Trilobites, and a few other Michigan fossils

2011 April 2

Michigan was not always the way it is now[1]. In fact, one could make the argument that Michigan has almost never been the way it is now, and will probably not be the way it is now for very long (geologically speaking[2]). This area has, at various times, been:
–  buried under glaciers, or
– covered in lava flows, or
–  seabottom/oceanic shoreline[3], or
– part of a relatively dry continental interior, or
– subjected to millions of years of conditions that are currently a mystery because the evidence of them has been scraped away by the glaciers[4].
One of the ways that Michigan was, is that much of it was partially submerged under a warm, shallow sea filled with coral and other life forms between about 514 and 306 million years ago[5]. Which, at the time, included a lot of these[6]:

This is your classic trilobite. They didn’t look exactly like this when they were alive, as they also had antennae and numerous legs (which aren’t normally preserved in the fossils). The mouth was on the underside, and isn’t visible here.


The underside is where all the fiddly, difficult-to-preserve bits like legs would have been, and they aren’t preserved in this specimen. It is clear that something was there, however. They probably had the standard little arthropod-type legs, and most likely walked along rather like woodlice.

Here’s a sketch I made of approximately what I think it looked like when it was alive:

I also found a slab of trilobites (embedded in shale) here in town, in the gift shop for the Seaman Mineral Museum (they were having a sale in preparation for their upcoming move to a new building). They aren’t collected from Michigan; they are from an outcrop of the Wheeler Shale in Utah, which is a formation that is about 505 million years old. Anyway, I think the slab was a good deal on a cost-per-trilobite basis: it has five and a third trilobites for $10, or slightly under $2 per trilobite.

These are probably the same species as the loose trilobite, and are from the genus Elrathia, most likely Elrathia kingii, which are the most common North American trilobite fossil (and pretty much what immediately comes to mind when someone says “trilobite”). The circled trilobites are the ones that we will be looking at in more detail in subsequent pictures. First, we’ll look at the tiny one in the lower right corner, which is only about 4 mm long:

and the medium-sized one on the far left, which is about 8 mm. long:

They were clearly “direct developers” – they looked essentially the same as new-hatched trilobites as they did when they got older, and just got progressively bigger and bigger. They have three distinct body sections: the broad, armored, shieldlike head section, or “cephalon”; a flexible, multi-segmented body, or “thorax”, and a hard, inflexible tail piece, or “pygidium”. They are also divided into three “lobes” running down the length of the body (hence the name “trilobite”); the “axial lobe” runs down the middle of the body from cephalon to pygidium, and the side lobes are the left and right “pleural lobes”.

For this particular species, the pygidium is smaller than the cephalon, and the whole trilobite is about 2.5x longer than the width of the pygidium:

This last one is just a pygidium from a trilobite much larger than the rest. Assuming that the proportions are about the same, this 14 mm pygidium would have been from a trilobite nearly 5 cm long, which is pretty good sized as arthropods go.

When I started putting this page together, I felt kind of bad that none of the specimens were actually local. Lower Michigan should have trilobite fossils too, after all – it was immersed at about the same time as the part of Utah that the above trilobites came from, and was in the sea around the same continent. Trilobite fossils just aren’t generally exposed near the surface in Michigan where they would be easy to get to, and I never found any myself when I was a kid.

But, we were visiting my aunt in Ann Arbor just a few weeks back, and I mentioned that I’d like to be able to confirm that there were, in fact, trilobites in Michigan. To which she replied that, years ago, she and my cousins had gone fossil-hunting in an abandoned limestone pit near Dundee, and found many, many fossils. Including some trilobites. She then said, “We don’t really have room for them any more, would you like them?” As a matter of fact, yes, yes I would! So, here they are. They aren’t as complete as the Utah fossils were (they are in limestone rather than shale, which doesn’t do as good of a job preserving fossils flat and intact), but they are positive confirmation that trilobites are found in Michigan:

While it isn’t obvious from most fossils (what with fossils being made of inflexible rock and all), trilobites were evidently fairly flexible in the thorax area, and some were able to roll up into a defensive ball, kind of like pillbugs do. Which can be seen in this next fossil from the Michigan deposit

Trilobites are essentially the lost 5th subphylum of arthropods. While the chelicerates (includes spiders and mites), hexapods (includes insects and springtails), crustaceans (includes lobsters, crabs, and shrimp), and myriapods(includes centipedes and millipedes) are all going strong, the trilobites hit their peak sometime around 444 million years ago[7] in the Silurian period, and then went into a decline until the last trilobites were wiped out in the Permian mass extinction 250 million years ago[8].

