How niche a topic can I make it? Yes, today's post is about hypostomes, and more particularly, the hypostomes of lichid trilobites (of the Amherstburg Formation).
Ever since stumbling upon my Amherstburg fill area back in August, I've certainly been fortunate to crack open many a rock to find the occasional fragments of lichids, from isolated pygidia, cranidia, librigena, and even a connected thorax (just the one time, recently). and hypostomes.
The thing about hypostomes is that they are very easy to miss for a lot of collectors in the field. They don't look trilobitic to anyone who isn't already familiar with the ventral morphology of a trilobite. When I first started several years ago, I probably left dozens of them in the field. There has been much discussion and speculation on the purpose of this hard, plate that appears just underneath the cephalon, and just as much variety in morphology (check out the spooky and fierce-looking Hypodicranotus hypostome in Ludvigsen's paper here!). It is generally agreed that they served a primary purpose as part of their feeding apparatus. On that discussion, I highly recommend Hegna, T.A. (2010) The function of forks: Isotelus-type hypostomes and trilobite feeding. Lethaia, Vol. 43, pp. 411–419.
This snip of four line drawing representative lichid hypstomes is from page 188 of the lichid bible, Thomas, A.T., and Holloway D.J. (1988). Classification and Phylogeny of the Trilobite Order Lichida Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, Vol. 321, No. 1205 (Aug. 26, 1988), pp. 179-262. There are similarities among the genera's conterminant hypostomes, but also distinct differences as seen above.
At present, I have been able to reasonably identify three lichid genera from this material: Acanthopyge contusa, Echinolichas eriopis, and Echinolichas sp. cp. hispidus. These are consistent with equivalent strata in New York (see Whiteley, Kloc, and Brett's Trilobites of New York, or "TONY" among us bug enthusiasts).
Here are three hypostomes I have found since autumn of last year, with the one on the far right being a recent (as of today) find. I would guess that the first hypostome belongs to an echinolichine, whereas the middle one is unmistakably Acanthopgye contusa. The third one is still undetermined to me as of yet. It seems to share the echinolichine shape, and yet also appears to possess the granular ornamentation of the Acanthopyge. It is also by far the largest one I've found, with a width of just under 20 mm. That would possibly make the original owner between about 70-100 mm in length. Not excessively large, but not tiny. Of course, the preservation could be much better on the last two, but I take what the rock will give.
Hopefully in future posts about the lichids I can start organizing some of the other body parts, too.