This is Mazon Monday post #146. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Astreptoscolex anasillosus was an annelid (Greek, annulatus "annulated" or "ringed) worm. It was described by Ida Thompson in 1979 in her paper "Errant polychaetes (Annelida) from the Pennsylvanian Essex fauna of northern Illinois" in Palaeontographica Abteilung A. Thompson described quite a few of the Mazon Creek polychaete worms from 1977 - 1979.
ABSTRACT
The Essex Fauna is the marine phase of the Maxon Creek beds; soft-bodied animals, including the polychaetes, are preserved as whole-body fossils in siderite concretions within the Francis Creek Shale. Descriptions of the polychaetes include details of the jaws, setae, prostomia, cuticles, and cirri. Nine of the species are placed in the order Phyllodocida, extant families Aphroditidae, Hesionidae, Phyllodocidae, Nephtyidae, and Glyceridae, and the new family Fossundecimidae. One species is placed in the order Amphinomida, extant family Amphinomidae. The Essex polychaetes comprise the earliest (and only Paleozoic) record for all the families. For the families Hesionidae, Nephtyidae, and Fossundecimidae this is the only fossil record. As a group, the polychaetes were large, active, predaceous, and epifaunal. This contrasts with Recent polychaete faunas on soft bottoms, where the dominant polychaete type is a sedentary deposit feeder. Possible reasons for this difference are discussed, none proving entirely satisfactory.
The following animals were all described in that paper. A. anasillosus belongs to the same phylogenetic family, Nephtyidae, as Didontogaster cordylina.
Astreptoscolex anasillosus
Didontogaster cordylina
Dryptoscolex matthiesae
Fastuoscolex gemmatus
Fossundecima konecniorum
Hystriciola delicatula
Levisettius campylonectus
Pieckonia helenae
Rhaphidiophorus hystrix
Rutellifrons wolfforum
Astreptoscolex anasillosus appears in both the ESCONI's "Keys to Identify Pennsylvanian Fossil Animals of the Mazon Creek Area" and Jack Wittry's "The Fossil Fauna of Mazon Creek". The magnificent artwork in the "Keys" book was done by Don Auler.
A. anasillosus appears on Page 24 in Wittry's "The Fossil Fauna of Mazon Creek".
This worm is a stout, medium-sized (median length 36 mm) polychaete, with a rounded head and body tapering toward the tail. Its name (from the Greek astrepto meaning inflexible) aptly describes the stiff appearance. An eversible proboscis carries conical jaws, which are totally different from those of any other family except its living relatives of Nephtyidae. The parapodia contain short, dense bristles. Gills are sometimes visible among the bristles. Astreptoscolex anasillosus appears to have had a tough hide of cuticle. Concretions containing molds of this fossil often reveal very fine detail. The body structure of A. anasillosus suggests a creeping, predatory lifestyle. Prior to receiving its formal designation, it was known as the "Plain Worm".
The "Richardson's Guide to the Fossil Fauna of Mazon Creek" mentions A. anasillosus in Chapter 7A "Polychaete Worms". That chapter was written by Kirk Fitzhugh, Steven D. Sroka, Samuel Kruty, Michael D. Henderson, and Andrew A. Hay.
Family Nephtyidae Grube, 1850
Body long, slender. Two pairs of small antennae. Peristomium fused to anterior segments. Eversible proboscis with one pair of small, sub- terminal jaws. Parapodia biramous, well developed; aciculae present; noto- and neurosetae simple; interramal respiratory cirri usually present.
Astreptoscolex anasillosus Thompson, 1979 Figures 7A.21, 7A.22
Description. Body medium sized, with 29 to 42 segments; total length 16-55 mm (Figures 7A.21A, 7A.22). Anterior end blunt, rounded; posterior end slightly tapered. Prostomium small, poorly preserved; no appendages observed. Proboscis sometimes everted, two small conical jaws present at about midlength (Figure 7A.21A, 7A.21B); in specimens with proboscis inverted, jaws lie between segments 8 and 13. Bases of jaws deeply concave (Figure 7A.21B).
Parapodia biramous, with lamellae in anterior segments (Figure 7A.21C); notosetae very slender, at least 30 per fascicle, shorter than neurosetae; neurosetae 2-4 mm long. Noto- and neuropodial aciculae present, though not always preserved. Interramal cirri sometimes present (Figure 7A.21C).
Remarks. The presence of a pair of small, conical jaws medial on the proboscis, and interramal cirri are characteristics of extant nephtyids. As well, nephtyids have two pairs of very small antennae along the anterior margin of the quadrangular-shaped prostomium. These structures probably have not been preserved well enough to be seen in A. anasillosus.
Although extant nephtyids are usually considered motile, epi- or infaunal predators, feeding on small invertebrates, some species are deposit feeders (Fauchald and Jumars, 1979). Thompson (1979) believed A. anasillosus to have been an epifaunal carnivore or omnivore since no specimens observed contained sediment in their guts.
Specimens
From Wittry's "The Fossil Fauna of Mazon Creek".
