Middle Devonian Worm Burrow?

[MarleysGh0st]

I've already posted photos of a nice group of trilobites I collected at Portland Point (Middle Devonian, Moscow formation) back in April. I've finally gotten around to taking some decent photos of an interesting trace fossil that I found in the same layer.

I've often found short segments of these, which I believe are infilled worm burrows. Frequently, they're oriented vertically, so when I split a piece of shale, I can see a pyritized cross-section on both sides of the slab.

When I split this slab, I found a surprise: the burrow ran horizontally for at least 7 inches (18 cm), with gentle loops up and down, almost like the worm was stitching the layer together with thread!

Here's a overview of the two pieces of shale, showing the part and counterpart of the fossil:

And here are two close-ups of the trail:

[mikeymig]

I find these as well and would like to hear what the others have to say about them. What I would also like to know is, how did you get this piece home without it falling apart?

mikey

[MarleysGh0st]

What I would also like to know is, how did you get this piece home without it falling apart?

Well, little bits of the burrow may have come loose when I first split the shale. But after I saw what I found, I set both pieces aside and resisted the urge to trim them any further, a path that has led to ruining countless other nice specimens!

Actually the left side of the larger piece of matrix is a little higher than the line of the burrow, which might just continue for a few more inches beneath that layer!

[Auspex]

Looks like the critter found a good feeding horizon.

[MarleysGh0st]

Since starting this thread a week ago, I've been studying about trace fossils. The KU Ichnology site served as a good introduction to the field, complete with a glossary ot terms and glossary ichnotaxa. Then I started downloading journal papers, looking for descriptions and photographs that matched what I found. A full report might require a paper of its own, but to jump to the conclusion, I believe this specimen is Planolites montanus Richter, 1937.

My primary source for this was Pemberton and Frey, “Trace Fossil Nomenclature and the Planolites-Palaeophycus Dilemma.” which lumped a number of ichnospecies names together. (My apologies if my layman's description of their work isn't expressed properly.) P. montanus was the best match for my specimen, particularly on the basis of its small diameter. But in combining a number of species descriptions, I think Pemberton and Frey may have skipped over some details that may be more relevant to the specimens I've found at Portland Point.

In particular, in Marintsch and Finks, “Lower Devonian Ichnofacies at Highland Mills, New York and Their Gradual Replacement Across Environmental Gradients.” I found their diagnosis of Planolites ballandus Webby, 1970 (one of the species Pemberton and Frey combined into P. montanus) to be a better fit:

Unbranched, parallel to bedding but may be inclined to vertical. Straight to sinuous burrow with smooth walls. Diameter varies between .75 and 2 mm.

Edited July 16, 2013 by MarleysGh0st