This is Mazon Monday post #333.  What’s your favorite Mazon Creek fossil?  Tell us at esconi.info@gmail.com.

---

Way back in Mazon Monday #10, we referenced the paper “Mazon Creek-Type Fossil Assemblages in the U.S. Midcontinent Pennsylvanian” written by G. C. Baird, S. D. Sroka, C. W. Shabica, T. L. Beard, A. C. Scott and F. M. Broadhurst in October 1985. The paper lists and discusses the various Pennsylvanian fossil sites known to produce fossil bearing concretions similar to what is found at Mazon Creek. Although the list of concretionary sites has grown since the paper was published, it’s still relevant and a good read. Check it out, you can download it from ResearchGate.

Abstract

Terrestrial plants, the non-marine Braidwood Fauna, and the euryhaline Essex Fauna, best known from the Francis Creek Shale Member within the Mazon Creek area of northeastern Illinois, are found to recur in analogous deltaic lithofacies elsewhere both in Illinois and in adjacent states. An important new Essex-type fossil locality is reported from deposits coeval with the Francis Creek Member in Missouri, and occurrences of Braidwood animals from additional stratigraphic units are discussed. These assemblages occur in estuarine-deltaic deposits juxtaposed on coals; the significance of the association of these fossils with transgressive inundation of coastal peat swamps is discussed and a predictive model for the occurrence of Mazon Creek-type assemblages is presented. We believe that fossil associations comparable to those at Mazon Creek, occur in certain coastal deposits ranging in age from Pennsylvanian to, at least, the Triassic.

This paper does mention Okmulgee in section 6b, “Recurrent Mazon Creek -type biotas : U.S. midcontinent region”.

(b) Braidwood-type fauna, Henryetta and Morris, Oklahoma

Sideritic concretions containing Braidwood-type animals and plants occur in the lower part of an unnamed mudstone-siltstone division of the Senora Formation above the Croweburg Coal in Okmulgee County in eastern Oklahoma (figure 1). As with the Windsor-area deposit, this unit is both temporally equivalent to the Francis Creek in Illinois (W right 1965) and analogous to it as a lobate deltaic unit containing distributary and bay-fill facies

Mine spoils both at Henryetta and nearby at Morris yield a fauna characteristic of the Braidwood association though of lower diversity and with different proportions of common taxa. Septimyalina, the most common animal, is the only taxon to suggest the presence of minimally saline water; it occurs along the boundary between Braidwood and Essex areas in the Mazon Creek area. These occur with small bivalves resembling non-marine forms. Branchiopods sometimes occur in clusters within concretions; both these and bivalves commonly occur broken and jumbled as hash in coprolites indicating predation. One of the most common elements is Palaeoxyris, the problematic structure most recently interpreted as a chondrichthyian egg case (McGhee 1984). Although not collected by the authors, rare forms including insects and arachnids were obtained from Henryetta mine dumps by the late Dr E. N. Kjellesvig-Waering.

For a more complete view of this site, have a look at “The Okmulgee, Oklahoma fossil flora, a Mazon Creek equivalent: Spatial conservatism in the composition of Middle Pennsylvanian wetland vegetation over 1100 km” by Lillien C. Moore, Jack Wittry, and William A. DiMichele. This paper was published in 2013 in the journal Review of Palaeobotany and Palynology.

Abstract

Temporal compositional conservatism of late Middle Pennsylvanian wetland vegetation and persistence of its dominance–diversity structure through time spans of millions of years have been documented from many places in North America and Europe. This conservatism occurs within the context of glacial–interglacial fluctuations that force spatial migration of that vegetation or its restriction to refugia during periodic sea-level changes on the craton. Although known from long temporal sequences, there have been few studies of spatial variability in these wetland floras over more than relatively small distances, generally much less than a km. Here we report a late Middle Pennsylvanian (Desmoinesian) flora from above the Henryetta Coal, near Okmulgee, Oklahoma, in the American Midcontinent. This flora is exactly correlative with the Mazon Creek flora, above the Colchester Coal of northern Illinois; the Colchester Coal and its equivalents may have been the largest, contiguous peat accumulating swamp of the Pennsylvanian. Okmulgee and Mazon Creek are separated by 1100 km, but the depositional context of the two floras is the same and they can be considered isotaphonomic. Though a much smaller sample, and thus of lower overall biodiversity, the known composition of and dominance–diversity structure of the Okmulgee flora is convergent with Mazon Creek. The overwhelmingly dominant elements of both the Okmulgee and Mazon Creek floras are pecopterid tree-fern foliage mostly attributable to Lobatopteris vestita
(Lesquereux) Wagner and the pteridosperm Macroneuropteris scheuchzeri (Hoffman) Cleal, Shute and Zodrow, Both of these floras likely represent wetlands that were being drowned during the early phases of sea-level rise associated with ice melting. Each sampled a broad area, and a variety of subhabitats that fringed the Pennsylvanian coastline.

This paper compares the exceptionally preserved fossil flora from Okmulgee with the flora of Mazon Creek. The authors concluded that these two floras, separated by approximately 1100 km (680 miles), were remarkably similar in composition. This suggests that the wetlands of the North American coastal plain of the Middle Pennsylvanian Period (some 307-309 million years ago) were extraordinarily uniform, consisting of nearly identical wetland vegetation, with the less diverse Okmulgee flora being essentially a subset of Mazon Creek.

The Oklahoma fossils occur in ironstone nodules formed in shale above the Henryetta Coal, which is similar to Mazon Creek, where nodules formed above the Colchester Coal in the Francis Creek Shale. Furthermore, the Henryetta Coal is stratigraphically equivalent to Colchester Coal.

So, why was diversity lower at Okmulgee? The study used a collection of about 1600 nodules from Okmulgee, which were collected systematically with little collecting bias. The small size of the collection would naturally contain fewer rare species while representing a sample of the dominant flora. The authors concluded the lower diversity was due to less extensive collecting compared to Mazon Creek, which has had intensive attention for more than 150 years.

The study identifies 22 whole plant species, with the flora dominated by:

• the seed fern Macroneuropteris scheuchzeri
• the tree fern Lobatopteris vestita
• other pecopterid tree ferns
• lycopsids
• calamiteans (horsetails)
• cordaitaleans

From this, the authors concluded that the Okmulgee flora is a compositional subset of the Mazon Creek flora rather than a distinct plant community.

Additionally, rather than representing classic river-delta deposits, the authors argue that both fossil assemblages formed during the early stages of marine transgression as sea level rose following glacial melting. Wetland forests bordering the coast were drowned and rapidly buried beneath fine muds, preserving their plants in siderite concretions.

This paper implies a few things for the Mazon Creek fossil flora. First, it was not unique, but it is a relatively complete representation of the widespread Pennsylvanian coastal wetland. Secondly, the Colchester Coal and its equivalents may have formed one of the largest contiguous peat-forming swamps of the Pennsylvanian. And lastly, these coastal “coal swamps” were likely inundated during periods of sea level rise. The fossils from both Mazon Creek and Okmulgee probably consist of a mixture of classic “coal swamp” vegetation and surrounding wetland communities on higher ground.

This paper paints helps us visualize how Mazon Creek fits into the larger picture of the Pennsylvanian Period. Mazon Creek can serve as a benchmark against other Middle Pennsylvanian floras as it represents one of the best windows into an ecosystem that once stretched across much of the ancient equatorial continent.

Fossil Photos