July 13, 2013 | 3
Yes, Indiana has coral reefs. More on that in a bit.
A couple days ago I entered a trilobite doodling spree, and have come up with a sort of technique for drawing them. Their diversity is astounding, but this is the basic body plan, which gets tinkered by the different species. Haven’t figured out how to draw a curled up one yet; working on it. But here’s a basic body plan, if you’d like to fiddle with it.
Let’s look at some Indiana coral reefs, starting with the Upper Ordovician. Then, the Midwest, including Indiana, was a shallow tropical sea close to the equator. Thankfully, rocks float, so now it’s only hellishly hot in the summer. Also, there’s not much ocean going on in the Midwest these days, in case you didn’t notice. But instead of cornfields and forests, imagine dense stands of crinoids swaying in the current, encrusted with boldly branching and protruding bryozoans, amongst a bed of brachiopods with trilobites running around between them. An occasional snail, Platyceras, is perched on the feathery top of a crinoid, leeching off the currents its echinoderm perch generates. A nautiloid swims by. Below is a chunk of the seafloor, now shale, with fragments of a giant trilobite Isotelus, a couple bryozoans and a fragment of a straight nautiloid cast (the sediment filled in its shell chambers, leaving behind a sliced sausage pattern after the shell material decayed away).
Here’s a large coral shelf. Reminiscent of pictures of the Caribbean reefs, don’t you think? Though this shelf hasn’t ever experienced bright colourful tropical fish swimming around it. Fish weren’t quite fish-like yet.
Bryozoans weren’t just meek crusty patches on algae — they formed massive reefs of their own right, and grew to large sizes. At first, they look like corals and it’s a common mistake to think of them as such.
This is probably a more familiar state of bryozoan for us, encrusted upon a brachiopod shell.
More dangerously coral-like bryozoan branches.
Horn corals, members of the now-extinct rugose corals. They look like tusks and grew upside-down, as in, pointy end rooted in the seafloor. The coral on the right is more or less ‘normal’, with a few traces of something drilling in it. The middle coral is covered by a thriving bryozoan colony, presumably after it died and got dislodged — the bryozoan covers the entire thing. On the left is a partially diagenised horn, meaning its original fossilised material got replaced by a different mineral, leaving behind that peculiar texture you see below. All are from the same site, just south of Brookville, IN along SR1.
A glimpse of the overall seafloor, littered with crinoid stem plates and brachiopod shells, as well as fragments of trilobites (including spines), and bryozoans.
Moving up to the Upper Mississippian now. I need to take more pictures of the fossils, but here’s a couple for now. First is the Mississippian (Lower Carboniferous) seafloor. You should see how the bryozoans are starting to look different, as a net-like form has evolved by then. There are a lot more crinoids, including some base plates from the calyxes (the base where the feathers come from). This is from the massive roadcut at the intersection of i-64 and SR37, in southern Indiana.
One of the strangest things about Upper Mississippian seas was the abundance of corkscrew bryozoans called Archimedes. The central stalk wound up like a screw, with sheets of bryozoans sticking out — like a christmas tree or a serpulid worm (also called a christmas tree worm).
And last for today, a tooth! From a Mississippian shark! They’re not very big in that era, and are rather hard to detect. But here it is!
There are more, but it will take a while to get around to taking pictures of each of them. Here’s a sampler, including yet-uncracked iron-stone nodules — with Pennsylvanian (upper Carboniferous) plant fossils!
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