March 11. Ichnology

Today’s topic is ichnology – the study of ick?  Nope, it’s Greek ichnos for track, plus logia for study. It’s the scientific study of traces of life, from footprints and burrows to feeding and resting marks, coprolites which are fossilized feces, and more.

Ordovician trace fossils (borings) from Kentucky.

Sometimes the traces are all we have providing evidence of ancient life, and sometimes, even when we have good body fossils of such life, the traces give us a lot more information about the critter than the body alone. How did it move around? Was it a crawler, or a swimmer? What did it eat? Was it a grazer, or an attacker? How did it live? Buried in the sand, in a constructed burrow, or just hanging out on the ocean floor?  All these questions are useful in terms of understanding life throughout earth’s history. And especially as long ago as the Ordovician, trace fossils can be pretty informative.

When I was in college, we learned about two trace fossils – Cruziana, which we were told were trilobite tracks, and teonuris, supposedly the marks in sediment caused by a plant or animal rooted to the sea floor, swirling around in the waves – or maybe the grazing trace of some bottom-dweller. You still find the term Cruziana, and I think it generally is thought to represent trilobite activity. As for teonouris, I don’t even know for sure how to spell it and I don’t think it’s a word that’s used any more.

Today the study of these things is much more advanced, enhanced by careful comparisons between fossil marks and the traces of life that we can see today. There’s even an International Congress on Ichnology.

Tony Martin, a professor at Emory University in Atlanta, is a specialist in ichnology, and among his particular interests is the trace fossils of the Ordovician. His blog (see the links below) has some great photos, including the resting mark of an Ordovician sea star.

I can really understand the fascination with these kinds of fossils. Where a trilobite fossil is cool, and of course there are things to figure out about it, on the whole, most of the time, you know when you’ve got a trilobite, or a brachiopod, or whatever. With trace fossils, you have more of a mystery story, and a fun challenge to figure out what it means. I remember being on a field trip in West Texas, and seeing these weird depressions in the rock, maybe a couple feet across, with narrow things like tentacles all around the rim – maybe 20 or 30 of those narrow branches focusing into the depression. No one I was with had a clue what it was. A giant jellyfish seemed unreasonable, but that’s what it looked like.

Several years later, on a different trip in a different place, along a modern river, we saw the same thing in modern sediment – and like a flash, it was obvious. It was essentially a sink, a sump, where the last little pool of water on a drying riverbank collected. The tentacle-like marks were the runnels that had taken the last flow of water, from a rainstorm or the last high stand of the river, draining into the depression, leaving 20 or 30 little drainage channels. Even though that one wasn’t due to life, to my mind, that’s ichnology – a challenging mystery story, trying to figure out the non-obvious cause of something that may be quite obvious in the rock. And that can be a lot of fun. I’m a little envious of Professor Martin.

—Richard I. Gibson

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Today, March 11, in 1902, was the birthday of Marland Pratt Billings, in Boston, Massachusetts. Billings was a structural geologist, focusing on things like the way folding and faulting work in rocks. He spent most of his career at Harvard, and in 1942 published his textbook on Structural Geology, which became the bible of structural geology for a couple generations of geology students.

Photo by Mark Wilson via Wikipedia, public domain. 

Further reading:
Ediacaran ichnofossils on Tony Martin’s blog
Cambrian ichnofossils on Tony Martin’s blog
Ordovician ichnofossils and modern traces on Tony Martin’s blog
Dinosaurs without Bones by Anthony Martin