There were a lot of different kinds of trilobites, but overall they were evidently mostly bottom-feeders, like a lot of crustaceans are today. In fact, a lot of marine isopods look like they have pretty much taken over the lifestyle vacated by the trilobites. While trilobites were undoubtedly common, they were probably not quite as common as the huge numbers of trilobite fossils would lead us to believe, because some of the fossils are molted skins. Like other arthropods, trilobites had to molt to grow, and so it was quite possible for a single trilobite to leave multiple fossils in the form of cast skins. In general, the complete fossils one finds are trilobites that died, while those that are broken between the cephalon and thorax are molted skins.

So, why did the trilobites die out? Well, it was probably at least partly because other things got better at eating them. Between the ancestors of squids and octopuses developing hard beaks that can crack shells, and certain fish ancestors developing powerful, can-opener-like jaws, the trilobites’ shells probably ceased being as good of protection as they had been. It has recently been suggested that this might have been aggravated by trilobites’ molting habits, they evidently shed their whole calcified shell in a rather awkward way. Modern crustaceans also have calcified shells, but they reabsorb as much as they can from their skins before molting so that they can recalcify the new skin as quickly as possible. In contrast, the trilobites evidently shed the whole thing, calcification and all, and had to start from scratch (which also explains why their cast skins tended to fossilize so well). This would have left them vulnerable to predation for much longer than necessary. A few of them were actually getting a leg up on the eater vs. eaten arms race at the end of the Permian, and might have been able to restore the ascendancy of their subphylum given enough time[9]. But, they were unfortunate enough to be in a precarious position when Bad Things Happened[8]. So, overall it was mostly just a lack of appropriate adaptations ending in some spectacularly bad luck.

And, while we are on the topic of fossils, here are a few more that I’ve had for a long time. This Brachiopod shell cast is one that I found next to the barn one day when I was about 13 or so. It was where the rain dripped off of the roof and washed away the dirt, exposing the gravel.

I also used to periodically find bits of Petoskey stone, like this one, which are remnants of the coral reefs that used to be common where Lower Michigan is now. They turned up pretty regularly in the gravel that Dad bought to mix concrete.

And this next one is one that I inherited from my great-uncle. It’s a Crinoid stem.

I don’t know where he found it. My great-uncle grew up in Michigan, but I understand he traveled quite a bit after he learned to fly airplanes sometime around World War I, and he ultimately ended up in Northern California. So, I think he could have found it pretty much anywhere in North America. It was in a box with a rattlesnake rattle, though, so I expect he probably found it out west somewhere.

The other common fossil I used to find was the conical Rugose coral, like this one[11]. When they were alive, the small end was attached to some surface, and the large end had the coral polyps projecting out their tentacles to collect food.

One of these days, the whole family needs to go someplace where the rocks are less than about a billion years old and there are actually fossils, so I can show the girls what fossils look like in their natural habitat. They really like the box of fossils from my aunt (which includes lots of stuff in addition to the trilobites), but it just isn’t the same as actually finding them oneself. Which is certainly not going to happen up here in the western Upper Peninsula. According to this site, we might be able to find some if we go south or east of here, but only if we go at least a hundred miles. Actually, we could probably start with rummaging around in the yard next time we go downstate to visit grandparents. Assuming that we go there sometime that there isn’t any snow on the ground.

[A note: Originally I was just going to put up a couple of pictures of that first trilobite as an April Fools joke, but I got a bit too much into it and things got out of hand. You can tell, can’t you?]
[1] Current conditions are significantly cooler, damper, and snowier than other parts of the continental interior, but without becoming quite as cold in the winter. This is mainly because the presence of the Great Lakes has a pretty powerful effect on the climate – they are tremendous heat sinks and moisture reservoirs. But, the Great Lakes aren’t permanent, and if they don’t get plowed up by glaciers again in a new Ice Age, then if left to themselves they will fill with sediment in a few tens of thousands of years.

