|Left it in the oven too long, again (University of Minnesota specimen).|
What's that? Looks kind of like a fish, doesn't it? If you get in there close, you'll see what looks like fine, gently-curved rods making up the herringbone pattern. These rods are made of calcium phosphate, a family of minerals which includes bone mineral. The stuff between the rods is interpreted as having been some kind of leathery material. Rather than being bones, the rods actually supported larger, triangular surfaces.
|Smaller, but there's the triangle (University of Minnesota specimen).|
In fact, a complete, 3-dimensional specimen would be shaped sort of like a flat-sided ice cream "cone" or a tall, skinny pyramid. There are some excellent figures and discussion at Wooster's Fossil of the Week, but that's essentially the upshot: a four-sided herringboned ice cream cone. (Most of the following discussion is after Leme et al. 2008.) Aside from looking like a dessert pioneered by someone with a fear of roundness, these fossils are the exoskeletons of an extinct group of invertebrates known as conulariids or conulatans (Conulata is the subclass, Conulariida is the order, but the critters involved are more or less the same). These creatures appeared by the very latest Precambrian (Vendian), persisted into the Late Triassic, and appear to have reached their greatest diversity in the Ordovician. Conulariids have long inspired debate about what exactly they were, because when you get right down to it they don't particularly resemble anything alive today. Most authorities today consider them in the Cnidaria, which we met last week, as an extinct subgroup or close relatives of Scyphozoa (you are probably more familiar with their common name: jellyfish). This isn't saying that they looked or acted much like jellyfish; they obviously had hard parts, and appear to have been anchored to the seafloor for at least part of their life cycle. Some have argued for them not being anchored but just laying there (recumbent), or having a free-swimming stage, but anchored and upright is the dominant conception today. The points linking them to the cnidarians include four-fold symmetry and tentacles. Given the tentacles and the broad resemblance to uptight horn corals, we can picture them living a similar lifestyle, with the narrow end of the structure anchored to the seafloor and a polyp extending out of the broad end, snagging tiny organisms and organic detritus.
Conulariids were never hugely common, but they stuck around into the Triassic, unlike many other marine Paleozoic groups, so clearly they were good at conulariiding (whatever that entailed). Stauffer and Thiel (1941), who listed their conulariids with the snails, reported two species and an undetermined species from four geologic units of the Twin Cities. Conularia quadrata was present in the regular Platteville, the combined Cummingsville and Prosser formations, and possibly the Decorah Shale. Conularia trentonensis was present in the Decorah and combined Cummingsville–Prosser. Finally, plain old Conularia sp. ("sp." means it's in that genus, but no species has been determined) was reported in the Glenwood and Platteville. A few years later, Sinclair (1952) described an additional taxon, Ctenoconularia obex, based on a specimen collected by our old friend Sardeson from the "Sprechs Ferry Member of the Platteville formation" (modern Carimona Limestone Member of the Decorah Shale) of Minneapolis.
To see additional photos of Ordovician conulariids, check out Fossils and Strata of the Type Cincinnatian and fossilid.info. A well-preserved and well-prepared conulariid is a striking fossil.
Leme, J. d. M., M. G. Simoes, A. C. Marques, and H. Van Iten. 2008. Cladistic analysis of the Suborder Conulariina Miller and Gurley, 1896 (Cnidaria, Scyphozoa; Vendian-Triassic). Palaeontology 51(3):649–662.
Sinclair, G. W. 1952. A classification of the Conularida. Fieldiana Geology 10(10):135–145.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.