This is Mazon Monday post #52. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Drawing from George Langford Sr.
This week, we are going to look at a syncarid shrimp called Acanthotelson stimpsoni. It's one of the most common shrimp found amongst the Braidwood fauna. We had a very nice one from the Mazon River a few weeks ago for Fossil Friday #42. A. stimpsoni was described in 1865 by F.B. Meek and A. H. Worthen. Fielding Bradford Meek and Amos Henry Worthen were both prolific namers of taxa back in the middle 1880's. They collaborated on many papers through the years. Meek worked at the USGS. A.H Worthen was the second state geologist of Illinois and the first curator of the Illinois State Museum. He has an extensive fossil collection, which now resides at the Prairie Research Institute PRI at the University of Illinois. Acanthotelson was named for another early contributor to American science - William Stimpson. Stimpson worked at the Smithsonian Institution and was later the director of the Chicago Academy of Sciences.
Syncarida is a superorder of crustaceans. It consists of two extant orders Anaaspidacea and Bathynellacea, and one extinct order Paleocaridacea. There are 59 living genera, in six families. They generally live in fresh water, but a few species do tolerate brackish conditions.
The book "ESCONI Keys to Mazon Creek Animals" has some great detailed drawings by Don Auler and a good write up by Andy Hay.
Scotland, 1835, was the site of the first reported discovery of a fossil shrimp. A concretion bearing the fossil shrimp, Acanthotelson stimpsoni, was reported from the Mazon Creek area, about 1860. Prior to the opening of Pit 11 during the 1940's, the diversity of the shrimp-like fauna in the Francis Creek Shale of Mazon Creek and the surrounding coal mines proved remarkably small. The opening of a strip mine, Pit 11, near the town of Essex, Illinois, ushered in a new age of crustacean and soft-bodied animal research.
There are two distinct faunas within the Mazon Creek area: the non-marine Braidwood Fauna and the marine Essex Fauna. Numerous shrimp-like animals, filter-feeders, algal-feeders, scavengers, and predators were found in the Essex biota. A great diversity of form and genera were described and recorded. Shrimp of the Braidwood and Essex biota have separate ecologies and relationships.
Terminology used in their formal descriptions varied widely among early writers. The labeled drawing utilizes terms used most often by present-day authors. They will be used in our descriptions. Another fact comes to light in a study of fossil shrimp. Shrimp, as indeed with all arthropods, are a prisoner of their exo-skeleton: their body armor. A periodic molting is required to allow growth of the animal. Thisis a source of present day confusion. Preservation of Mazon Creek/Pit 11 shrimp varies from excellently preserved specimens showing hair-line detail, to concretions containing nothing more than a color difference arising from the animal's body juices. To facilitate reading, "Shrimp" will be used to designate these interesting animals, whose shapes may resemble present-day shrimp, crayfish, crabs, or lobsters.
It should be noted, and strongly emphasized, at the risk of being boringly repetitious, that the preservation of soft-bodied animals is a highly singular event in the fossil record. Reconstruction of the animals depicted in this chapter was only possible through the study of numerous concretions. The concretion held by the reader
will not contain every detail in the individual sketch.This moderately sized crustacean was first described in 1865. Original specimens were found along Mazon Creek. Concretions bearing Acanthotelson in the Mazon Creek. area are abundant in the Braidwood Fauna, but uncommon and poorly preserved in the Essex.
The cephalon (head shield) is short, grooved, with a small rostrum (beak) and insignificant optic notch. The eyes are small and stalked. First thoracic segment is short, next three are progressively longer, the other body segments are rather equal in length. Telson and uropods are slender with no pronounced bristles. First thoracopod (leg) very small, second and third are adapted for seizing prey; the other five for walking Last five "legs or pleopods are flap-like paddles with bristled margins.
Due to poor preservation this arthropod was shifted between various classes In some older literature it is referred to as Eileticus aequalis. The generic name Eileticus today reserved for a myriapod.
A. stimpsoni is mentioned in the Crustacea chapter in the "Richardson's Guide to the Fossil Fauna of Mazon Creek". That chapter was written by Frederick R. Schram, W. D. Ian Rolfe, and Andrew A. Hay.
Acanthotelson stimpsoni
Meek and Worthen, 1865First thoracic limb markedly reduced; outer ra mus of thoracic limb as a single flaplike segment; second and third thoracic limbs robust and spiny; abdominal limbs with two flaplike branches; telson and uropods styliform, with telson equal to or slightly longer than the uropods.
The cuticle of these animals was apparently very well sclerotized. As a result, these fossils are often well preserved with distinct segmental rings in evidence along the length of the body. This species has been the subject of much taxonomic confusion. In the older literature, it was often referred to by the incorrect generic name Eileticus. It is quite abundant in the Braidwood assemblage, but relatively rare and poorly preserved in the Essex assemblage.