This is Mazon Monday post #29. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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This week's "Mazon Monday" is a species (or class) spotlight on the Holothurians otherwise known as Sea Cucumbers. They belong to the phylum Echinodemata, which includes starfish, sea urchins, sand dollars, and seal lilies. Current feeling is there were two species of holothurian in Mazon Creek. The most common, and maybe only, one is called Achistrum sp.
There's a nice page about Sea Cucumbers over on wikipedia.
Sea cucumbers are echinoderms from the class Holothuroidea. They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian (/ˌhɒləˈθjʊəriən,ˌhoʊ-/[2][3]) species worldwide is about 1,717[4] with the greatest number being in the Asia Pacific region.[5] Many of these are gathered for human consumption and some species are cultivated in aquaculture systems. The harvested product is variously referred to as trepang, namako, bêche-de-mer or balate. Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter after which bacteria can continue the degradation process.[5]
Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are usually reduced to isolated microscopic ossicles (or sclerietes) joined by connective tissue. In some species these can sometimes be enlarged to flattened plates, forming an armour. In pelagic species such as Pelagothuria natatrix (Order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring.[6]
Sea cucumbers are named for their resemblance to the fruit of the cucumber plant.
Here's how modern day sea cucumbers look.
Chiridota heheva, abyssal species. By n - n, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11363281
Holothurians appeared in Andy Hay's Creature Corner in May 1986. Here is the text.
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.
Ina 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.
Here is the text from "The Mazon Creek Fossil Fauna", by Jack Wittry.
Achistrum sp.
This holothurian,or Sea Cucumber, is a member of the echinodermsand has a worm-like, soft and leathery body. Like most echinoderms, the body is divided into five sections. They are not uncommon in the Essex Fauna, but are extremely rare in the overall fossil record. It was assumed at one time that about a dozen species existed in the Mazon Creek Biota. Today, it is felt that, at most, two species are found.
Achistrum has a cylindrical form that is sac-like, and dries during preservation, leaving dessication cracks. These cracks are replaced with calcite, giving the fossil a septarian or mud-crack appearance. A ring of approximately 15 small calcareous plates at the head end is a prominent feature of Achistrum. (See Insets on pages 122 and 170.) This ring of plates forms the mouth support ing the esophagus, and serves as the anchor point for five radial body elements that run the length of the animal. Radial symmetry is a uniting feature of all echinoderms and is occasionally seen in Achistrum such as the one shown in Figure 122.1 on page 122. Embedded throughout the body wall are small fish-hook shaped Figure 121 after Shab Scale bara supports called sclerites
Though found in modern species, no feeding tentacles surround-ing the mouth have been observed in the Mazon Creek forms. The calcareous mouth rings, fish-hook structures, and body cracks are the field marks of this genus.
Care must be taken when cleaning Achistrum specimens. Using any form of acid, such as vinegar, will quickly destroy any traces of the mouth ring plates and sclerites.
