesting thing - on the surfaces of the bioherms or reefs
themselves, we find maybe mats of corals and so forth. We also find a lot
of the attachment structures, and only the attachment structures of the
crinoids. This thing that looks like a caterpillar is not a stem per se,
but is actually a holdfast. And the surfaces of these little lumps and
mounds provide an excellent area for the animals to colonize - they were firm,
in a nice high energy environment, which crinoids like for their feeding.
But, because it is also a high energy environment, the only thing that is left
of any of these reefs is the roots. They were anchored in place so
toughly that they stayed there. And around the flanks, this is the kind
of stuff you see - rubble of material that was shed off the reef, and sometimes
even in graded beds, where they’re coarse to fine, coarse to fine, representing
storms. And occasionally here you’ll find well preserved things, but it’s
rare, because of high energy and reef reworking. As we go out on the
limestone platform, we see skeletal material that was worked over by the
backflowing gradient currents, and sometimes ripples. And you’ve seen
these kinds of deposits nearby - some of the Fairview Formation and even some
of the Kope Formation would be this. And these are storm-dominated
limestone shelf layers and these can have some very interesting features.
........... all of a sudden sometimes, at least their last deposition was
sudden, and they may have debris that was worked over for millenia, but the
last thing which put the ripples on was one particularly bad day. And,
sometimes as a result, on the tops of these, or even within them, you can find
very well preserved material. Mostly what you may find is just worked
over debris. These are not as thick as the shoal limestones - they may
have shales in between them that represent quieter conditions, and breaks where
we go from these very violent storms that put in ripples and so on and may have
knocked down crinoids to a kind of a quieter, normal condition. Sometimes
the tops of these limestones are exposed for a long time and they become
frozen, as it were, or cemented, to form hardgrounds. Then these
limestones can support a whole new set of animals - many echinoderms that are
anchored by a holdfast, such as these little crinoids that are anchored to a
hardground. This means that after these limestones are dumped and rippled
in some cases, there’s a long period of time where they just sit there and they
become cemented as hard as concrete. And then you get this special set of
echinoderms on them - edrioasteroids and these little guys which were anchored
in place. And in most cases, of course, those just fall apart into little
plates, but even though, there may be at these surfaces millenia, thousands of
years of no deposition, in some cases, you have one final, big, bad day and the
whole surface is snuffed by a plume of mud from another big storm somewhere
else and the things are preserved right where they live, still anchored in
place. And that’s a remarkable feature of our hardgrounds, or hard
seabottoms. But, if we get still farther out, sometimes we get into an
area that’s lower energy and maybe not so much of the good rapid sedimentation
we need, so in many cases, echinoderms may simply fall apart in place - they
won’t move very much, but they’re also not buried quickly. So, you might
get some pretty good preservation, but not as spectacular as some of these more
proximal settings, except in some cases. In some cases, you do actually
find what are actually carbonate mud flows, lime mud that was carried out and
dumped fast as these little graded beds. And there too, occasionally, we
can get some pretty interesting preservation. Many of the local soft bottom
echinoderms which were living out in deeper water can occasionally be caught up
and preserved pretty well. .................. So, sometimes in
deeper water you can well preserved things. A lot of times, just things
falling apart in place - they’re not moved much, they’re not abraded, they’re
not broken, they’re also not that beautifully preserved.
Now,
to shift over to the sand to mud gradient, we’re not in limestones anymore, we
have some cases, many cases, where we have a little bit of a steeper slope,
still fairly gentle, less than one-half of a degree, where you have deltas
dumping out in the seaway, or at least a lot of sediment coming out of
rivers. And you generally go from coarse sand in the nearshore, out into
silts, and finally, all pure mud. And when you see them in a cycle,
sometimes you go from mudstone to sandstone that indicates shallowing.
Now, take it from the shallow end, the shallow end here is what I call
storm-dominated shoreface. That means right at the lower end of wave base
and you get sometimes some spectacular, cross-bedded sands. These are
lousy places to look for echinoderms in general. Sediments are shifting
around fast, and are deposited quickly, they’re unstable bottoms, and in
general, these are not very good situations for preservation, or even for
living. But, there are exceptions. These are sand dollars. If
you lived in that kind of high energy environment, there are whole sea biscuits
here, preserved in a sandstone, but some of those are even abraded.
