Sunday, November 18, 2018

Decorah gastropods (and some things that look like gastropods)

The snails* of the Decorah Shale are a lesser component of the fauna than bryozoans, brachiopods, or crinoids. My personal experience is that snails are uncommon except for certain beds, which feature abundant and diverse snails. One of these beds is perhaps 20 ft (6 m) above the top of the Carimona in St. Paul; this bed produced the plate in the "Equatorial Minnesota" box near the top of the page. For whatever reason, this bed also hosts abundant trilobite pieces, particularly of Eomonorachus. There's probably a facies thing going on, such that the original depositional environment was favorable to snails and trilobites; it's not quite as stark as, say, McKee (1938)'s mollusk and open marine facies of the Kaibab Formation (the very durable rock at the top of the Grand Canyon stack), but there's certainly some kind of difference. It might be a carbonate thing; both the underlying Platteville and overlying Cummingsville/Prosser, which have more limestone and dolomite, also have more diverse mollusks than the muddy Decorah (Sloan and Webers 1987). On the other hand, the relatively limited diversity and abundance of snails, combined with most genera having visually distinctive appearances, make it possible to summarize them in a reasonably brief guide.

*and things that look a whole lot like snails, and things that people argue about, such as Sinuites

Several snails in a small section of a slab: C for Clathrospira, S for Sinuites, and L for "lophospiroid".

Sunday, November 11, 2018

Rebbachisauridae

Diplodocoidea contains three wings: Diplodocidae, where the popular diplodocoids such as Apatosaurus, Barosaurus, Brontosaurus, and Diplodocus hang out; Dicraeosauridae, somewhat undersized and short-necked sauropods that are seemingly content to be represented in the public eye by Amargasaurus and its magnificently strange vertebrae; and Rebbachisauridae, also generally known for undersized and short-necked sauropods represented in the public eye by one exceptional taxon. In the case of rebbachisaurids, it's Nigersaurus, famous for its skull, which looks kind of like the animal habitually slept with its snout pressed against a wall.

Yeah, like that. Photo taken at a traveling exhibit at the Science Museum of Minnesota back in May 2014.

Sunday, November 4, 2018

Cumberland Bone Cave

There are a handful of notable Pleistocene "bone caves" in the Mid-Atlantic states. We've stopped at one of the classic sites already, Port Kennedy Bone Cave. Another classic site is Cumberland Bone Cave in western Maryland. Its scientific history began a few decades later than Port Kennedy, but both sites are of Irvingtonian age (between approximately 1.9 and 0.25 Ma), rather than a younger age like most Pleistocene sites, and the faunas from the two sites are often mentioned together.