This is Mazon Monday post #68. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Dryptoscolex matthiesae is an annelid worm, which are part of class Polychaeta. It is known informally as the "Rat Worm". Ida Thompson described D. matthiesae in 1979 in the paper "Errant polychaetes (Annelida) from the Pennsylvanian Essex Fauna of Northern Illinois", which was published in the journal Palaeontographica. That paper included descriptions of multiple Mazon Creek Polychaetes taxa, including Fossundecima konecniorum, Didontogaster cordylina, Rutellifrons wolfforum and Pieckonia helenae. Additionally, Ida Thompson was the main author of "The Audubon Society Field Guide: North American Fossils".
The holotype of D. matthiesae is part of the Field Museum's Mazon Creek collection. That specimen and its labels are displayed on the Field Museum website. D. matthiesae was named for amateur Mazon Creek collectors Catherine and Harry Matthies.
DRYPTOSCOLEX MATTHIESAEDryptoscolex matthiesae is a long (median length 60 mm), narrow polychaete with a squarish head and eversible proboscis that contains four jaws, usually seen superimposed in the retracted posi tion. These four large, triangular, almost pendulous-appearing jaws are distinctive. In specimens found with the proboscis everted, the jaws are located at the very tip of that organ. The parapodia bear thick setae bundles and short cirri. This worm's long narrow body, setae, and distinctive jaws are the identifying features of the Rat Worm, the informal name for D. matthiesae.
Dryptoscolex matthiesae Thompson, 1979Description. Elongate, large-bodied species (Figure 7A.14), total length 2-10 cm, 30 to 52 segments. Prostomium large, rounded, with two antennae (Figure 7A.13A). Proboscis eversible; four stout, triangular-shaped jaws present (usually only two visible; Figure 7A.13B), each with six to eight teeth along inner margin and hook near distal outer margin, proximal margin with two equal-sized lobes. In specimens with proboscis inverted (typical condition), jaws 2-4 mm from anterior end. Parapodia biramous (Figure 7A.13C); noto- and neurosetae long, fine, in dense bundles; notosetae simple, presumed compound neurosetae observed in several specimens. Aciculae rarely preserved. Some specimens with parapodial (presumed dorsal) and two anal cirri.Remarks. Our placement of D. matthiesae in the Sigalionidae is tentative, owing to the questionable presence of compound neurosetae observed in two specimens by Thompson (1979). None of the specimens we have examined provide evidence that neurosetae were compound. The most substantial hindrance is the coarse-grained nature of the rock matrix, which easily obscures any potential sign of an articulation.The only other aphroditiform family with compound neurosetae is the Pholoidae Kinberg. Extant members of this family and sigalionids tend, however, to have distinct differences in body dimensions. Pholoidids are short and ovate, whereas sigalionids are usually elongate and slender. The body shape of D. matthiesae accords well with the general dimensions of sigalionids. Thompson's (1979) interpretation of the prostomium as bearing a pair of antennae should also be considered tentative; the palps are also located in this area and are usually considerably larger than antennae.Thompson (1979) suggested that D. matthie sae might have been an epifaunal predator.