They’re so tough that they just got rolled around and knocked around and didn’t
break, but they abraded. But sometimes also in these sandy environments,
not so many of the anchored echinoderms like crinoids, but mobile things like
starfish are there, and once in a great while, they can get caught up in a sand
sweep during a big storm, and basically be very well preserved in these very
shallow, sandy environments. On the whole, though, I don’t recommend
spending a lot of time prospecting for echinoderms in these kinds of deposits.
Thick sandstones can yield beautiful stuff, but it’s a rare accident if you
find it because mostly, they don’t ................ As we go out into the
area where we have storms sweep layers of sand out onto the mud very quickly,
we get sediments that look rather like this - it doesn’t look too different
from some rocks around here, but these are sandstones, instead of
limestones. And these can be pretty good areas for preservation.
I’ve got one of the famous ones, where we’ve got this hummocky cross-bedding -
sort of mounded layers of sand that were dumped by storms, currents, and
waves. Sometimes the bottoms of these beds can show spectacular
preservation. This is indeed the starfish bed - this is what is looks
like in a side view - the Jurassic of England along the Dorset coast - and
that’s what’s on the bottom of that sandstone. So sand sweeps get, again,
depending on how shallow you are, the shifting sand makes a pretty harsh
environment, but some echinoderms, especially the more mobile ones, can survive
there, and once in a while, they’re going to get caught up in these deposits
and preserved very well. As we go still deeper now, we’re getting more
mud and less of the coarser sediment. And this, of course, is a familiar
scene to you all. This is actually more skeletal sand and debris than it
is really quartz sand, but there is some quartz silt layers in here, but
dominantly shale. Storm-influenced deeper shelf - now you’re getting to
the place where right down near the lower end of where storm waves actually
touch the bottom, in even the biggest storms. Yes, the biggest storms
will still touch down, you’re talking about 50 meters, 150 feet, or so of
water, or in that order of magnitude. It’s still storm-influenced,
though. The occasional storm does rip through. And storms way back
up in here have an influence because they stir up the muds and silts which are
carried down by gradient currents and dumped in these settings. And here,
you may see the influence of storms, mostly, again, you’ll see where it simply
rotted and fell apart in place. But the occasional big storm may leave
its imprint in this way. Here, for example, are aligned Ectenocrinus
stems - one of the famous logjam occurrences we see at several levels in the
Cincinnatian; they’re also present down in the Point Pleasant Limestone.
They’re aligned by the storm gradient currents - they’re very, very fast.
These Glyptocrinus, or are they Pycnocrinus?
Colin Sumrall: Pycnocrinus.
Yeah.
Whatever. These crowns were caught up probably in something very, very
quickly by a distal mudflow in about this type of setting. Now, again, I
want to emphasize that I’m showing the extraordinary circumstances, and they
are extraordinary. But, but normal things - to find echinoderms in these
settings that have fallen apart, more or less in place. And we also saw
the same analogous thing in the limestones ramp, where you get into the deeper
water. Most of the time, you see things aren’t really broken or abraded -
they’ve fallen apart - the problem is they usually haven’t been buried quickly
enough. But, on the other hand, I would argue that one of the most
favourable environments for preserving occasional event signatures - but this
is, of course, from Rochester Shale, a cystoid, I think Caryocrinites,
that’s virtually fallen over in place. This is its holdfast - it’s
slapped over, and covered by mud. Now, here’s an interesting little twist
where paleontologists can make a contribution to understanding the dynamics of
how sediments accumulate. A sedimentologist, a guy who studies silts and
muds, looking at this shale would have no clue that this package of mud was
deposited any faster than any other. The clue comes from the fossils -
there’s no question that this thing was covered by probably no less than 10
centimeters of mud, all within a few hours. No doubt about this.
This was a very bad day, and this guy got toppled over. These specimens
are even current-aligned in some cases. And what we’re seeing here is the
effect of those real distal, those real dying-out ends of the mudflows that
were stirred up way back up on the shelf where the storms hit bottom, and then
this mud got carried out and dumped very fast. It’s kind of an
interesting question - how do we get clays deposited that fast?
But, we think we have an answer to that as well.