[2] When a geologist says “a short time”, this doesn’t mean the same thing as when most other people say it. A short time to a geologist is anything less than about a million years. I have a book on clay chemistry that talks about the transformation of volcanic ash to bentonite clay, and says the reaction is “almost immediate”. Followed by an estimate that it takes about a thousand years.

[3] Most of Michigan is the Michigan Basin, and spent a lot of time underwater after the Cambrian, finally mostly filling in sometime in the Carboniferous (although there’s a patch in the middle that didn’t fill in until the Jurassic). But, as far as I can tell the western Upper Peninsula of Michigan was only on the margin of the basin and probably hasn’t been submerged at least since it became part of Rodinia in the late Precambrian[5].

[4] The northwestern part of the Upper Peninsula of Michigan, in particular, has essentially had most of its geological history surgically removed. We have a base layer of Precambrian rocks that mostly run over a billion years old, and a light frosting of what has been deposited in the last 10,000 years or so since the glaciers melted, and in between – nothing. The glaciers scraped away everything else. Oh, one finds the occasional fossil (generally a piece of coral or something like that), but only because whatever was much further north got dragged down to here and dumped. But overall, for this area it is as if the last billion years, almost the entire period that organized multicellular life has existed, never actually happened.

[5] The whole story about how the continents rose and fell, split and merged over billions of years is kind of a fascinating one to me. Particularly since the theory of Plate Tectonics was “born” about the same time that I was, and we have sort of grown up together. I remember when I was a kid, our school science books really didn’t have a good explanation for why there were mountains, aside from some unconvincing analogies with baked apples, and I’ve really enjoyed seeing how plate tectonics explains so many, many things that used to be mysterious. For a long digression about how Michigan specifically was affected by movements of the Earth’s crustal plates, see footnote [12] at the bottom of this page.

[6] I put it down as “Found in House”, because I did, but only because I bought it years ago from Ward’s Natural Science. I was buying some mineral samples from them for use in research that I was doing at work, and when I saw that they sold trilobites, I couldn’t help myself.

[7] Just for the sake of comparison, the first animals identifiable as dinosaurs didn’t appear until about 230 million years ago, a good 20 million years after the last trilobite died. Dinosaurs and trilobites were in no way contemporaries. Which brings up a point: when I was a kid, I had one of these prehistoric animals playsets (mostly dinosaurs) which was actually pretty good so far as using accurate reconstructions of animals that actually existed. It included some “cavemen”, as well as a woolly mammoth and, I think, a Glyptodon (both of which are mammals more or less contemporary with humans). Now, we’ve all had drilled into us that mixing humans and dinosaurs (or, for that matter, most mammals and dinosaurs) is an “anachronism” and worthy of ridicule, and so this bothered me. But then, I realized something: The Mesozoic era lasted a long time (started 250 million years ago, and ended 67 million years ago, so total duration was about 185 million years). So, it’s true that early humans didn’t appear until 65 million years after, say, the last Tyrannosaurus died at the end of the Mesozoic. However, the last Stegosaurus died somewhere around 145 million years ago. So the time that passed between Stegosaurus and Tyrannosaurus was around 77 million years. In other words, having Stegosaurus and Tyrannosaurus together in the same playset was more of an anachronism than having Tyrannosaurus and humans together! And let’s not even get started on the anachronism when the set also included Dimetrodon (lived 280-265 million years ago) and Moschops (about 255 million years ago), which were both from before the Permian extinction and predated even Stegosaurus by around 105 million years (and Tyrannosaurus by close to 182 million years). All in all, I think I can bite my tongue and live with the inclusion of cavemen now.

