This is Mazon Monday post #191. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Velvet worms, also known as Onychophora, are a phylum of terrestrial invertebates. They are soft-bodied, with many short thick legs, and a velvety body. Modern day examples live in tropical environments. They are very rare in the fossil record and appear unchanged since the Cambrian. Some researchers believe they might be the missing link between annelid worms and arthropods.
The one Mazon Creek species of Onychophora was described by Ida Thompson and Douglas S. Jones in 1980 in the paper "A Possible Onychophoran from the Middle Pennsylvanian Mazon Creek Beds of Northern Illinois". It was named Helenodora inopinata after Helen Piecko. It is one of many holotypes donated by Helen and her husband Ted.
H. inopinata was later reclassified as Ilyodes inopinata in 1982 by a group of researchers studying the Carboniferous fossils from Montceau-les-Mines, France. The researchers, which included W. D. Ian Rolfe and Frederick Schram, concluded that Ilyodes was identical to Helenodora. Ilyodes was described by Samuel Scudder in 1890 as a myriapod. Scudder published his description in "New Carboniferous Myriapoda from Illinois".
The "Richardson's Guide to the Fossil Fauna of Mazon Creek", edited by Charles Shabica and Andrew Hay, has a good discussion of the reclassification in Chapter 15C Onychophora.
Mazon Creek
There has been some confusion in the Mazon Creek literature regarding multilegged (serially repeated) wormlike fossils. The affinities of two genera, Ilyodes Scudder (1890) and Helenodora Thompson and Jones (1980), have been reexamined in recent papers. Specimens discussed in the original descriptions were conceded to have been poorly preserved. Specimens of the same or a similar animal discovered at Montceau-les-Mines in France prompted a review of those earlier articles (Rolfe et al., 1982).
Scudder described Ilyodes (Figure 15C.3) as being chilopodiform, composed of a large number of similar segments each bearing a single pair of legs. Jointing of the short legs is uncertain. Each segment is composed of a series of about eight transverse ridges having a beaded appearance; front margins of the segments bear tubercular bosses. Scudder assigned Ilyodes to the Myriapoda, a classification then frequently utilized for multilegged, wormlike, invertebrates.
In the original description of Thompson and Jones, Helenodora inopinata (Figure 15C.4) was described as being a wormlike lobopod fossil with 21 or more pairs of short, tapered legs located ventrolaterally. The legs bear hooklike claws. The body is annulated with about nine annulations per ring. Small papillae are seen on the annulations. Specimens occasionally display a dark brown spot that may be a jaw. Assignment to the phylum Onychophora was considered tentative until details of the head (not preserved in the two originally described specimens) could be examined on fossils yet to be discovered.
Rolfe et al. (1982) in their description of the fossils found at a Carboniferous (Stephanian B) locality in Central France (Mazon Creek is Westphalian D) stated, "The Mazon Creek onychophoran Ilyodes Scudder, 1890 (=Helenodora Thompson and Jones, 1980) is matched by a virtually identical onychophoran at Montceau." Scudder, unwittingly, had described the first fossil onychophoran; it took the discovery of other fossil onychophorans and 92 years to establish the fact. The name Ilyodes will have to replace Helenodora, according to the rules of zoological nomenclature, if the more recent studies correct. prove
Extant onychophorans are terrestrial. Cambrian species are presumed to be marine, but "the habitat of Ilyodes is uncertain" (Robison, 1985). The type specimens of Ilyodes were in all probability found in the natural exposures along Mazon Creek, in whose bed and banks the majority of concretions were found during the last century and the early part of this century (Richardson, 1980). Baird et al. (1986) noted that both Braidwood (nonmarine) and Essex (estuarine) animals are found in the concretions from Mazon Creek. The type and paratype specimens of Ilyodes (Helenodora Thompson and Jones, 1980) were found at Pit 11, considered to have been an estuarine environment, although fossil terrestrial plants and animals are also found here.
The ongoing discussions of the affinities and systematics of the diverse and sometimes oddly constructed Cambrian onychophoran-like animals is in sharp contrast to the assessment of Ilyodes. A marked similarity between the structural plan of Ilyodes and that of the extant genus Peripatoides (Figure 15C.5) suggests that the Onychophora had become a conservative group by the Pennsylvanian; their living descendants being little changed structurally from Paleozoic ancestors.
Ilyodes inopinata appears on page 33 of "The Mazon Creek Fossil Fauna" by Jack Wittry.
The Mazon Creek Biota has a variety of animals whose modern descendents, sometimes referred to as living fossils, are little changed from their Carboniferous ancestors. Among them is the onychophoran, a seeming combination of worm and millipede. Except for its greater size, Peripatoides, a present-day denizen of the Central American jungle, is a virtual duplicate of its Mazon Creek relative, Ilyodes inopinata.
Ilyodes was described by Samuel Scudder in 1890 from a specimen found in a concretion from the bed of the Mazon River. Thompson and Jones (1980) described an onychophoran found in Pit 11, naming it Helenodora inopinata. The finding of the same or similar species in a French Upper Carboniferous location led to a restudy of these animals, and Ilyodes took precedence over Helenodora. The soft, worm-like body is covered by tiny papillae, bristles, and scales. This body covering has earned the extant animal the informal name of Velvet Worm. Locomotion is achieved via limbs that appear more like protuberances than legs. The head bears fleshy antennae with eyes at their bases. Present-day onychophorans (literally meaning claw bearers) are equipped with strong jaws that can bite through the body armor of their prey. They feed on small animals, such as insects, which they catch by squirting an adhesive slime.
Onychophorans are a group that shares features of both arthropods and annelids. They have been referred to as a missing link between them. Like annelids, they have unjointed appendages and a soft, noncalcified body wall without any articulating plates. On the other hand, they shed their body wall as they grow, and share many internal features with arthropods. Most zoologists currently agree they are more closely related to arthropods.
The holotype as it appears in "The Mazon Creek Fossil Fauna". Collected by and donated to the Field Museum by Helen Piecko.
From Dave's Down to Earth Rock Shop in Evanston, IL. Collected by Ralph Jewell
