This is Mazon Monday post #44. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Monurans are an extinct order of wingless insects. They are known from the Mazon Creek biota, other Carboniferous localities, and have been found in later deposits that date to the Permian. They get their name for the single tail spike. Their closest living relative today is the Silverfish. In fact, some recent research suggests they may even be silverfish, instead of a distinct order as they are today. The species Dasyleptus is somewhat rare in the Mazon Creek biota. It was described in 1957 by Sharov. It's found in the more terrestrial localities, but might also be present in concretions from Pit 11.
Monurans (and silverfish) were mentioned in Creature Corner accompanied by a couple of drawings by Don Auler.
Monura and Silverfish
Among restaurateurs and homemakers, the sight of a silverfish scurrying over the plumbing engenders panic and revulsion. Yet this very ancient insect, a Thysanuran, and its sister group, the Monura (extinct), are prize finds for the paleontologist. Both creatures were members of the Mazon Creek biota.
The taxonomy of animals that are descended from an ancient arthropod-like creature can get quite "sticky". Authors have developed various phylogenetic charts. It is sufficient for our purpose to state that the wingless (Apterygote) and the winged (Pterygote) insects evolved from a common myriapod-like ancestor. Discoveries within the last 15 years have tentatively pushed the antiquity of the Hexapoda (Insecta) back to the Silurian. Myriapod-like animals, somewhat replaced by fluorapatite, were discovered in a dolomite quarry near Milwaukee, Wisconsin in 1985.
One scenario for Insect development goes in order to escape predators the ancestors of insects used their clawed "legs" to climb the primitive low-growing plants near shore, acclimating themselves to a terrestrial existence. Myriapods (millipedes) and Hexapods (insects) split off from the parent group becoming inhabitants of swampy, then moist soil. Differentiation continued until the Monurans split off from the group that would produce Thysanura and winged insects. All three groups are found in the concretions of the Mazon Creek area.
Monuran Silverfish (primitive insect) Collection of Kathy Dedina
Reconstruction 1987 by Kukalova-Peck J.
Dasyleptus was mentioned in "The Mazon Creek Fossil Fauna" by Jack Wittry.
Monurans are an extinct primitive group of wingless, swamp- dwelling insects. Paleoentomologists consider them to be relatively plentiful in the fossil record. In fact, they are the most common insects in the Mazon Creek Fauna.
Monurans have three pairs of stout, clawed, walking legs that are found on the thorax, with a pair of short bristly legs at each abdominal segment. A single tail-like process, as long as the bristled body, extends from the end of the abdomen, hence the mon (Greek meaning one) in the name.
There still needs to be a considerable amount of taxonomic Figure 58.1, monuran Dasyleptus modified after Shabica and Hay (1997). Scale bar = 10 mm. work done on this Mazon Creek form. No species name has ever been given, though an attempt was started but not completed by Dr. Frank Carpenter before his death. It has been suggested.and may turn out to be, that the Mazon Creek form should be removed from 'Dasyleptus' and placed into a different genus altogether.
To make matters even more confusing. the whole suborder of Monura has recently been reinterpreted. It is now believed by some authors that the members of 'Dasyleptus' represent juvenile silverfish, albeit of relatively large size. If this is so, it would remove them from their present position among the order consisting of bristletails and place them into the order composed of silverfish.
Preservation of the Apterygote insects, Monura and Thysanura, in concretions is usually poor. Distinguishing the two groups is not too difficult. Body segments of the Monurans are fairly uniform in size with no distinct thorax. Abdominal segments bear a pair of jointed, clawed "legs". Silverfish body has distinct thoracic segments and abdominal segments bear narrow, jointed bristle-covered appendages, "legs". Both groups have three pair of sturdy, jointed, clawed, walking legs on the front segments. Long antennae adorn the head. Conspicuous jointed bristles on the last segment of the Monura and several long bristles on the last segment of silverfish served as feelers and predator warning system. Thysanura ("tassel-tail"), silverfish, had found its niche early in the Paleozoic. It exists today virtually unchanged.
No post would be complete without a few pictures of real concretions from ESCONI members. The first specimen belongs to Andrew Young and is from the Mazon River. The second hails from Pit 2 and belongs to Rich Holm.
