This is Mazon Monday post #118. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Last Friday (Fossil Friday #114), we featured Mazonova helmichnus, which are eggs - possibly amphibian or lungfish eggs. Here's a little more information about them.
Mazonova helmichnus were classified in 1995 by Stephen Godfrey in the paper "Fossilized eggs from the Pennsylvanian of Illinois". The paper appeared in the journal Ichnos - an International Journal for Plant and Animal Traces. For more on Dr. Godfrey, see his website - Paleo Skullptures.
Nine siderite concretions from the Middle Pennsylvanian, Francis Creek Shale (Carbondale Formation, Desmoinesian Series, West‐phalian C‐D), in the Mazon Creek area, Will‐Kankakee Counties, Illinois, U.S.A. preserve clusters of impressions of small eggs. Differential staining of the matrix suggests that the eggs were originally spawn within gelatinous strings containing 1 or 2 rows of eggs. Unfortunately, these egg impressions lack the diagnostic features needed to identify the zoological taxon (taxa) from which they originated.
"Nine siderite concretions"... did we say rare?!?
M. hlmichnus appears in "The Mazon Creek Fossil Fauna" by Jack Wittry, with a few nice examples.
Mazonova helmichnus Godfrey, 1995
Egg cases other than those of sharks from the Paleozoic are extremely rare in the fossil record. However, many unique eggs have now been discovered in Mazon Creek fossils. The exact types of animals that laid these eggs are unknown, but there must have been several. The eggs are likely from both fish and amphibians. They have been found as strings, as clusters, and as masses of individual eggs, from six to fifty in number. No internal structures have been seen in the egg impressions.
Mazonova helmichnus eggs are found as blackened spheres often encased in lighter color material, usually in the form of a string. Individual eggs range in size from 3 to 6 mm.
The "Richardson's Guide to the Fossil Fauna of Mazon Creek" have a nice description by Dr. Godfrey in chapter 21 "Coprolites and Trace Fossils (Biogenic Structures)".
Fossilized Eggs from Pit 11
Mazonova helmichnus
Apart from the egg capsules of chondrichthyans (sharks and their kin; Zidek, 1976), fossilized eggs from the Paleozoic are exceedingly rare. Nine nodules from the spoil heaps within the Pit 11 area are an exception, as they preserve strings of eggs that closely resemble those spawned by some living fish and amphibians. To my knowledge, fossilized eggs of this type have never been found elsewhere. The known specimens are currently held by the Royal Ontario Museum (Toronto, Canada), Mary Pat Smith (Chicago, Illinois), and the Mazon Creek Project (Northeastern Illinois University, Chicago, Illinois). These unique trace fossils have been named Mazonova helmichnus (Godfrey, 1995).
Differential staining of the iron carbonate (Figure 21.8) immediately around the egg impressions indicates that they were originally spawned within a gelatinous sheath containing one or two rows of eggs. The preserved remnants of the sheath are almost always a lighter color than the eggs, ranging from tan to white. What is now interpreted as the sheath remnant does not appear to be a preservational artifact. In ROM 47490, for example (Figure 21.8), some of the egg impressions are seen to zig-zag within the ribbon. Had the decomposing eggs altered the appearance of the surrounding matrix, one would expect the outer margin of the sheath to follow more closely the alignment of the eggs within. Furthermore, in some specimens, short segments of the sheath are devoid of eggs. In the known specimens, the diameter of the sheath ranges from 3.0 to 6.0 mm.
The number of eggs per nodule varies from 6 to 50. The diameter of the eggs ranges from 1.4 to 1.7 mm. Adjacent ova frequently abut; others are separated by 1-2 mm. In most specimens, the ova are about the same color as the iron carbonate matrix, although they can be either darker or lighter. Embryological differentiation within the egg impressions has not been observed.
Discussion. The Pit 11 fauna consist primarily of the marine Essex type, but freshwater Braidwood elements are also found. In the Pit 11 area, a 2-5 km wide zone of mixing occurs between the Braidwood and Essex, particularly at the northern end (Schram, 1979). Baird (1979) has suggested that the Braidwood faunal constituents within this area were washed into the sea during distributary flooding of the neighboring swamps and delta levees. Unfortunately, as a result of this localized faunal mixing, it is uncertain whether M. helmichnus eggs were spawned by a marine or freshwater or ganism. Nevertheless, on the basis of the spawn morphology in living organisms, the fol lowing observations are given: Among the invertebrates, some opisthobranch molluscs extrude eggs in gelatinous ribbons or sheets (Thompson, 1976). However, opisthobranch eggs range in diameter from about 40 to 400 μm, with most about 100 μm in diameter, much smaller than those described here. The diameters of both the eggs and surrounding sheaths lie within the vertebrate range. Among extant vertebrates, at least five species within the Teleostei spawn egg ropes or ribbons (Breder and Rosen, 1966). A greater number of living amphibian species spawn egg strings that resemble those figured here (Duellman and Trueb, 1986). One should note however that amphibians assignable to the extant orders and teleosts do not appear in the fossil record until the Mesozoic. Nevertheless, the Mazon Creek fauna is not without fish or amphibians that could have spawned these eggs.
Specimens
From "The Mazon Creek Fossil Fauna"
From ESCONI member Ralph Jewell
