Sunday, December 17, 2017

Decorah crinoids revisited

Here in the Land of 10 Billion Crinoid Columnals, it's nice to be reminded that we do occasionally find a little more of the beast. I touched on the background of the metro crinoids in the crinoid post a couple of years ago. To quickly summarize: for whatever reason the paleontologists of the Minnesota Geological and Natural History Survey did not include echinoderms in the great monograph summarizing their work, although there were clearly plans to describe some crinoids, and Edward Oscar Ulrich did get to describe Cremacrinus punctatus; Frederick Sardeson described portions of the crinoid fauna in the first few decades of the 20th century; and the whole shebang was done up by Brower and Veinus in 1978. Their work was based around the University of Minnesota collections, largely collected by Sardeson. In his several decades of collecting, he amassed about 50 crinoid crowns and cups and several thousand plates (Brower and Veinus 1978), so if you're wondering why you haven't found any, it's because he got them all first. Here are a few highlights of the collection.

These first few photos, of Carabocrinus, require some explanation. As noted by Brower and Veinus, carabocrinids are particularly well-represented in the collection; they observed more than a thousand plates. There are two species of carabocrinids in the Decorah of Minnesota, Carabocrinus dicyclicus and C. magnificus. (These are the crinoids that have been restored as having big calyces and short stalks.) As usual, there are also a couple of synonyms. One of them is a bit of a problem. Back in 1899 Sardeson named a new genus and species Strophocrinus dicyclicus. He thought it was transitional between "cystocrinoideans" and carabocrinids, while everyone else thought it was just an abnormal Carabocrinus. This seems to have irritated him, and in 1925, he decided to take positive action against people referring to his Strophocrinus dicyclicus as Carabocrinus dicyclicus. His solution was to describe a new species, which he named... Carabocrinus dicyclicus. If nothing else, he certainly succeeded in confusing people. Brower and Veinus came to the conclusion that Strophocrinus dicyclicus was indeed an abnormal Carabocrinus dicyclicus. This leaves us with the odd situation that, technically speaking, Carabocrinus dicyclicus Sardeson 1925 is a junior synonym of Carabocrinus dicyclicus (Sardeson 1899). Photos of the type specimens of both follow.

The type specimen of Sardeson's Carabocrinus dicyclicus, UMPC 9207.

The type specimen of Sardeson's Strophocrinus dicyclicus, UMPC 9187, reconstructed by Sardeson himself.

If you are more like me, the following is more along the lines of what you might get on a good day: an isolated partial plate:

A Carabocrinus plate, probably a basal based on the striations. The "bubbly" marks are impressions of brassy ooids, so although the plate is not complete, we've got some stratigraphic information included.

The rest of these photos are of some of the species named by Brower and Veinus. Like the Carabocrinus specimens, they come from the Decorah of the St. Paul area. They have a variety of shapes and sizes. More than a dozen crinoid species can be identified from the Decorah of Minnesota, although they didn't all live at the same time. The list from the crinoid post is repeated below, with a few Platteville-only species removed, leaving us with Carimona Member (Ca, the base) and the lower, middle, and upper main body of the Decorah (De4, De5, and De6):

Archaeocrinus sp. (De4)
Carabocrinus dicyclicus (De4–6)
Ca. magnificus (De4–6)
Cremacrinus punctatus (De4–5)
Cupulocrinus jewetti (Ca, De4–5)
Cu. canaliculatus (De5)
Glyptocrinus tridactylus (De6)
Grenprisia billingsi (De4–5)
Isotomocrinus tenuis (De5)
Palaeocrinus angulatus (De4)
Periglyptocrinus spinuliferus (De4)
Porocrinus pentagonius (De4–5)
Po. elegans (De5)
Pycnocrinus sardesoni (De5–6)
Py. multibrachialis (De5)
Lichenocrinid holdfasts (Ca, De4–5) (resemble small rough-surfaced rings attached to brachiopods)
Lobate/digitate holdfasts cemented to bryozoans (De4–5)
Massive conical attachment disk (De6)
Stem with grasping cirri on bryozoan (De5)
Tree-stump cirrus root (De5)

The type of Pycnocrinus sardesoni, UMPC 9263.

The type of Glyptocrinus tridactylus, UMPC 9261.

The paratype of Periglyptocrinus spinuliferus, UMPC 9260b.

The type of Cupulocrinus canaliculatus, UMPC 9286.

References

Brower, J. C., and J. Veinus. 1978. Middle Ordovician crinoids from the Twin Cities area of Minnesota. Bulletins of American Paleontology 74(304):372–506.

Sardeson, F. W. 1899. A new cystocrinoidean species from the Ordovician. American Geologist 24(5):263–276.

Sardeson, F. W. 1925. Ordovicic Crinoidea. Pan-American Geologist 43(1):55–68.

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