Sunday, April 8, 2018

Phycodes: bundles of burrows

I'm going out of the office again, so, like last year, I'm tossing up a few pictures of something I find interesting, in this case an invertebrate trace fossil called Phycodes (not to be confused with Phycodes the moth). I touched on Phycodes briefly a few years ago, using the image included below:

That pale gray color is characteristic of the Brickyard in Lilydale, for whatever reason(s).

The whole piece looks like this:

"Licrophycus ottawaensis" in older literature.

I collected it on a Geological Society of Minnesota visit back in 2006 and it has since become one of the pieces I like to take to events because it's a great teaching fossil. I ask people what it is and let them explain their choice if they want to, and then I identify it. I get a lot of plant-based guesses (which of course is what a lot of paleontologists and geologists thought this kind of structure was decades ago). What Phycodes really is is an invertebrate trace fossil recording the behavior of some kind of wormy animal probing in the mud for food and returning to a central point. This resulted in splayed bundles of burrows, giving the trace fossil a characteristic root-like or mop-like appearance. It doesn't have quite the oomph of a dinosaur bone, but it looks interesting, it's good for conversation, and worms are more familiar than, say, crinoids.

When space is an issue, I have a more compact specimen.

Phycodes turns up every so often in the Decorah. It's not as common as Rauffella (which has turned up in a half-dozen posts so far), but it certainly makes a striking fossil.

Suitable for framing: this chunk is almost entirely Phycodes, with little matrix, which also makes it more fragile than the first two specimens (too bad, because it's also the best). The individual tubes are a bit smaller in diameter than those of the first two as well.

The makers of Decorah Phycodes differed from the makers of Rauffella in a couple of notable ways: Phycodes-makers were smaller (a few mm in diameter versus finger thickness for many Rauffella) and apparently smoother (no surficial striations in Phycodes).

This piece is one of the group from the construction site last year. I'm not certain what kind of ichnofossil it is. It resembles Phycodes templus, but it's also kind of poorly preserved and it's not clear if the burrows are bundled, so it might not be Phycodes at all. (Another possibility is poorly preserved "Camarocladia".) Note the brassy ooids.