This is Mazon Monday post #281.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

There’s quite a bit of Mazon Creek fossil research happening.  Last week, we posted a paper that redescribed Palaeocampa (see Mazon Monday #280), some of our friends at the Field Museum had a paper about Sphenophyllales in June (see Mazon Mionday #277), Cal So did a presentation about amphibamiforms in June (see Mazon Monday #276), and Arjan Mann has more than a few things cooking… This week, we look at a new paper that reexamimes groundbreaking work on the environmental setting of the Mazon Creek area by Gordon Baird, Charles Shabica, Steven Sroka, Eugene Richardson, Jr., and others in the 1980s.

The paper’s references lists many of the important recent and older publications about Mazon Creek.  Some of the most relevant are Gordon Baird’s work.  If you’re looking for some interesting Mazon Creek reading, dig in…

• Baird, G. C. 1979. Lithology and fossil distribution, Francis Creek Shale in northeastern Illinois. Pp. 41–67 in Nitecki, M. H., ed. Mazon Creek fossils. Academic Press, New York.10.1016/B978-0-12-519650-5.50010-2 CrossRef Google Scholar
• Baird, G. C. 1997. Geologic setting of the Mazon Creek area fossil deposit. Pp. 16–20 in Shabica, C. W. and Hay, A. A., eds. Richardson’s guide to the fossil fauna of the Mazon Creek. Northeastern Illinois University, Chicago.Google Scholar
• Baird, G. C., Shabica, C. W., Anderson, J. L., and Richardson, E. S.. 1985a. Biota of a Pennsylvanian muddy coast: habitats within the Mazonian Delta Complex, Northeast Illinois. Journal of Paleontology 59:253–281.Google Scholar
• Baird, G. C., Sroka, S. D., Shabica, C. W., and Beard, T. L.. 1985b. Mazon Creek-type fossil assemblages in the US midcontinent Pennsylvanian: their recurrent character and palaeoenvironmental significance. Philosophical Transactions of the Royal Society B 311:87–99.Google Scholar
• Baird, G. C., Sroka, S. D., Shabica, C. W., and Kuecher, G. J.. 1986. Taphonomy of Middle Pennsylvanian Mazon Creek area fossil localities, northeast Illinois: significance of exceptional fossil preservation in syngenetic concretions. Palaios 1:271–285.10.2307/3514690 CrossRef Google ScholarBaird, G. C., Sroka, S. D., Shabica, C. W., and Kuecher, G. J.. 1986. Taphonomy of Middle Pennsylvanian Mazon Creek area fossil localities, northeast Illinois: significance of exceptional fossil preservation in syngenetic concretions. Palaios 1:271–285.10.2307/3514690 CrossRef Google Scholar

The new paper is published with open access in the journal Paleobiology and can be found on the Cambridge University Press website.  A nice summary of the paper can be found on Phys.org.

283,821 concretions, how do you measure the Mazon Creek? Assessing the paleoenvironmental and taphonomic nature of the Braidwood and Essex assemblages

by James Schiffbauer, Gordon C. Baird, John Warren Huntley, Tara Selly, Charles W. Shabica, Marc Laflamme and A. Drew Muscente

Abstract: The Mazon Creek Lagerstätte (Moscovian Stage, late Carboniferous Period; Illinois, USA) captures a diverse view of ecosystems in delta-influenced coastal settings through exceptional preservation of soft tissues in siderite concretions. The generally accepted paradigm of the Mazon Creek biota has been that of an inferred paleoenvironmental divide between what have been termed the Braidwood and Essex assemblages, wherein the former represents a freshwater ecosystem with terrestrial input and the latter a marine-influenced prodelta setting with abundant cnidarians, bivalves, worm phyla, and diverse arthropods. Here, we revisit the paleoecology of the Mazon Creek biota by analyzing data from nearly 300,000 concretions from more than 270 locations with complementary multivariate ordinations. Our results show the Braidwood assemblage as a legitimate shoreward community and provide evidence for further subdivision of the Essex assemblage into two distinct subassemblages, termed here the Will-Essex and Kankakee-Essex. The Will-Essex represents a benthos dominated by clams and trace fossils along the transition between nearshore and offshore deposits. The Kankakee-Essex is dominated by cnidarians, presenting an ecosystem approaching the geographic margin of this taphonomic window. These new insights also allow a refined taphonomic model, wherein recalcitrant tissues of Braidwood organisms were subject to rapid burial rates, while organisms of the Essex assemblage typically had more labile tissues and were subject to slower burial rates. Consequently, we hypothesize that the Braidwood fossils should record more complete preservation than the Essex, which was exposed for longer periods of aerobic decomposition. This is supported by a higher proportion of non-fossiliferous concretions in the Essex than in the Braidwood.