And
finally, you down into the real distal floor, you see some real interesting
sorts of preservation, where you’re really down into well below the direct
influence of storms. Again, mostly it’s pretty poor. There’s bits
and pieces of echinoderms that fall apart. But, occasionally, one of
these gradient currents has enough momentum to carry muds way out and you get
probably some of the most spectacular preservation of this form. Now
you’re really talking about shales that are quite dark, in some cases, dark
gray. You will find some spectacular things. One of the things that
happens out in the deep water is that, under the conditions of these deepwater
muds, its often anaerobic and anoxic and this may lead to the development of
pyrite - pyrite, which can coat fossils very quickly. And this from the
famous Hunsruck Shale of Germany, and that’s one of these deeper basinal
deposits where mudflows came down and buried things very quickly. Yes,
there were things living on the bottom, but down in the sediment a little ways,
there were colonies of anaerobic bacteria, and they’re the very sort of thing
that can produce pyrite. Somewhere, I had a picture of the actual
specimen. This is an x-ray photograph of one of these beautiful
brittlestars. And again, its from the very deep water that we get truly,
or what seem to be appear very, very low-oxygen conditions on the
seabottom. And here, you’ve got really strange situations where echinoderms
are almost not found because the conditions are so bad, but then occasionally
we see some bonanzas like these long-stemmed crinoids attached to logs as shown
in this little cartoon from a German museum. And in some of these really
deep basinal settings, we do found that these have beautifully pyritized
fossils, well-preserved because those scavengers didn’t knock them apart, and
perhaps also the very distant plumes of mud came down to cover these. And
the pyrite formation then began and you get this extraordinary
preservation. And, of course, we have many debates about - some people
have argued for years that these specimens grew attached to floating logs on
the surface. And the reason you’re getting echinoderms out there is
because they were swept down into a toxic environment from floating logs at the
surface. But you’ll see that I have deliberately put the slide in the
other way to show these as though they were growing up off sunken logs, rather
than growing on floating logs. They grew a little above the bottom
supported on logs that had become waterlogged and sunk down and provided a hard
substrate and elevated them just enough to get started. That’s my side of
the story. If you want to argue that, we can argue it later. [New
slide] That’s the brittlestar which is pyritized there, this is one that is
very similar to the one that we showed in the x-ray photograph. And these
are famous dark slates from Germany - but they’re slates! That stuff has
been heated and metamorphosed. They’re used to put on roofs of houses in
Germany. But, somehow, miraculously, because of the pyrite formation, the
echinoderm fossils have escaped the destruction of all of the pressure, and
they come out of some of these slates. The guy who studied these fossils had
invented an x-ray truck that he could take into the field. He had a
portable x-ray machine, and he’d simply take slabs and stick them in, under the
x-ray. He couldn’t even see sometimes the fossil specimens in
there. The x-rays would reveal, because these are pyritized, they would
reveal these beautiful fossils contained inside. So this is where you’re
getting down into the deeper distal environments where the final dying-out end
of the mud plumes go, where strange geochemical conditions conspire to make for
some very spectacular preservation at times. You’re really down here at
the far, distal end, maybe 100 meters or more of water in these deeper basinal
settings.
I
don’t want to say too much about this. But what I wanted to say here,
though, is that, when we look at whole sequences of rock - whole cycles, as I
would say - there may be some predictability in the way in which echinoderm
assemblages will occur. Then we might find that there are certain places
within a cycle of sedimentation where we’re more apt to get certain kinds of
fossils. I’m working toward a predictive model where we can use this kind
of - we can help prospect for new occurrences. For example here - what
we’re doing is starting out with a package of sediment, perhaps 30 feet thick,
in which we are starting with a deep water deposits down here, a rise in sea
level here, and then as water deepens, we get this sort of deeper water shales
and so forth. And then, gradually shallowing upward within this idealized
cycle to sandier deposits at the top. And once again, one interesting
fact that stands out is when we shift over from shallowing to the beginning of
deepening, sediments from the land are starved compared to the times when there
is so much sediment coming out. And even when we have shales and
sandstones the rest of the time, you’re going to get limestones during the
transgressive times, during the deepening times. Now there are some
interesting things in this higher-energy shallow water that occurs near the top
of the cycle - most echinoderms are going to be in pretty poor shape, with rare
exceptions. You’re going to see shoal features, the high-energy things,
mostly broken apart. But a very interesting setting occurs here, where
water depth is suddenly rising or deepening. This may cut off
sedimentation and allow the bottom to cement, forming a hardground right
there. And a hardground may be exposed for a long period of time and
accumulate some interesting critters, like edrioasteroids in the Early
Paleozoic, which may be tremendous. And then there comes a day when the
sediment starvation ends with a big plume of mud and this may preserve
beautiful things. So the flooding surface, this time of rapid deepening,
may be a very interesting place to look for certain kinds of echinoderms.
