Milwaukee Public Museum

by Rodney Watkins

EDITOR'S NOTE: This article is reprinted from LORE magazine (vol 39, no. 2, (Summer, 1989), p. 26-30), a benefit of museum membership. ©1996 Milwaukee Public Museum, Inc.

More than four hundred million years ago, Wisconsin was covered by a vast inland sea that stretched from New York to Nevada. This interval of geologic time is known as the Silurian Period. During the Silurian Period, North America was located near the equator, and life on Earth was much different from that of today. The land was inhabited only by small plants and tiny arthropods. However, the Silurian seas teemed with life, and one of the world's richest concentrations of Silurian sea life is preserved as fossils in the bedrock of eastern Wisconsin.

In the early 1850s, James Hall, a geologist from New York, was brought to Wisconsin by the state legislature to conduct the first geologic survey of the state. In Racine, at a locality now preserved as Quarry Lake Park, Hall discovered a distinctive formation of dolomite bedrock that is literally packed with fossils. He dated these fossils as Silurian in age, and he named their enclosing rock the Racine Formation. The Racine Formation extends from Kenosha through Manitowoc counties, and it can be seen at the ground surface in quarries, roadcuts and natural outcrops.

It is famous for its ancient reefs. Indeed it was from the Racine Formation in Wauwatosa that T.C. Chamberlin published an 1877 description of the first known fossil reef in North America. Reefs in the Racine Formation are mound-shaped bodies up to 100 feet thick and as much as 900 feet in breadth. The reefs are massive, very fossiliferous structures that stand out from the surrounding layered dolomites. The outcrop at County Stadium is an example of this type of structure, although unlike typical reefs of the Racine Formation, the County Stadium reef is very poor in fossils. The Racine Formation contains dozens of known reefs, and probably hundreds more are buried beneath glacial drift.

Reefs throughout geologic history share a common set of environmental features, including tropical to subtropical location beneath warm salt water. The water above reefs are generally less than 100 feet deep and relatively free of suspended sediment. This last feature is important, as it allows sunlight to penetrate to the reef surface where it is used for photosynthesis by algae and many types of microbes.

During the past five years, I have conducted an extensive research project on the reef fauna of the Racine Formation. This project involved careful study and identification of over 10,000 specimens in MPM collections. A major result of the project was the tabulation of 191 fossil species. This represents the highest recorded diversity for any Silurian reef community in the world.

Species inhabiting ancient and modern reefs can be grouped into four categories according to their role in the reef community: constructors, binders, bafflers, and dwellers. Constructors build the reef by forming a framework of hard parts. the major constructors in Wisconsin reefs wee an extinct group of sponges called stromatoporoids. Stropmatoporoids built hard, calcarious skeletons shaped like domes, and they reached sizes of three feet. Tabulate corals, another type of constructor, are an extinct group of colonial animals distantly related to modern corals. Tabulates in the reefs are generally less than two feet in size, and their colony shapes include domes and chain configurations.

Binders are organisms with a sheet-like form that grow over loose sediment and skeletal remains and literally bind them together. The skeleton of a binding organism may be only 1/10 inch think but many feet in width. Binders in the Silurian reefs include stromatoporoids, tabulate corals and bryozoans, a still- living type of colonial animal.

Bafflers are organisms with an upright, branching growth form that interferes with currents and causes sediment to settle from the water and accumulate on the reef. Small, bush-like colonies of tabulate corals and rugose corals, another extinct group, represent bafflers. In spite of their role in reef building, constructors, binders and bafflers comprise only 9% of the reef species.

As in modern reefs, the majority of species in the ancient reefs of Wisconsin fit into the category of reef dwellers. The dwellers comprise a highly varied assortment of organisms that live in and among the larger colonies. Usually the dwellers are only a few inches or less in size. Small sponges, solitary rugose corals, and bryozoans lived by filtering food particles out of the water. These were immobile animals that were either attached to the larger colonies or nestled in patches of soft sediment.

The most diverse group of reef dwellers were brachiopods, which accounted for nearly 23% of the total reef species. Brachiopods still live in modern seas, although they are much less abundant than in the geologic past. Brachiopods live in a shell superficially like that of clams, although the animal inside the brachiopod is quite different from a clam. Brachiopods circulate water through the inside of their shell, where they filter out small food particles with a filament-like organ. Some brachiopods were attached to the reef surface by a fleshy stalk, and others nestled into soft sediment.

Reef dwellers which have changed the least from Silurian to modern times are snails and clams. By analogy with living snails, those of the Silurian probably crawled about the reef surface grazing on algae, filtering food particles with their gills, or preying upon other animals. Most species of clams in the reefs burrowed into soft sediment and filtered food particles from the water above.

Cephalopods are another important group of reef dwellers. Living cephalopods, including the squid, octopus and pearly nautilus, are actively swimming predators, and this can be said of the extinct cephalopods of the Silurian. Thirty-three species of cephalopods have been recorded from the Wisconsin reefs, where they range in size from less than an inch to six feet or more in length.

Trilobites are an extinct group of arthropods that crawled about the reef in search of food. Like modern arthropods, trilobites had distinct heads with eyes and antennae, as well as dozens of tiny legs.

Crinoids and related echinoderms were an abundant type of reef dweller, and they contributed more skeletal particles to the reef sediment than any other group. Crinoids still live today but were much more common in the geologic past. Known popularly as "sea lilies," crinoids consist of a long, upright stem that is rooted to the seafloor and topped by a crown-shaped head and feathery arms. They are not plants, however, and they use their arms to filter small food particles from seawater. Some crinoid species grew to heights of three feet above the reef surface.

Wisconsin's ancient reefs represent the most diverse ecologic community of Silurian seas, and they share much in common with reefs of the present day. However, significant differences exist between Silurian and modern reefs. Whereas 191 species have been recorded from reefs of the Racine Formation, modern tropical reefs contain thousands of species. Growth forms of colonies in the modern reefs are much more diverse and complex that those of the Silurian, and modern reef dwellers are more specialized for life in the reef environment.

NOTE: Readers interested in the technical background for this article should consult the following publication: Watkins, R, 1993, The Silurian (Wenlockian) reef fauna of southeastern Wisconsin: Palios, v.8, p. 325-338.

ALSO NOTE: A recreation of a Silurian reef is on display in MPM's Third Planet Geology Hall.