Non-technical Summary: Recognized since the nineteenth century, the Mazon Creek (pronounced “muh-ZAHN”) fossil sites in northeastern Illinois have become some of the most well-known and well-collected exceptional fossil deposits in North America. Their popularity with avocational fossil hounds and research paleontologists alike grew substantially after the 1940s, when strip-mining practices began uncovering the Colchester Coal, leaving abundant fossil-bearing siderite concretions in their spoil piles. And they became even more popular with discoveries of enigmatic icons of evolution like the Tully Monster (Tullimonstrum gregarium Richardson, Reference Richardson1966). The work by Gordon Baird and colleagues in the 1980s provided an extensive understanding of these sites, their preservation, and their paleoecology—which they suggested to represent deposition along a muddy Carboniferous-aged coastline, capturing a distinct delta-to-offshore transition in biotic assemblage composition. Sites containing the more terrestrially influenced Braidwood assemblage hosted abundant plants, washed-out terrestrial arthropods, and freshwater organisms, while the Essex assemblage was characterized by marine bivalves, abundant Essexella cnidarians, and other unequivocally marine taxa. While the distinctness of these two assemblages was recently challenged, we provide here an assessment of nearly 300,000 concretions and illustrate that there are instead three discrete biotic assemblages—the nearshore Braidwood assemblage and two marine Essex assemblages represented by more abundant benthic taxa–like worms and bivalves versus cnidarian-dominated localities—an idea that Baird and colleagues had postulated in an early paleoecological summary four decades ago.

Conclusions: In resurrecting the raw concretion data and associated metadata from the publications of the mid-1980s by Baird and colleagues (Reference Baird, Shabica, Anderson and Richardson1985a,Reference Baird, Sroka, Shabica and Beardb, Reference Baird, Sroka, Shabica and Kuecher1986), we have been able to statistically corroborate the appropriateness of two regional distinctions in Mazon Creek fossil assemblages, the nearshore Braidwood and marine Essex assemblages. The Braidwood assemblage is characterized by terrestrial fauna and flora, which exist as allochthonous components washed in to an estuarine coastal system, but there is a real, underlying, but low-diversity and significantly understudied aquatic assemblage that signifies deposition in a fresh-to-brackish setting. Moreover, our investigation supports the further division of the Essex assemblage into two subassemblages: (1) the benthic Will-Essex, dominated by clams and traces and likely representing a transitional assemblage between nearshore and offshore deposits; and (2) the Kankakee-Essex, instead largely consisting of cnidarians, and representing an undoubtedly marine ecosystem.

The differences in paleoenvironment between the Braidwood and Essex localities should impart some taphonomic distinctions as well, largely related to depositional geochemistry, burial rate and decay exposure, and tissue histology. The organisms captured in the Braidwood assemblage are composed of more resistant tissues on the whole, although their burial should be the most rapid as well. Both of these factors should help ensure that the Braidwood fossils are exceptionally well preserved. Moving seaward, however, a larger constituent of the Essex assemblages becomes composed of more labile, easily decayed tissues, and the burial rate is presumably slower, leaving more time for aerobic decomposition. Simultaneously, the Essex localities show a higher proportion of dud concretions, which is the apparent record of excessive decay beyond the point of taxonomic resolvability. This paleoecologically refined taphonomic model thus helps to holistically explain the distribution of organisms, concretions, and fossil fidelity among the Mazon Creek localities.