This is the "Fossil Friday" post #110. Expect this to be a somewhat regular feature of the website. We will post any fossil pictures you send in to [email protected]. Please include a short description or story. Check the #FossilFriday Twitter hash tag for contributions from around the world!
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For this Fossil Friday, we have a very sweet sea cucumber (Achistrum sp.) from Mazon Creek. For more information, we looked at these echinoderms back in Mazon Monday #29. Holothurians appeared in Andy Hay's Creature Corner in May 1986.
Holothurians (sea cucumber)
As has been noted previously in these articles, the not uncommon fossil finds of Pit 11 are a great rarity in the fossil record of the rest of the world. So it is with the "sea cucumbers", more accurately the members of the class Holothuroidea which are found at this location.
In a recent journal it is stated "complete fossil Holothurians are rare". During the last 100 years only a few such specimens have been found. Following a page describing the complete specimens known, a paragraph much further on makes note of Dr. Eugene S. Richardson, Jr.'s study of "entire specimens of apodus holothurians" from Pit 11.
Holothurians possess leathery to gelatinous body walls, which are strengthened by Ossicles and spicules -tiny bone-like structures of various shapes. The body is more or less cylindrical, mouth at one end, anus at the other. Feeding tentacles surround the mouth. These tentacles are usually retractable. Some holothurians are burrowing creatures, ingesting the bottom sediment and filtering it for food.Others feed on tiny animals swimming by ensnaring them in the fan like tentacles. A calcareous ring of calcite plates joined together (often called radula by fossil collectors) is a support for the esophagus and body muscles. This ring plus the tiny "fish-hooks" (ossicles) are the field marks of Pit 11 sea cucumbers. During preservation, the sac-like body dries and cracks. Spaces being filled by calcite give the fossil body a septarian or mud-crack appearance. The various genera and species, one dozen plus or minus, are under study and will be described by S. Sroka (personal communication). Gelatinous body with few skeletal elements that could be preserved make the study of fossil holothurians difficult. Pit 11 holothurian specimens may change this condition.
This particular specimen was found by ESCONI member Sue Diblee. She sent along this story to describe her discovery.
Here is the Achistrum sp. that you confirmed for me last Sunday. I found the nodule on March 1, 2022. The nodule opened by itself while it waited in line for some freezer time! I initially thought it was a coprolite but it matched quite closely with Jack Wittry's description and photo in his Mazon Creek Fossil Fauna book. The desiccation cracks and the circular mouth are essential features. Dr. WIttry reminds us to avoid vinegar when cleaning a sea cucumber fossil because acid can dissolve the calcite in the cracks.
Thanks for the contribution, Sue! That's a very nice fossil! The mouth can be seen on the upper right of the bottom half shown below.
