Cambrian crinoids
Bruce Stinchcomb

True crinoids—that is an echinoderm with a stem and a holdfast (pelmatozoan) which is not a cystoid or a paracrioid—are rare or non existence in the Cambrian Period, that period of geologic time which held the first "flowering" of invertebrate animals, many possessing distinctive hard parts. Two questionable crinoid-like fossils are known from the famous Middle Cambrian Burgess Shale otherwise Cambrian crinoids are unknown!. Crinoids (as well as most other echinoderms), when they die disarticulated and their hard parts, consisting of the head (calyx) and stem (pelma), scatter but may also the concentrate and form so called crinoidal limestones. Crinoidal limestones are particularly common and widespread in the late Paleozoic Mississippian Period (Lower Carboniferous) where sizeable beds of limestone can be almost entirely composed of their disarticulate hard parts. The well known Burlington and Keokuk limestones of the Midwest U S being good examples as well as Lower Carboniferous limestones of other parts of the world such as found in the Urals of Russia and the Brooks Range of northern Alaska.

Middle Ordovician limestones can also contain pelmatozoan fragments similar to those of later periods of the Paleozoic Era; Ordovician "crinoidal" limestone (pelmatozoan limestones) do have crinoids as a major contributor however disarticulated plates and stems of cystoids and paracrinoids are also major contributors. Crinoid-like fragments, including stem fragments similar to those found in Ordovician limestones also occur in Upper Cambrian limestones such as the Flathead Formation of Wyoming and the Bonneterre Formation of Missouri. In limestones of these Cambrian formations can occur pockets of what look very much like crinoid stem fragments—their being from true crinoids is doubted although they do look like small crinoid stems. For one thing, complete crinoids are unknown from localities where complete, articulated Cambrian echinoderms do occur. Both the Spence Shale of southern Idaho and the Wheeler Shale of western Utah yield excellent complete echinoderms but crinoids have not been found. What is found (and desirable) are eocrinoids, a class of stemless crinoid-like echinoderms which went extinct after the Cambrian and cystoids—an extinct phylum of echinoderms which have a stem. Cystoids represent a sizeable category of echinoderms which supplied the raw material in the 1980’s and 90’s for a number of "new" classes (viz. body plans) of echinoderms., Echinoderm classes proposed included "new" classes like the homostealians and the stylophorans. These (presumed) echinoderms (stylophorans have been placed as an extinct phylum by some paleontologists) lack a pelma (stem) and were apparently free swimming. Cystoids on the other hand, have stems which were often attached to hard grounds. The issue of Cambrian echinoderms is still a "muddy" one as well as a complicated one.

My interest in these fossils was tweaked by the finding a Cambrian stemmed echinoderm which resembled a crinoid as well as finding various "crinoid" stem-rich limestone pockets in Cambrian age rocks. In many ways the Cambrian Period was a strange, archaic world whose fossils are representative of body plans different from those of later geologic time. Fossils shown here are (presumably) not crinoids—rather they are echinoderms, probably cystoids representative of that time designated as B C—that is before crinoids.

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Cambrian echinoderm
Unidentified Cambrian echinoderm

Unidentified-stalked Cambrian echinoderm. This fossil was tentatively placed as a crinoid? in my 2008 book "Paleozoic Fossils"; It is most likely an undescribed stalked cystoid from the Davis Formation, Ste. Francois Co., Missouri.

cystoid stem fragments
crinoid stem like fossils

"Crinoid stem fragments". Cambrian age rocks were scoured when a huge rush of water spilled from a failed pumped-back-electrical storage facility, (Taum Sauk Reservoir) in southern Missouri. Exposed in this potential catastrophe were shale beds which yielded these small crinoid stem like fossils. If these fossils were Ordovician or younger they would be considered as crinoid stem fragments—being Cambrian in age they probably are from some sort of cystoid. Bonneterre Formation, East Fork of Black River, Missouri.

holdfasts Cambrian
Cambrian holdfasts

Holdfasts. These button-like holdfasts (Echinoderm pelma attachment structures), can locally be abundant on what are referred to as hardgrounds (hard limestone surfaces which existed on the ancient sea floor). The crinoid-like stem fragments of the previous picture may have been attached to something like these. Crinoid stem holdfasts usually resemble the root of a plant rather than having a button-like shape like these. Davis Formation, Ste Francois Co., Missouri.

crinoid-like stem fragments
cystoid stem fragments

Crinoid like stem fragments from the Big Horn Mountains of Wyoming. The Flathead Formation of Wyoming’s Big Horn Mountains can locally contain limestone beds full of these fossils. Such fossils, if younger than the Cambrian, would be considered as being from crinoids. As with the Bonneterre fossils these, if they were younger would be considered as parts of crinoid stems. They are probably from cystoids—stemmed echinoderms which predated the crinoids back in B C---that is, before crinoids.

Peculiar Cambrian fossils
crinoid stem-like fossils

Peculiar crinoid stem-like fossils from the lower Bonneterre Formation Ste. Francois Co., Missouri. These look like crinoid stems except that they have small perforations in them. They may be a type of sponge or again they may be parts of peculiar cystoids—a number of small, odd fossils occur in the Bonneterre Formation.

Kinsabia sp. Cambrian
Kinsabia sp.

Kinsabia sp. A small coral-like fossil associated with "crinoid stem" like fragments from the Bonneterre Formation of Missouri.

Cambrian crinoids
Bruce Stinchcomb
MAPS Digest, April 2009; Vol. 32, No. 1

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