[8] The Permian Extinction wiped out much, much more than the last few trilobites. According to some estimates, somewhere around 95% of all living things died at the end of the Permian, through some major, world-wrecking disaster of a kind that paleontologists and geologists are still arguing about. Trying to reconstruct the exact nature of something that happened 250 million years ago takes a lot of investigating, but everybody agrees that whatever happened was really, really bad, much worse than the mass extinction that did in the dinosaurs 185 million years later. The theories of what happened generally involve asteriod impacts, the massive volcanic eruptions that formed the Siberian Traps, large climate variations, deoxygenation of the oceans, and even massive releases of methane and hydrogen sulfide from the oceans. There are even some suggestions that all of these things happened in a chain reaction, such as this:

    1. Massive asteriod impacts the Earth, causing the sort of destruction one would expect.
    2. Shock of the asteroid impact triggers the massive flood basalt eruption of the Siberian Traps, which directly lays waste to a large tract of ocean. It also both releases a lot of carbon dioxide in its own right, and pours a lot of lava into the ocean depths where it volatilizes a lot of methane clathrates from the sea floor.
    3. The CO2 and methane from the eruptions warm the planet, causing the ocean to warm up enough that even more methane clathrates are released, reacting with atmospheric oxygen and reducing the oxygen content.
    4. The combination of lower oxygen in the atmosphere, reduced oxygen solubility in warm water, and decomposition of things growing in the ocean results in large parts of the ocean getting deoxygenated, and nearly every non-microscopic organism in the oxygen-poor regions dies.
    5. The low oxygen allows anaerobic sulfate-reducing organisms to take over and grow rapidly. These organisms convert sulfur compounds in the water to hydrogen sulfide, which is a highly toxic gas. The hydrogen sulfide in the atmosphere then kills most of the living things that are left.

So, whether all of these things happened, or just some of them (or even just one or two of them), it is pretty clear that the Permian Extinction was probably the worst ecological catastrophe the world has ever seen. And not something we’d like to see repeated. Which is why it would be useful to figure out just what did happen, so that we can be sure not to trigger something like that by accident.

[9] It is actually quite common for a large group of animals to mostly die off, with only one or two lineages actually surviving, but then this surviving group goes through an evolutionary radiation and becomes very successful in its own right. For example, the synapsids (or “mammal-like reptiles”) were very widespread and numerous before the Permian extinction, but then they were ultimately all wiped out except for the immediate ancestors of mammals[10]. The surviving mammalian lineage hung on by its teeth for a long time, but then they also managed to make it through the K/T extinction that killed the dinosaurs. This was the mammals’ big chance to expand and diversify into a whole bunch of suddenly-vacant ecological niches, and now the mammals, as the only surviving synapsids, are a very numerous and successful group. Similarly, the only “dinosaurs” that survived the K/T extinction were the birds, but now they are, if anything, even more successful than the mammals.

[10] Apparently being a successful survivor of a mass extinction doesn’t guarantee that you’ll continue to be successful, though. One of the survivors of the Permian extinction was Lystrosaurus, a synapsid. They were one of the larger land animals (about the size of a pig) to survive the disaster, and in the absence of either predators or herbivorous competitors they proceeded to become outrageously common. There are estimates that, for a time, they accounted for 95% of all land animals(!). But, eventually new predators and new competitors evolved, and Lystrosaurus eventually died out without descendants, leaving the Cynodonts to give rise to the mammals.

[11] I believe that the Rugose Coral were the inspiration for H.P. Lovecraft’s Great Race of Yith, which he often referred to as “Rugose cones”. They were supposed to be a highly-developed civilization back in the Cambrian. He liked to use terms like “rugose” and “squamous” to describe various unspeakable horrors. They sound nicely unpleasant, until you look them up in the dictionary and find out that they just mean “rough” and “flat”.

[12] And now for a long digression about plate tectonics, and in particular how Michigan gradually emerged from beneath the waves (Paleographic maps are by C. R. Scotese,
PALEOMAP Project (, images used by permission) (Note: Ron Blakey also has a set of paleographic reconstructions. Some of his reconstructions of the vicinity of Michigan are pretty substantially different from Scotese’s, particularly in the more distant past. When you consider the massive geological studies that would have to go into making such reconstructions, especially considering that large swatches of the earth’s crust from back then simply don’t exist any more, this is not too surprising. I expect there are some pretty bitter fights about this sort of thing if you read the right papers).

