This is Mazon Monday post #121. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Rhaphidiophorus hystrix is a polychaete worm. It was described by Ida Thompson in 1979 in the paper "Errant polychaetes (Annelida) from the Pennsylvanian Essex fauna of northern Illinois.", which was published in the journal Palaeontographica Abteilung A Palaeozoologie-Stratigraphie.
AbstractThe 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.
Rhaphidiophorus hystrix Thompson, 1979Rhaphidiophorus hystrix is a short (median length 22 mm), blocky polychaete, with a seldomly well-defined head, no jaws, and few body segments. The parapodia are not prominent. Setae bundles on the segments are the most distinctive feature of R. hystrix; they are short at the head end and grow progressively longer until the most posterior bristles are longer than the body.Informally named the Oliver Hardy Worm, R. hystrix is placed in the family Amphinomidae. Extant members of this family prey on soft animals such as sponges. Apparently lacking jaws, this could have been the feeding behavior of their Mazon Creek relatives.
Rhaphidiophorus hystrix Thompson, 1979 Figures 7A.2, 7A.3
Description. Body short, broad; total length 10-35 mm, with 9 to 16 segments (Figures 7A.2A, 7A.3). Anterior end usually poorly preserved; well-preserved specimens with anterior prostomium margin rounded, bilobed, three short antennae present. Anterior parapodial dorsal cirri present in some specimens. Parapodia biramous (Figure 7A.2B). Noto- and neurosetae stout; short in anterior segments, becoming very long in posterior segments; setae hollow, outer surfaces ornamented with fine, longitudinal ridges. Anal cirri not observed.
Remarks. Thompson (1979, pl. 6, fig. 7) described a small, teardrop-shaped depression or elevation in the anterior end of some specimens that she thought might be a caruncle. The caruncle is a sensory lobe behind the prostomium, characteristic in extant amphinomids (e.g., Figures 7A.1C, 7A.2A). In addition to the three "antennae" Thompson (1979, p. 191) also observed in four specimens "[s]ome what longer projections... located below the outline of the prostomium," which she suggested were "peristomial or parapodial cirri." Since extant amphinomids do not have peristomial cirri, it is possible that these structures are palps.
Thompson (1979) hypothesized that R. hystrix was an epifaunal scavenger or possibly pelagic. Extant amphinomids are carnivores (Fauchald and Jumars, 1979) and typically epifaunal.
