Our hard-working and observant Estonian colleagues (Olev Vinn and Ursula Toom) recently made a remarkable discovery among Estonian early Silurian fossils: an attachment structure of a stalked crinoid that apparently bioeroded its way into a calcitic stromatoporoid skeleton.
There’s a lot packed into that opening sentence. (And you’ve noticed that “remarkable” is a relative term depending on your interests!) For orientation, a crinoid is an arm-bearing, filter-feeding marine echinoderm with a very long evolutionary history to the present day. They have skeletons of calcite typically consisting of a flower-like crown with feeding appendages, a stem (or stalk) made of stacked disks, and some sort of holdfast structure. That holdfast can be quite variable, from roots in muddy sediments to thick disks cemented to hard substrates.
Generalized diagram of a crinoid drawn by our coauthor Bill Ausich.
Bioerosion is the biological erosion of a hard substrate by either mechanical means (grinding, rasping, scraping, biting, drilling) or chemical (such as acidic dissolution) producing structures like borings, grooves, drillholes, etc. Until this work, no crinoids living or extinct have been known to bioerode their substrates. Here, though, as you can see in the top image, we appear to have a crinoid that has done just that way back in the Silurian.
The top image is of a thin-section cut through a crinoid holdfast attached to a stromatoporoid, which is a kind of calcareous sponge with a dense, layered skeleton of calcite. The whitish vertical portion is the crinoid stem partially buried in sediment. The yellow arrows show the radices (essentially “roots”) of the crinoid penetrating into the stromatoporoid skeleton, cutting through its topmost layers. It is clear that the crinoid was not simply embedded in the skeleton by stromatoporoid growth — the radices cut through the layers. This crinoid bored into the stromatoporoid (probably chemically) and held on tenaciously. Remarkable. The first indication that a crinoid could do this sort of thing.
Thus far it is only one specimen, so there is the inevitable question mark in the title of this paper. We put it out there as a first report hoping that other such cases may be found. Or, of course, someone could show that our interpretation is not correct.
I love these paleoecological stories! Thank you again to our Estonian friends and colleagues Olev Vinn and Ursula Toom. Bill Ausich and I treasure our friendship and collaboration with them.
For the record, this is the location of the boring crinoid find (from the paper).
And this is the stratigraphy (from the paper).
Reference:
Vinn, O., Ausich, W.I., Wilson, M.A. and Toom, U. 2022. Did stalked echinoderms bioerode calcareous substrates? A possible boring crinoid attachment structure in a stromatoporoid from the early Silurian (Telychian) of Estonia. Paläontologische Zeitschrift (https://doi.org/10.1007/s12542-022-00637-3)
