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IGS - The Winneshiek Lagerstätte

The Winneshiek Lagerstätte

by Huaibao P. Liu, Robert M. McKay, Brian J. Witzke


Recently, a new stratigraphic unit was recognized in northeast Iowa near Decorah (Fig. 1). This unit comprises greenish-brown to dark-gray laminated sandy shale, with significant organic carbon and pyrite content (Figs. 2 & 3).

 Map showing location of fossil site in US and Iowa.

Photo of shale outcropping.

Close-up photo of polished shale sample.

Stratigraphically this unit occurs between the Tonti and Readstown members of the St. Peter Formation, a widespread quartz-rich sandstone formation in the Midwest. Well records and core samples show that the new shale unit ranges in thickness from zero to 35 meters in the study area (Figs. 4 & 5), and a Middle Ordovician age is suggested (Fig. 6).

Photo of core drilling rig.

Photo of boxed shale core samples.

Diagram of stratigraphic position and lithology.

 

From the shale unit, an unusual fossil fauna has been discovered. It comprises both vertebrates and invertebrates, including conodonts, jawless fish, arthropods, brachiopods, mollusks, and a number of enigmatic fossils (Fig. 7).

Bar graph of specimen types and number.

Most fossils from the fauna are well-preserved, some associated with soft tissues and body impressions, or even in 3-dimentional preservation, indicating that the unnamed shale unit is a Konservat-Lagerstätte. Fossil Lagerstätte (plural Lagerstätten) is any deposit containing abundant and/or extraordinarily preserved fossils, some of which preserve animal soft bodies or tissue impressions. Fossils from Konservat-Lagerstätten are important because they provide more details than normal fossils concerning animal taxonomy, anatomy, their living environments, as well as taphonomic information. However, fossil Lagerstätten are very rare because this preservation requires special physical and chemical conditions. Figure 8 shows well-known Lagerstätten found in the Early Paleozoic strata. As the only known Middle Ordovician Lagerstätte with diverse biota, the Lagerstätten discovered in northeast Iowa has been named the Winneshiek Lagerstätte, name taken from the county where the fossil site is located.

Diagram of well-known Lagerstätten showing age and location.

The Winneshiek fauna is dominated by conodonts which are preserved as either individual elements or partial to complete apparatuses. Conodonts from this fauna display high variations in color and levels of dissolution (Fig. 9).

Close-up photos Archeognathus sp.

At least 5 different kinds of natural conodont assemblages have been recovered, including the first known apparatus of Coleodontidae, indicating that Archeognathus and Coleodus are synonymous (Fig. 10).

Photo of conodont apparatus of coleodontids.

Conodont assemblages associated with high-carbon soft tissue remains also occur in this fauna (Fig. 11). Although more definitive evidence is still required, these specimens may represent parts of early conodont animals.

Photo of conodont elements associated with impressions of soft-bodied material.

Jawless fish are another important vertebrate group found in the Winneshiek fauna. Most fish fossils are articulated head shields of Astraspis sp. (Fig. 12).

Photo of partial head shield of Astrapsis sp..

As in most Paleozoic Konservat-Lagerstätten, a diversity of arthropods forms another important characteristic of the Winneshiek fauna. Among them, chelicerates and crustaceans are the most common groups. Chelicerates are mostly eurypterids (Fig. 13), and the fauna contains some of the oldest known taxa. Possible xiphosuran and aglaspidid-like fossils also occur in the collection. 

Photo of eurypterid leg specimen.

Phyllocarid crustaceans are abundant in certain layers of the shale, most of them are caryocaridids (Fig. 14). In addition to Caryocaris sp, other taxa are noted. Although rare, ostracods also appear in the fauna. Other invertebrate fossils occurred in the Winneshiek Lagerstätte include linguloid brachiopods and mollusks.

Photo of phyllocarid crustacean.

Many indeterminate forms and problematic fossils also occur in the Winneshiek Lagerstätte, including several soft-bodied forms (Fig. 15).

Photo of an indeterminate arthropod fossil with high carbon soft tissue.

Three-dimensional vermiform fossils (Fig. 16) are common in the collection and their classification requires further study.

Photo of three-dimensionally preserved vermiform fossils.

Trace fossils include bromalitic materials (Fig. 17) and thread-like fossils. The former was common in the Late Ordovician Soom Shale Lagerstätte of South Africa and the latter was reported from the Middle Ordovician Douglas Dam fauna in Tennessee.

Photo of bromalitic material.

Many Ordovician normal marine biota, such as trilobites, graptolites, corals, echinoderms, and bryozoans, are absent from the Winneshiek fauna. The fauna component, exceptional preservation, and sediment lithology and structure suggest the Winneshiek Lagerstätte was deposited in a restricted marine environment with limited benthic oxygenation and low energy, likely a shallow brackish estuarine embayment of the transgressing Middle Ordovician sea.

In brief, the Winneshiek Lagerstätte opens a unique window into the Middle Ordovician biotic community which lived along the margin of the Laurentian cratonic seaway. Well-preserved fossils provide exceptional materials for studies on the taxonomy, anatomy, affinity, and taphonomy of several extinct taxa. For details, please refer to the related publications listed below.
 


Huaibao P. Liu, Robert M. McKay, Jean N. Young, Brian J. Witzke, Kathlyn J. McVey, and Xiuying Liu, 2007: The Winneshiek Lagerstätte. Acta Palaeontologica Sinica, 46 (Suppl.), Proceedings of the 10th Int. Symposium on the Ordovician System/the 3rd Int. Symposium on the Silurian System IGCP 503 Annual Meeting, p. 282-285.

Huaibao P. Liu, Brian J. Witzke, Jean N. Young, Robert M. McKay, 2007: Conodonts from the Winneshiek Lagerstätte, St. Peter Sandstone (Ordovician) of northeast Iowa (abstract). Geological Society of America Abstracts with Programs, v. 39, no. 3, p.63.

Huaibao P. Liu, Robert M. McKay, Jean N. Young, Brian J. Witzke, Kathlyn J. McVey, and Xiuying Liu, 2006: A new Lagerstätte from the Middle Ordovician St. Peter Formation in northeast Iowa, USA. GEOLOGY, v. 34, no. 11, p. 969-972.

Liu, Huaibao P., McKay, Robert M., Young, Jean N., Witzke, Brian J., McVey, Kathlyn J., and Liu, Xiuying, 2005: A new soft-bodied Middle Ordovician fauna from the St. Peter Sandstone in northeast Iowa (abstract). Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 116.

Young, Jean N., McKay, Robert B., and Liu, Huaibao P., 2005: Unusual sections of the Readstown Member, St. Peter Formation, at Decorah, northeast Iowa (abstract). Geological Society of America Abstracts with Programs, v. 37, no. 5, p. 78.