This is Mazon Monday post #174. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Convexicaris mazonensis
Concavicaris georgeorum
Thylacocephala are considered problematic. They are known from some species in the Paleozoic and a few more from Mesozoic, with no known extant forms. There are two species of Thylacocephala, often referred to as "flea shrimp", from Mazon Creek - Convexicaris mazonensis and Concavicaris georgeorum. They are both exceedingly rare with Concavicaris georgeorum being the most common. Either would fulfill a bucket list item on any serious Mazon collector's want list. They were both described in 1990 by Frederick Schram in the paper "On Mazon Creek Thylacocephala", which was published in the Proceedings of the San Diego Society of Natural History.
There is a description of Thylacocephala in Chapter 15B "Euthycarcainoids and Thylacocephalans" in the "Richardson's Guide to the Fossil Fauna of Mazon Creek". That chapter was co-authored by Frederick Schram and W. D. Ian. Rolfe, both men are giants in the subject of fossil arthropods.
Class Thylacocephala
The Thylacocephalans (flea shrimp) are another strange group in the Arthropoda (subphylum uncertain), with a range that extends from the Cambrian to the Cretaceous. Long known from individual shells or carapaces, only recently have fossils been found that supply information on other aspects of the body. The best preserved material is from the Jurassic of France and includes a strange thylacocephalan that appears to have been a rather formidable predator. "Flea shrimp" are common in the Mazon Creek Essex fauna, though they may not in fact be shrimp or crustaceans.
Both species appear in Jack Wittry's "The Mazon Creek Fossil Fauna", pages 98 and 99.
Concavicaris georgeorum Schram, 1990
Concavicaris georgeorum is the most common flea shrimp in the Mazon Creek Fauna. It has large compound eyes that are set back in a notch in the body. The body is roughly oval in shape and about as tall as it is long. A single ridge runs along the sides.
C. georgeorum is from an extinct group of Arthropoda commonly called Flea Shrimp. They are neither fleas nor even likely true shrimp, but it now seems probable that thylacocephalans are crustaceans. They have three sets of raptorial appendages that point toward the mouth. Certainly, they appear to have been a for- midable predator for their size.
Convexicaris mazonensis Schram, 1990
Compared to its body size, Convexicaris mazonensis has enormous compound eyes that seem to protrude from the creature's anterior end. The front appendages are long and heavy. Two weak ridges running parallel to each other are found along the upper sides of the animal's carapace. Unlike Concavicaris georgeorum, the body tapers abruptly toward the rear into a blunt point.
This flea shrimp looks rather like a bicycle helmet with a large headlight. It is the second most common Mazon Creek flea shrimp, with large examples being the norm.
Specimens
All photos are from "The Fossil Fauna of Mazon Creek" by Jack Wittry.
Concavicaris georgeorum
Convexicaris mazonensis