Even though the Earth has existed for something greater than 4500 million years, we don’t really know much about what it looked like back then, what with first being “resurfaced” by asteroid impacts during the Late Heavy Bombardment 3800 million years ago, followed by long-term reshuffling of the surface by plate tectonics (sometimes referred to as “continental drift”). The motions of the continents are very slow; I’ve seen it estimated that the speed is roughly comparable to the speed that a person’s fingernails grow (about 1.5 inches/year). But, if your fingernails grew for a few million years, it all adds up to quite a distance – at 1.5 inches per year, that amounts to 2360 miles per 100 million years.

The configuration of the continents has really only been reconstructed back to the supercontinent Rodinia, which formed up somewhere around 1100 million years ago and started to break up around 750 million years ago (at which point, over 3/4 of the time that Earth has existed had already passed). The formation of Rodinia is probably about the time that the terrain I am currently standing on got lifted out of the ocean, since a lot of the surrounding rocks are Banded Iron Formation. These are metamorphosed from oceanic sediments that were laid down sometime around the time of the Great Oxygenation Event about 2400 million years ago.

By the Late Proterozoic, about 650 million years ago (and about the time that multicellular animals were starting to become common), Rodinia’s breakup was well underway. To start with, Michigan was near the south pole and pretty thoroughly glaciated,

but by the Late Cambrian (514 million years ago) we had made it practically all the way to the Equator.

By that time, the Michigan Basin had sunk, and so most of Michigan was a shallow tropical sea, rich in things like coral which, by about 350 million years ago, formed the coral reefs that ultimately became Petoskey Stone. However, the western Upper Peninsula of Michigan (which is where I am) was basically the seashore at that time. And, being up out of the water, it was mostly eroding into the sea, and not getting fresh material laid down on top of it the way that sea-bottom areas do. So, while the Lower Peninsula is fantastically rich in aquatic fossils, the Upper Peninsula is practically fossil-free.

In the Ordovician, almost everything around us was kind of sinking:

And by the Middle Silurian (425 million years ago) the western UP and part of Wisconsin and Minnesota were an island that was all of the Midwest that was still above water (at least, according to this reconstruction. Ron Blakey says we were just at the base of a long, branching peninsula.)

It is easy to see the effects of the Western UP being the beach while the Eastern UP was submerged when we drive from Houghton down to the Mackinaw Bridge leading downstate. Starting from Houghton, the exposed rock outcroppings beside the road are mostly Precambrian basalt and banded iron formation up until about Marquette. But then, the rocks change to the mostly Cambrian sandstones (basically the remains of a long, sandy beach) by the time we reach Munising (where they are exposed as the Pictured Rocks). After that, the next visible rock outcroppings are the Silurian limestones (solidified ocean bottom sediments) at St. Ignace

We didn’t reconnect to the mainland until the Devonian.

And we were still mostly surrounded by shallow ocean through the Carboniferous.

It wasn’t until the Late Carboniferous that the rest of Michigan got lifted back up above sea level, probably due to the continental collision that raised up the Appalachian mountains and created what is variously known as the Old Red Sandstone Continent, Euramerica, or Laurussia.

After that, it looks like the whole state has stayed mostly above sea level ever since.

So, the lower peninsula was below sea level from the Cambrian to the late Carboniferous (514-306 million years ago), which includes the entire period of the heyday of the trilobites (from the Cambrian through the Devonian). The trilobites are there, but the lower peninsula is buried under a few hundred feet of Glacial Till, which means that you’ll hardly ever see any of the real bedrock downstate. There are a lot of fossils on the surface (mostly various kinds of coral), but they tend to be the sorts of things that can survive in a recognizable form even after being mauled severely by glacial action. And trilobites are much too fragile to survive that kind of abuse. One really needs to find an actual quarry digging down to the bedrock to have very good odds of finding trilobites in Michigan.

16 Responses
  1. April 2, 2011

    Is this going to be on the test?

  2. April 2, 2011

    Mmmmaaaaayyyybe . . . 🙂 When should I schedule the exam?

  3. April 4, 2011

    Sigh. I just wrote several paragraphs about the Petosky stone I have that used to belong to my grandfather, and about the crinoid stems and horn coral (probably the Rugose coral you reference) and brachiopods I found when I was living in Cincinnati, along with reminiscences of buying polished rock along with thimbleberry jam during a childhood trip to the UP….but I scrolled up to see where your aunt lived again, and accidentally clicked on the photo of the trilobite, and then all my work vanished. Which is sort of like “the dog ate my homework.”

