Cornulites

The Decorah Shale is the king of Twin Cities rock formations in terms of fossils. You don't have to take my word for it; try these reviews:

• "Nearly optimum conditions for marine life must have prevailed, inasmuch as a large number of marine macro- and microorganisms, as represented by nearly all phyla of marine invertebrates found in the Ordovician of Minnesota, attain maximum abundance and diversity in the Decorah Shale" (Webers 1972);
• "The Decorah Shale is the best formation in southeastern Minnesota in which to collect fossils" (Mossler and Benson 2006);
• "The Decorah Shale contains the most diverse and abundant fauna of the Ordovician System in Minnesota (Webers, 1972). Because of the abundance of fossils and the ease with which they can be extracted from the shale, numerous biostratigraphical and paleontological studies have been conducted on the Decorah Shale" (Mossler 2008).

Fossils are commonly found either loose, having eroded out of the shale, or concentrated in thin limestone beds. One of the fun things about the limestone fossil hashes is how you can keep finding new things in them; there's just so many fossils in some pieces that you can't notice everything at once. The average small sample of Decorah fossil hash, say a rock chunk about as thick as a finger and about palm-size across, will probably have specimens of brachiopods, bryozoans, and crinoids (the typical Decorah "BBC") and often also includes snails and fragments of trilobite exoskeletons. Breaking out a magnifying glass or hand lens sometimes reveals other things. A surprisingly common type of "accessory" fossil are millimeter-scale conical objects, which were once the home of the enigmatic extinct cornulitids.

Cornulites (as a "tubiculous annelid") attached to a specimen of the bryozoan Diastoporina flabellata from the upper "Galena shales" (Decorah Shale) of Cannon Falls (Ulrich 1895:Plate II, fig. 2).

Cornulitid tubes are never going to win any prizes as popular fossils. They are one of several types of humble "tube worms" found mostly in Paleozoic rocks. Broadly speaking, the tubes look quite a bit like "Bugles" snacks, especially when flattened. They are not instantly obvious to the untrained eye (and given the size, a hand lens comes in quite handy), but once you have seen one, the darn things start appearing all over the place.

The evolutionary relationships of cornulitids are not certain. They are currently thought to have been closest to things like tentaculites and microconchids, other extinct enigmatic "tube worms". In terms of groups that are still alive, annelid worms were the default for many years. Sometimes, mollusks are preferred, although the referral seems kind of half-hearted. Newer research suggests that the various Paleozoic "tube worms" were lophophores (members of the group containing bryozoans, brachiopods, horseshoe worms, and others). The Paleobiology Database includes cornulites with tentaculites as mollusks; the Baltoscandian fossil database has them with the tentaculites as unclassified animals; the online guide to Cincinnatian fossils includes them and the tentaculites as mollusks, but in the text suggests that they are actually closer to horseshoe worms. This is all rather dry, but it does go to show that the farther you go back in time, the more enigmatic extinct things you find, and the harder it gets to pin down who is related to who. The Wooster Geologists Blog has a good write-up with a diagram of the hypothetical anatomy of a living cornulitid.

An unattached cornulitid, ~3 mm long.

Cornulitids were predominantly encrusters, although occasionally you find them unattached. I've seen them stuck onto both bryozoans and brachiopods, and doubtless those two groups were not the limits of their talents. Sometimes they are found one-to-a-host, while in other cases you may see a veritable infestation of the little things. Like practically everything in the Late Ordovician, they were filter feeders that found themselves a good place as larvae and stuck there for the duration. (The Late Ordovician was The Age of Homebodies) The likely explanation for their habits is that by building onto a larger animal, they would be above the bottom mud and could also horn in on whatever their hosts were doing to get by, i.e. mooching off the feeding currents generated by the larger filter feeders (see Meyer and Davis 2009 and references therein for more).

How to make friends, Decorah-style: a brachiopod shell encrusted by a bryozoan, encrusted by three cornulitids.

References:

Meyer, D. L., and R. A. Davis. 2009. A sea without fish: life in the Ordovician sea of the Cincinnati region. Indiana University Press, Bloomington and Indianapolis, IN.

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Mossler, J., and S. Benson. 2006. Minnesota at a glance: fossil collecting in the Twin Cities area. Minnesota Geological Survey, St. Paul, Minnesota.

Ulrich, E. O. 1895. On Lower Silurian Bryozoa of Minnesota. Pages 96–332 in Lesquereux, L., C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(1). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

Webers, G. F. 1972. Paleoecology of the Ordovician strata of southeastern Minnesota. Pages 25–41 in G. F. Webers and G. S. Austin, editors. Field trip guidebook for Paleozoic and Mesozoic rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook 4.