Oklahomacystis

Something I'd hoped to find at Chickasaw National Recreation Area was an example of Oklahomacystis. The Bromide Formation is one of the great rock units for echinoderm diversity, and Oklahomacystis is one of the classic forms. Unfortunately, we didn't end up getting into the Bromide in the recreation area, although it should be there, so it will have to wait for another time. I did see plenty of Oklahomacystis in the collections of the Sam Noble Museum, as well as one slab featuring a group of them at the Travertine Nature Center in the recreation area. Here are some photos of the slab:

Oklahomacystis tribrachiatus

Other photos of the same slab are on the Internet, but my photos may be the only ones without the glass, so they've got that going for them. The scale is important, because for some reason, when taken without it, the fossils look to be about the size of dates, when they are actually more like the size of the first joint of a finger. The fossils represent the body, or "theca".

Mysteriously larger

These are visually unusual fossils, looking something like a pine cone, or a pineapple, or a cluster of berries, or a grenade. A close look shows that their surfaces are covered by shared circular arrangements of six (usually) wedge shapes, kind of like Trivial Pursuit pies that share wedges with other pies. If you're familiar with the kinds of words that are frequently used in scientific names, you may have already guessed that we're dealing with an echinoderm. If you guessed "paracrinoid", congratulations, either you are really good with echinoderms or you already know something about Oklahoma's fossils.* A paracrinoid is one of the numerous varieties of stalked echinoderms that flourished for a relatively brief span of time during the Paleozoic and then went extinct. The name suggests affinities with crinoids, but it should be noted that sometimes they are classified more distantly, with relatives to blastoids ("sea buds") or as their own group distinct from either crinoids or blastoids. Like those two groups, a complete paracrinoid would have also featured a stalk attaching it to the seafloor. Presumably Oklahomacystis would have had a fairly simple feeding apparatus compared to crinoids, like the similar paracrinoids Amygdalocystites or Comarocystites (which looks quite a bit different because its thecal plates are strongly concave, giving it that "puckered hexagon" look that was such a craze in the Ordovician). Oklahomacystis had recumbent arms which are part of the thecal structure; as the species name "tribrachiatus" indicates, there are three of these (tri + brachi, meaning arm, which of course we know from Brachiosaurus, the "arm lizard". Snuck in a dinosaur!). Like the other two genera mentioned above, Oklahomacystis's filter feeding apparatus would have consisted of pinnules attached to the arms. If you'd like more technical information on paracrinoids, there's a monograph on the subject available here (no. 288; it just happens to name Oklahomacystis as well).

(*Oklahomacystis is sometimes identified as a cystoid echinoderm, as under the "Bromide Formation" page at Wikipedia. I hinted at this a little bit in a previous post, but in the past people have occasionally had recourse to shovel various enigmatic extinct echinoderms into the cystoids. I'd just stick with paracrinoids here.)

I'll be at an event for Interstate State Park next weekend, so expect some Cambrian in the next post. After that, I'll try to do something dinosaurian; otherwise they'll take away my card!