Corals of the Twin Cities

The Ordovician seas of the Twin Cities would have been unfamiliar in a lot of ways. There were no sharks, no bony fish, no marine mammals, no seabirds. No driftwood bobbed in the water. There were no octopuses or true squids, no lobsters or crabs scuttling about. The kings of the echinoderms were not sea stars, brittle stars, and sea urchins, but crinoids. A diver would see a Lilliputian sea-scape featuring cm-scale brachiopods, profusions of bryozoans, and "forests" of sea lilies, traversed by trilobites. Snails, of course, would provide a common point of reference. "Are there no corals?" you ask, thumbing through your waterproof guidebook as you try to figure out if you have just stepped on an inarticulate brachiopod, a bivalve, or a monoplacophoran. Well, yes, there are corals; it's just they are all representatives of groups that have been extinct since the end of the Permian (that pesky Permian–Triassic extinction).

If you've never had reason to consider the lot of a coral, here's the thumbnail version. Corals are part of Cnidaria, a phylum they share with jellyfish, hydroids, and similar such critters. Their particular wing is known as Anthozoa. Corals are famous for building giant reefs, but not all corals are colonial. Individual coral animals are known as polyps, and are cylindrical animals with a crown of tentacles at one end surrounding a mouth. Unlike sponges, they have distinct tissues and organs. Coral polyps form hard surrounding skeletons of carbonate minerals, which are what we find as fossils. Polyps can act as predators, securing tiny prey with the stinging cells on their tentacles (like jellyfish do, except corals are stuck in one place). Many corals also host symbiotic photosynthetic algae, which provide an alternate source of energy. Finally, here are a few bits of anatomical terminology that may come in useful: a skeleton of an individual polyp is a corallite, while a compound skeleton of multiple corallites is a corallum (some authorities use these terms somewhat differently); vertical partitions in a corallite are septa, and horizontal partitions are tabulae.

The corals of the Twin Cities Ordovician were not reef-builders; in fact, a lot of the corals were solitary, and we don't have reefs of anything. They are represented by a handful of species belonging to two extinct groups, the Rugosa and the Tabulata. Rugose corals are best known for their solitary forms, which for reasons that quickly make themselves apparent are called horn corals. "Rugosa" comes from the irregular, rugose surface of their corallites. In addition to septa and tabulae, rugose corals featured a central column within the corallite. The septa are the most obvious of these internal features, and a well-preserved horn coral will show them running along the inside wall of the corallite. Horn corals are reputedly fairly common in the Decorah Shale, but you would never know it by me. I can scarcely find a coral to save my life, and I have no idea how I found the one example I do have; probably saw part of it and thought it was a snail or something. Although you may find them clustered together, such clusters are not true coral colonies. Some other rugose corals were truly colonial, but they don't appear to be represented in the Twin Cities. Instead, we get the horn corals Lambeophyllum and Streptelasma.

Spot the corals! (nuts if I can)

Behold, the fruits of a life's labor. They aren't even complete.

Better examples from the collections of the University of Minnesota (although I think these are natural casts).

Tabulate corals are sometimes known as honeycomb corals, again for reasons that become plain when you see a well-preserved example: the individual corallites are hexagonal and packed together, making them look like, well, honeycombs. Tabulate polyps apparently got on better with each other than rugose polyps, so almost all tabulates were colonial. The "tabulate" part of the name refers to the fact that tabulates emphasized the horizontal tabulae instead of the vertical septae. If you section a tabulate, you'll see lots of "ladders", with the "rungs" being the tabulae within the corallites. Tabulate colonies can be large masses, but they can also be thin encrustations of other objects, or even something that looks kind of like a runner with "shoots" coming off of it. The Twin Cities rocks feature examples of these different types: Foerstephyllum made masses, Lichenaria and Protaraea encrusted, and Aulopora made "shoots" and "runners".

As per the previous article on sponges, we've got Stauffer and Thiel (1941)'s listing to go by. They have a better time of it with the corals than they did with the sponges. The following species were listed as known from the Ordovician rocks found in the Twin Cities area. Because I'm kind of shy of pictures on account of not having any examples, I have some external links to pinch hit. As before, here are the codes for the rock units:
Pl is the Platteville Formation (their McGregor Member of the Platteville)
Ca is the Carimona Member (their Specht Ferry Member of the Platteville)
De is the Decorah Shale (their Decorah Shale Member of the Galena Formation)
Gp is the Cummingsville and Prosser formations (their Prosser Member of the Galena Formation)

Rugosa:
Streptelasma breve (Pl), cf. breve (Ca)
Streptelasma corniculum (De, Gp)
Streptelasma (Petraia) profundum (Pl, Ca, De, Gp) (inconsistent use of subgenus) (now known as Lambeophyllum profundum)
Streptelasma? parasiticum (Pl, Ca, De)

Tabulata:
Aulopora? trentonensis (Ca, De)
Columnaria halli (Ca) (now known as Foerstephyllum halli)
Lichenaria minor (Gp)
Lichenaria typa (Ca, De)
Protaraea vetusta (Gp)

Also listed is Tetradium fibratum (Ca), which has recently been reclassified as red algae; under such a classification, it would be renamed Prismostylus, because it would now be in plant territory and Tetradium is also in use for a tree genus.

The rest of Winchell and Schuchert's plate G. 7 to 9 are Hindia parva, which we met previously. 10 to 13 are Lichenaria typa (an encrusting large colony, a small colony, a longitudinal cut, and a free mass). 14 to 16 are Columnaria (?) halli, now Foerstephyllum halli (a surface, a vertical section, and a longitudinal section). 17 to 19 are Streptelasma profundum, now Lambeophyllum profundum (internal cast, specimen with broken wall, and view into the cavity). 20 and 21 are Streptelasma corniculum (exterior and interior). 22 and 23 are Streptelasma rusticum (exterior and interior). 24 and 25 are Protaraea vetusta (encrusting colony and detail view). 26 to 28 are Aulopora (?) trentonensis (a colony, a detail view of same, and another colony).

Species and genera of corals are usually differentiated by details of the corallites, which require sectioning of the specimen in question. To be honest, you probably aren't going to be able to differentiate species or sometimes even genera in the field, unless you happen to be out with Batman, and he's sportingly agreed to let you use his miniature batsaw, batsection-polisher, and digital copy of the Treatise of Invertebrate Paleontology (you'd be surprised how often these come in handy when fighting crime). If the Caped Crusader is otherwise occupied, you'll be able to say "horn coral" but probably not which genus or species. Of the tabulates, Aulopora is pretty distinctive as chains and networks of cups, and Foerstephyllum is the only one to make big properly coral-looking masses, but both Lichenaria and Protaraea have the encrusting mat thing going on. Protaraea corallites have prominent short septa that give the corallites a fancy appearance, kind of like a mutant asterisk, unlike Lichenaria corallites, so that might help if it's well-preserved. If you're worried about distinguishing encrusting corals from bryozoans, the size of the chambers for the individual animals is the clue: bryozoan pores are typically less than a millimeter across, while corallites are several millimeters to a centimeter across.

References

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

Winchell, N. H., and C. Schuchert. 1895. Sponges, graptolites, and corals from the Lower Silurian in Minnesota. Pages 55–95 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.