Sunday, September 10, 2017

When ammonites got bored

I'm heading out of the office for a few days, so nothing particularly profound for this entry. It seemed like a good excuse to highlight some variety in the fossil record, so here are some ammonites that decided not to look like ammonites.

If you're reading this, you probably know what an ammonite is. You might even have an automatic response along the lines of "extinct squid-like cephalopods in coiled chambered shells" queued up. To be perfectly fair, most ammonites did go for the flat spiral look. A handful, though, rebelled. Most of them conveniently belong to the same suborder, the Ancyloceratina. These non-conformist cephalopods are called heteromorph ammonites, "other forms". They are particularly a feature of all of those Cretaceous-age shallow continental seas. Here are a few examples.

Baculites grandis, image from Wikimedia Commons.
Baculites is the walking-stick ammonite, as well as a throwback to the old familiar orthoconic nautiloids of the Paleozoic. It is notable as a source of iniskims ("buffalo stones"), produced by the shell breaking up along the complicated sutures into single chambers resembling tiny buffalo, and was common in the Western Interior Seaway. There is another ammonite, Ptychoceras, that resembles a Baculites which has been neatly folded on itself one or more times, as if for better packing.

Hamites as restored by Neale Monk, image from Wikimedia Commons.
Hamites is an example of the loosely coiled heteromorph ammonites; it was sometimes more circular, sometimes more oval, for lack of a better word.

Hyphantoceras orientale, image from Wikimedia Commons.
Hyphantoceras, the stressed Baculites; kind of a helix expanding as the animal gets larger, sometimes ending with a hook curved back on the structure. (Or, sure, some kind of unicorn horn.)

Several Nipponites mirabilis, image from Wikimedia Commons.
Nipponites: a collection of U-bends operated by a small tentacle beast.

Nostoceras malagasyense, image from Wikimedia Commons.
Nostoceras resembles a snail that lost the plot partway through life, or some kind of failed cinnamon roll. I can see some problems with the whole "grow back upon the older part of the shell" thing.

Scaphites, from Wikimedia Commons.
Another common heteromorph ammonite of the Western Interior Seaway, Scaphites went with a partly coiled, partly open shell resembling a "6" or "9" (although not always as nicely as the pictured specimen).

Turrilites costatus, about as unfair a fossil as you'd ever want to see; image from Wikimedia Commons.
Turrilites is the ammonite that inspired this post. It is one of a few ammonites that went with the snail look, specifically the high-coiled Turritella look.

Some of these heteromorph ammonites are easier to figure than others. Baculites, for example, could probably do more or less what the orthoconic nautiloids of the Paleozoic did, and Turrilites seems to have cleverly combined the powers of a squid with the powers of a snail. Others are more difficult. What in the world was Nipponites up to? Presumably, whatever the stranger heteromorphs were doing did not rely on speed and streamlined forms.

By the way, if you're in the neighborhood in a couple of weeks, I'll be doing an event for the Minnesota side of Interstate State Park at 10 AM, September 23. We'll be looking at the Cambrian rocks, so be prepared to hear me say "Mazomanie" a lot.

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