Rostroconchs: Paleozoic taco shells

The Paleozoic was full of invertebrate groups that didn't quite hack it. The kings of extinct invertebrates are, of course, trilobites. The next tier down, widely known to paleontologists, geologists, and fossil collectors, is where we find things like tabulate corals, rugose corals, and eurypterids ("sea scorpions"). Then there are some that linger in obscurity; the aglaspids (Cambrian critters that were something like trilobites with poor-quality horseshoe crab disguises), for example, or cyclocystoids (disc-like echinoderms), which of this writing don't even have a Wikipedia article. Rostroconchs are another group that hangs out near the aglaspid/cyclocystoid end of the scale.

Rostroconchs were mollusks, but their exact evolutionary relationships with the other mollusks has been a matter of debate (as are the relationships of essentially all groups of mollusks). Early researchers included them with bivalves, and they do resemble small clam-like things, but they have been recognized as a separate group (relatively recently, in Pojeta and Runnegar [1976], the first place to go if you need a scholarly overview of this group). More recent work suggests that they were closer to the scaphopods, or "tusk shells", mollusks with long skinny conical shells that make a living with their head end buried completely and the narrow end of the shell sticking out of the sea floor to exchange gases and excrete wastes. This does not seem like an exciting arrangement, or conducive to great works of the intellect, but it does beat waiting in traffic. The layout of rostroconch anatomy suggests a similar lifestyle to this. Rostroconchs appeared during the Cambrian, diversified in the Ordovician but then contracted, occupied small areas of Paleozoic seafloor for a couple of hundred million years, and then departed the stage at the end of the Permian, joining the great extinction bandwagon that closed the Paleozoic.

Rostroconchs are known from their shells, which as mentioned look superficially like those of bivalves, apparently with two halves. Closer inspection reveals that the "shells" are actually a single shell folded over, like a taco. Opposite the "hinge", the two sides are close together, but at one end, which is considered anterior, there is a substantial gap that could have been used by the animal's foot. Opposite this gap, on the posterior end, is a tubular structure known as the rostrum, from which the rostroconchs get their name. This arrangement of parts could have allowed the rostroconch to function like a scaphopod, with the foot and anterior end in the substrate and the rostrum sticking out. Growth was probably permitted by fracturing in the layers of calcite that formed the shell.

Although the Ordovician seems to have been a good time for rostroconchs, they are still rare in the rocks of the Twin Cities, and if you spot one you should count yourself fortunate. Stauffer and Thiel (1941) reported a grand total of four rostroconch species, all under the genus Technophorus, and all classified as bivalves at the time. They are as follows:
Technophorus divaricatus (shale member of the Decorah Shale)
T. extenuatus (Carimona Member and shale member of the Decorah Shale)
T. filistriatus (shale member of the Decorah Shale)
T. subacutus (Platteville Formation)

All four of them were described by Edward Oscar Ulrich in 1892 (1892a, 1892b) and illustrated for his 1897 survey of local Paleozoic bivalves. None of them were very large; the specimens that Ulrich worked with reached sizes as big as 21 mm, or a little less than an inch at their greatest extent. T. extenuatus and T. subacutus were both named from Twin Cities specimens, but the other two were named from Cannon Falls. Pojeta and Runnegar (1976) accepted all four as valid species of Technophorus, which is refreshing after trying to track down representatives of certain other groups. Less conveniently, the type specimens of the first two species were reported as lost.

The lost type specimen of Technophorus divaricatus from the Decorah Shale of Minneapolis (Ulrich 1897: Pl. XL).

The lost type specimen of Technophorous extenuatus, from the Decorah Shale of Cannon Falls (Ulrich 1897: Pl. XXXVII).

From left to right: Technophorus subacutus from the Platteville Formation of Minneapolis (34 and 33), concentric marks on the surface of a specimen of T. filistriatus (36), and the whole of the specimen, from the Decorah Shale of Goodhue County (35) (Ulrich 1897: Pl. XL).

References:

Pojeta, J., Jr., and B. Runnegar. 1976. The paleontology of rostroconch mollusks and the early history of the phylum Mollusca. U.S. Geological Survey, Washington, D.C. Professional Paper 968.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Ulrich, E. O. 1892b. New Lamellibranchiata, no. 4. descriptions of one new genus and eight new species. The American Geologist 10:96–104.

Ulrich, E. O. 1892c. New Lower Silurian Lamellibranchiata chiefly from Minnesota rocks. Minnesota Geological and Natural History Survey Annual Report 19:211–248.

Ulrich, E. O. 1897. The Lower Silurian Lamellibranchiata of Minnesota. Pages 475–628 in Ulrich, E., W. Scofield, J. Clarke, and N. H. Winchell. The Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.