This is Mazon Monday post #150. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Fossundecima konecniorum was a medium sized polychaete worm that made up part of the fauna of Mazon Creek. It had two triangular jaws in its proboscis, which is probably evidence of a predatory lifestyle. It had short setae that extended from its body segments. It is the second most common "worm" found in the Mazon Creek biota, most commonly in Pit 11. Its informal name is the "Simple Jaw Worm".
F. konecniorum was named for Jim and Sylvia Konecny, who were both prolific fossil collectors and very active ESCONI during from the 1960's until the 1990's. Both wrote paleontology articles for ESCONI's newsletter and various other publications until well into the 2000's. F. konecniorum was described by Ida Thompson in 1979 in her famous paper "Errant polychaetes (Annelida) from the Pennsylvanian Essex fauna of northern Illinois. Palaeontographica Abteilung A Palaeozoologie-Stratigraphie". That same paper established many of the Mazon Creek worm species.
F. konecniorum appears on page 28 in Jack Wittry's "The Mazon Creek Fossil Fauna".
Fossundecima konecniorum is the second most common worm in the Essex Fauna. It is a broad, round-ended, medium-sized (median length 24 mm) polychaete. The head contains two large, almost triangular jaws mounted in a proboscis that could be thrust or rolled inside-out (everted) to feed. The number of body segments increase with the length of the animal. Each segment bears short, sturdy setae and long cirri on the parapodium. Its oval outline, jaw shape, and length of its setae and cirri are its identifying features.
F. konecniorum was called Simple Jaw Worm prior to its formal description. It is considered to have been a free-swimming or grazing predator. Some specimens are known to exhibit ostracodes preserved on the gut line.
The "Richardson's Guide to the Fossil Fauna of Mazon Creek" discusses F. konecniorum in Chapter 7A "Polychaete Worms".
Fossundecima konecniorum Thompson, 1979 Figures 7A.15, 7A.16
Description. Medium-sized, broad-bodied species, total length 9-46 mm, with 12 to 31 segments. Prostomium with two or three short antennae; peristomium with two pairs of tentacular cirri. Proboscis eversible but usually inverted (Figures 7A.15A, 7A.16). One pair of large, triangular jaws present, each with about 10 blunt teeth along inner margin (Figures 7A.15B, 7A.16). Parapodia biramous (Figure 7A.15C); long cirri (presumed dorsal) on first few segments, becoming shorter on median and posterior segments. Noto- and neurosetae stout, forming short, fanlike bundles. Pygidium with two long anal cirri.
Remarks. In the original description, Thompson (1979) tentatively placed F. konecniorum in its own family, the Fossundecimidae, though she did suggest that the species might be a member of either the Aphroditidae subfamily Hystriciolinae (discussed above) or the Nereididae. Membership in the Hystriciolinae was discounted since there were two rather than four jaws, as well as differences in jaw shape. Thompson (1979, p. 190) then argued that while F. konecniorum might be closely related to the Nereididae, the jaws of this species "are considerably broader and flatter." Based on Thompson's illustrations and our own observations, we find no outstanding differences between the jaws of F. konecniorum and those of extant nereidids. As well, the general arrange- ment of what appear to be tentacular cirri and antennae strongly suggests that this species is a member of the Nereididae. The most striking difference between most extant nereidids and F konecniorum is the latter species' short, very robust body and small number of segments.
Assuming F. konecniorum was a nereidid, we predict that at least the neurosetae were compound as opposed to simple as described by Thompson (1979). As we noted in the remarks for Dryptoscolex matthiesae, the inability to discern articulations in setae might be a taphonomic artifact.
Thompson (1979) hypothesized that F. konecniorum was an epifaunal predator. While most extant nereidids are thought to be carnivorous (Fauchald and Jumars, 1979), it is not at all uncommon to find them as members of the infauna.
Don Auler did some very nice drawings of Fossundecima konecniorum in the book "ESCONI Keys to Mazon Creek Animals".
Specimens
From the Richardson's Guide
From "The Mazon Creek Fossil Fauna"