And as you go through the cycle, water’s getting shallower, so we start out off
with ........... pretty interesting fossil assemblages, rather scarce, but
occasionally within some of the mudstones - rather beautifully preserved
layers. And depending upon the rapidity of the rate of sedimentation,
they may look really quite spectacular. As we get shallower still, of
course, now the problem is we’re getting more dumping of sand and we’re getting
higher-energy, and that’s generally militates against getting good
fossils. But in some cases, even here, the occasional sweep of sand, as I
say, may preserve a bonanza and it does not later get reworked. If I have
to say where the prospects for fossils in a cycle like this is, in a typical
cycle like this, for example in the Devonian, we’re starting off relatively
deep, coming up through and hitting through shales and mudstones and
siltstones, and getting some limestones up here, the hardground right on top
there, and we kick back into shales. So really, in fact the diagram is
based on this very place [gesturing to an outcrop slide], here we go, on up
through there in these limestones, and if you have to predict where to find
interesting things, my general feeling is this part of the cycle is some of the
most exciting. The best potential for getting things out, just at the
time when sea level that is just rising quickly, through there we may see some
of these hardground assemblages, and also in some of this early deposition
where water is still relatively deep, down in some of these shales, when you
get the occasional plumes of mud preserving things very well. The higher
you go, the shallower it gets, the more apt things are to be broken apart, or
in fact also, the more it tends to become a hostile environment for many
echinoderms. They don’t really like areas that are stirred up a lot or
have a lot of turbidity in the water - that might alos relate to their way of
locomoting, their water vascular system, and other things. But,
occasionally, again, you will find - I wouldn’t rule out any part of the marine
cycle - fossils, of course, are where you find them! (as people always
say). But I think there are some places in the depositional patterns
where you have a better probability of finding well preserved things and could
perhaps go prospecting.
So,
in the final slides, fossil echinoderm assemblages can be found in a whole
array of different styles, even if they’re made up of the same animal.
They may look like this, of course, these aren’t the same animals, but they may
look like this - pretty much broken up pieces. These have been pretty
much broken up in the near neighbourhood, probably, of where they lived - just
fallen apart in place. Or, you might find some things like this.
What you do find will tell you a good deal about what was happening in the
past. You might be able to prospect for new assemblages - and that’s very
valuable, because the whole specimens, as you know, are the most critical
things for describing echinoderms. In many cases, we can’t describe them
based on the individual small parts. So it’s critical to be able to try
to find these. But on another hand, when we look at these assemblages, we
are able to make a very nice deduction about what was going on in the environment.
Are the skeletal pieces broken or abraded? Then, we’re dealing with a
pretty shallow, high-energy, surgy environment. Are they simply falling
apart in place? Well, this would indicate slow burial, but rather quiet
water conditions. Or in fact, are you simply seeing something like
this? In any environment, whether it be shallow or deep, where you’re
getting the whole animal, and going back, all the way back to Dave Meyer’s and
other people’s experimental studies - things I talked about in the beginning -
from what we’ve learned, we know that this is an extraordinary situation.
This can only be the signature of an event. And so, as I say, “For us, a
very good day in the field is finding a very bad day” for ....... echinoderms
that presumably met their demise in a matter of hours, were slapped down, and
by some lucky twist of fortune, didn’t get dug up again by scavengers or by the
next storm that happens to appear. So, when you see these things, and I
know you have, and you will see more of them - even if you just see well
preserved stems - think of that. Think that you are seeing sometimes
frozen moments in time. And, at other times, seeing things like this, you
may be seeing many centuries, decades or centuries of time gone by when very
little was happening.
And
so, echinoderms are wonderful animals, wonderful fossils, and they’re very,
very exciting. But they’re also, like all fossils, can tell us a really
wonderful story about the way in which sediments accumulate and the wonderful
ancient environments where these critters actually lived.