    Anyway, the Petosky stone was my grandfather’s, and is chipped around the edges because some kids in my neighborhood when I was growing up wanted to break it open to see if it was a geode. I keep meaning to take it to a lapidary shop and have it smoothed and polished.

    I love the idea that Rugose coral was the inspiration for Lovecraft; never could get into him but he is implied on the “About” tab on my blog!

    This really is a wonderful, wonderful post and I’ve spent way too much time on it (and I’m not just saying that to improve my grade on the test.) One final note: Permian Extinction would be a great name for a band.

  4. April 5, 2011

    I’m sorry to hear that your comment got eaten the first time. I really hate it when that sort of thing happens, and I really wish that commenting functions had some option for saving a comment in progress.

    While writing this post, I was getting a bit worried that maybe it was growing too long, I’m glad that you liked it. It really is too easy for me to get overenthusiastic about paleontology.

  5. Della3 permalink
    April 17, 2011

    Paleontology and geology, Tim. Studying geology is wonderful because you start looking at the earth as an open book of planetary history. When you’re driving about in rural or wilderness areas, you find yourself reading that book.

  6. Della3 permalink
    April 17, 2011

    I’ve been using a rather awkward but reliable way of saving comment text before I post it. I highlight the text, hit my right mouse button to get the menu of options for the text, and select copy. Then I hit my windows button and select “Notepad”. I hit my right mouse button again, and select paste, and then I save it under a name, such as “Trilobite”. Notepad won’t put in the same paragraph margins, but when you copy and paste the lost text back into a comment posting, it will appear with the same margins that it originally had.

    Another trick, when doing multiple saves of a large or complicated project is to give each save a different name, i.e. Trilobite1, Trilobite2, Trilobite3, etc. Thus, you can refer back to your previous versions if you decide you don’t like the changes you have made.

  7. March 19, 2012

    Hey, thanks for the blog.Much thanks again. Fantastic.

  8. jack permalink
    November 1, 2012

    hi, i have a few trilobites in my fossil collection awesome to learn more about them cool stuff thanks again ,jack

  9. Laura Amatulli permalink
    January 7, 2013

    Thank you for the effort and research that went into this post. The images of Michigan through geologic time are really thought provoking. This is very helpful!

  10. January 8, 2013

    You’re welcome, Laura.

  11. Chris permalink
    October 12, 2013

    Great site. Excellent overview of the geological history of Michigan. The fossil history throughout Michigan, Ohio, and Ontario is fascinating. I really appreciate the work you have put into this page, especially the geologic maps identifying the location of Michigan through the millions of years. This really emphasizes the dynamic nature of Earth’s history.

  12. Kevin permalink
    January 22, 2016

    Thank you very much for providing so many interesting information here. As a beginner of fossil hunter living in Ann Arbor of Michigan, I would like to search the limestone pit your aunt went near Dundee for some trilobites which is so fascinating. Would you please give me some detailed information about the address.
    Thank you in advance.

  13. January 25, 2016


    Anything I could tell you about fossil hunting around Ann Arbor would necessarily be second- or third-hand information. A better bet would be to contact the Friends of the University of Michigan Museum of Paleontology:

    Among other things, they get permission to hunt in the local quarries, which for liability reasons are normally closed to the public.

  14. Kevin permalink
    January 25, 2016

    Hi Tim,
    Thanks for your prompt reply. The link you provided is very useful and I will email the UM ” the friends” to see if I could get the membership in 2016.
    Thanks again.

  15. Theresa permalink
    May 10, 2016

    You did a great job here. But I have one little correction. Your label of the mouth on the trilobite is incorrect. The “mouth” is on the underside. It is marked by the hypostoma on the ventral surface which opens posteriorly. Trilobites fed more like Crustacea. Interestingly, the frontal glabellar lobe which kind of looks like a nose is actually where the stomach was located.

  16. May 10, 2016

    Thanks, Theresa. I corrected the picture to remove the erroneous label.

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