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.
