This is Mazon Monday post #56. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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The first format descriptions of the Polychaete worms of the Mazon Creek biota didn't happen until 1979. One of the first was Didontogaster cordylina, commonly known as the tummy tooth worm. D. cordylina was described by Ida Thompson in 1979. Ida Thompson is also famous for writing the National Audubon Society Field Guide to North American Fossils. A book that has been in print since at least 1994.
The book "ESCONI Keys to Mazon Creek Animals" has a nice write up about Polychaete worms.
Pit 11 marks the earliest, and in some cases, the only appearance of polychaetes in the fossil record. Serious study and description of these previously unknown animals did not take place until the mid 1970's. Owing to the absence of a formal classification, collectors bestowed comic and semi descriptive names on their "worms". Upon examining collections of Essex fossils, it is quite common to read names such as Oliver Hardy, Simple Jaw Worm etc. One would not expect to see designations such as these in scientific literature. These titles did and do serve a function, therefore they will be included in the descriptive literature.
Polychaete worms of the Essex Fauna, as a group, were large, predaceous, and free swimming; in contrast to recent marine polychaetes, which tend to be generally small sedentary, and deposit or suspension feeders. The polychaete body is a cylinder of muscles in segmented units. Chitinous bristles (chaetae) are secreted by the epidermis: adorning the body, and extensions of the segments (the parapodia). Respiration is achieved via gills that may be an addition to the body wall, or an or an extension of the parapodia. The most forward segment contains the brain, sense organs, and feeding mechanism. Locomotion is aided by movement of the chaetae (cirri, aciculae, setae, etc). on the body wall and the parapodia. Typical feeding, locomotor, and respiratory apparatus are illustrated above.
Annelid worms were not an unexpected find in the Essex Fauna. Discoveries of numerous, diverse members of a typical inshore marine community, made the discovery of these fossil fossil worms and worm-like animals a definite possibility.
The information on D. cordylina appears on the next page.
This is the most commonly found Essex Fauna polychaete; a medium sized (median length about 49 mm) worm. cordylina, means club-like; a rather apt description of the body outline. Rear 2/3 of Its specie name of body is fairly narrow; swollen front section, tapered toward head and rear, earned it the "club" of its title. segments have prominent parapodia bearing short, fine setae.
D. cordylina was equipped with two conical jaws in the proboscis. Entire organ could be thrust out to feed. Most often the proboscis was preserved retracted, jaws prominently visible in the gut region of the fossil; earning it the designation Tummy Tooth Worm prior to the formal description. Body plan suggests a predator and a burrower. Ostracodes and plant material have been noted in the gut region of some specimens.
The "Richardson's Guide to the Fossil Fauna of Mazon Creek" has a whole chapter on Polychaete Worms. That chapter (7A) was written by Kirk Fitzhugh, Steven D. Sroka, Samuel Kruty, Michael D. Henderson, and Andrew A. Hay. It has a very nice complete diagram of Polychaete worm parts.
Didontogaster cordylina Thompson, 1979
Description. Medium-sized species with 29 to 68 segments; total length 12-87 mm. Anterior ends lightly inflated, tapering to narrow posterior end (Figures 7A.23A, 7A.24). Prostomium anteriorly rounded, no appendages observed. Proboscis present, inverted in most specimens. Two conical jaws present (Figure 7A.23A, 7A.23B); each jaw base with three shallow pits. Parapodia biramous; each ramus with two aciculae (Figure 7A.23C). Setal fascicles with short (less than 2 mm) and long (2 mm) setae; notopodia of well-preserved specimens with two distinct setal fascicles, one oriented dorsoventral to the other. Interramal and anal cirri not observed.
Remarks. This is the most abundant polychaete species in the Mazon Creek fauna. Thompson (1979) speculated that the three shallow pits located in the base of each jaw served as points of muscle attachment to the proboscis. Based on positions of some specimens relative to bedding planes, Thompson |1979) suggested that D. cordylina was a burrowing omnivore.
