This is Mazon Monday post #310.  What’s your favorite Mazon Creek fossil?  Tell us at  email:esconi.info@gmail.com.

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Cyclus obesus is one of four species of Cycloidea found in Mazon Creek. It was described in the paper “Mazon Creek Cycloidea” by Frederick Schram. The paper was published in the Journal of Paleontology in 1997. In that paper, Schram established three new species of Cycloidea… Cyclus obesus, new species,  Halicyne max, new species, and Apionicon apioides, new genus, new species. The new ones joined Cyclus americanus (see Mazon Monday #28), which was described by Alpheus Spring Packard (1839-1905) in 1885.

Abstract of “Mazon Creek Cycloidea“

The Mazon Creek Cycloidea contain four taxa: Cyclus americanus Packard, 1885, Cyclus obesus, new species, Halicyne max, new species, and Apionicon apioides, new genus, new species. We conclude, based on a cladistic analysis, that cycloids are specialized maxillopodan crustaceans and a possible sister group to the Copepoda. They may have filled a niche similar to modernday crabs.

Cyclus obesus is rare. It has only been found in the Essex Biota of Mazon Creek. It differs in body shape from C. americanus, as it is larger and wider with a distinct oval, cap-like, and decorated carapace.

Jack Wittry covers Cyclus obesus on page 94 in his book “The Fossil Fauna of Mazon Creek”.

Cyclus obesus Schram et al., 1997

Cyclus obesus, as its name implies, has a wider body than long. The carapace is oval shaped, and the length is generally three-quarters as long as the width. The upper surface is smooth, with an elevated center region forming a plateau surrounded by a ridge and a narrow shelf or brim. The carapace edges are smooth and lack a posterior notch, which distinguishes C. obesus from all other known members of the genus Cyclus. Limbs are unknown at this writing. All known examples of this rare species have come from the Essex Fauna of the Mazon Creek Biota.

Specimens

Field Museum PE 30630, holotype, collected by Gordon Baird

From Wittry’s “The Fossil Fauna of Mazon Creek”

Field Museum PE 39056

Field Museum PE 23041