This is Mazon Monday post #175. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Chitons are rare in the Mazon Creek biota. They are Mollusks, belonging to the same Phylum as oysters, clams, cephalopods, snails, and gastropods. Classified as polyplacophorans, they always have eight ornamented segments or valves.
Glaphurochiton concinnus was one of the first animal described by Dr. Eugene Richardson, Jr. He described it as Helminthochiton concinnus in the paper "Pennsylvanian invertebrates of the Mazon Creek area, Illinois: marine fauna", which appeared in the Field Museum's journal Fieldiana in 1956. It was moved into the genus Pterochiton in 1972. It was later reassigned as Glaphurochiton by Hoare and Mapes in the paper "The polyplacophoran "Chiton" carbonarius" in 1986.
The holotype of G. concinnus resides in the Smithsonian Museum of Natural History. It was collected by Mr. and Mrs John McLuckie.
G. concinnus appears on page 100 in Jack Wittry's "The Mazon Creek Fossil Fauna".
Glaphurochiton concinnus was one of the first marine ani- mals described by Dr. Eugene S. Richardson, Jr. This chiton or polyplacophoran mollusk is a long, thin mollusk with eight valves ornamented with concentric rows of tubercles and irregular growth lines. The valves are surrounded by a muscular girdle covered with spicules that overlap the lower part of each valve. The radula consists of over 100 rows, with 17 teeth per row forming a long band.
Generally, isolated valves or a rare soft part of the animal are found as fossils. Those from the Essex Fauna are almost always preserved as complete, intact specimens with the girdle and radula frequently present. Its occurrence in the Mazon Creek Biota is the first report of a radula from the Paleozoic. Chiton radulae from G. concinnus are very similar to those of modern animals and are reflective of a very conservative group.
Chitons are very ancient animals ranging from the Late Cambrian to the present. G. concinnus specimens were found in at least two concentrations at Pit 11, which were nicknamed Chiton Hills by collectors. Chitons are also known from other Pennsylvanian-age sites in the United States. Modern chitons spend much of their time in the dark, attached to the undersides of submerged objects, feeding on algae. They are considered to be the most primitive of all living mollusks.
G. concinnus is discussed in chapter 8B in the "Richardson's Guide to the Fossil Fauna of Mazon Creek".
Glaphurochiton concinnus (Richardson) Figures 8B.1-8B.5
The eight-valved shell of this lepidopleurid chi- ton is about five times as long as it is wide (Fig- ures 8B.1, 8B.2). The head valve is subovoid, broadly rounded in the anterior. The tail valve, rounded in the posterior, is subovoid, and bears a blunt mucro. In small, younger specimens, the tail valve is more subcircular than ovoid. Intermediate valves are subrectangular, being wider than long, with a fairly sharp jugum. Ornamentation of all valves consists of concentric rows of fine tubercles and irregular, fine, concentric growth lines (Richardson, 1956; Figure 8B.3). The girdle surrounding the shell laterally overlaps the lower edge of each valve, extending into interplate junctions (Yochelson and Richardson, 1979; Figures 8B.1, 8B.2). Small, straight, pointed aragonite spicules covered the girdle, except at its outer edge, where a dense fringe of larger spicules occurs (Figure 8B.4).
The ribbonlike radula, consisting of at least 100 rows of 17 denticles each, extends from the front of the head valve to the front of the third valve on the ventral surface (Figure 8B.5).
Richardson (1956) originally described this taxon as Helminthochiton concinnus. Hoare et al. (1972) placed H. concinnus into the genus Pterochiton and synonymized it with Chiton carbonarius Stevens. Yochelson and Richardson (1979) accepted the Pterochiton assignment but rejected the species synonymy. They believed that the majority of the Essex fauna specimens lie within the limits of P. concinnus, although a few individuals may represent a second, closely related species. Recently, Hoare and Mapes (1986) reassigned this taxon to the genus Glaphurochiton, and it is thought that G. concinnus was derived from G. carbonarius. All species of Glaphurochiton are considered to have been mud dwellers because they disappear rapidly from the fauna as the carbonate ratio of the sediment increases (Hoare and Mapes, 1986).
Specimens
From "The Mazon Creek Fossil Fauna".
