This post shows a series a pictures of a small pea sized (3-6 mm) fossil found in the Silurian Period Waldron Shale of southern Indiana. Is this some sort of small calyx, blastoid, or something else?
The top has a small 5 petal flower like pattern that reminds me of crinoid calyx. It sort of reminds me of a Pisocrinus crinoid but the base is circle enclosed in a triangle shape on these fossils. The tops on some of the fossils have points the bend inward which seems more like a blastoid.
If anyone has some identification ideas on these fossils, let me know. Thanks!
UPDATE (1/7/2011): After reading some great comments about these specimens, I e-mailed Dr. Colin Sumrall at the University of Tennessee in Knoxville for advice. He identified it as a member of the Coronoidea and it is called Stephanocrinus gemmiformis. It is somewhat of a confusing crinoid and he pointed out it is "basically a blastoid with erect arms".
Kenny has found some of these fossils as well and sent me some images he took with a microscope. One can see a line like pattern on parts of the fossil plus a magnified view of the triangle shape on the fossil bottom.
6 comments:
Cool fossil and tiny! I think it's a Crinoid but not sure what type. I've seen similar fossils somewhere before but can't remember where.
Have you seen crinoids with equilateral triangle shapes like that before?
I would venture that these are neanic (i.e "baby") blastoids. The genus should be fairly easy to narrow down, as there are only a few genera (three, according to the Treatise) of blastoids known from the Silurian (blastoids first appeared in the Silurian). Troosticrinus is one of the spiraculate group, Decaschisma and Polydeltoideus, of the fissiculate group, are two others. All three occur in the Silurian of North America.
Thing is, all 3 of these genera have adult thecas ("calyxes") that are fairly long and conical, with long, tapering basal plates. However, neanic/baby blastoids are often very different looking than the adult versions, and unfortunately, illustrations of neanic specimens are hard to find, so that may make it hard to identify them. If you can find a faunal list of the Waldron Shale, you may be able to narrow down a likely candidate.
Kenny's first photo shows a very clear radial plate, embracing the lower end of the ambulacrum (petal-shaped structure on the top), which is characteristic of blastoids. His first and second photos both show clearly the suture between the radial and basal plates. I am sure this is a blastoid plate arrangement. The rest of the photos aren't detailed enough to tell very much, but the way the ambulacra are formed, with a denser, star-shaped feature set in the middle with its points partially dividing each ambulacrum is more likely to indicate a fissiculate blastoid, like Polydeltoideus or Pleuroschisma. It's possible that as the theca grows bigger/older, the basal plates elongate to a more conical shape, characteristic of the adult forms.
I would say that the triangular stem attachment knob on the bottom is also evidence that it's a blastoid, as blastoids characteristically have 3 basal plates, and each plate would likely contribute one point of the triangle.
Cheers,
--Howard
Thanks for the comments all and nice research Howard.
The fossil is the crinoid Stephanocrinus gemmiformis as identifed by Dr. Colin Sumrall of the University of Tennessee at Knoxville. He pointed out it is "basically a blastoid with erect arms" in the class of Coronoidea.
Cool! I guess I was close, but no cigar! Looking it up in the Treatise, I see that it was at one time lumped in with the cystoids (by Jaeckel, in 1918). It definitely has a blastoid look about it, but I see now why it's classified as a crinoid.
--Howard
Howard,
You may still end up correct. I have e-mailed with Paul at Primitive Worlds. They are a quarry for the Rochester Shale fossils of New York which are synonymous with Waldron Shale fossils of Indiana.
They find the Stephanocrinus angulatus which is labeled in their book, The Silurian Experience, as a crinoid. He says that Dr. Carlton Brett (University of Cincinnati) believes they should be considered blastoids. So maybe in the near future this creature will change status.
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