Milwaukee Public Museum

by Peter M. Sheehan, Curator of Geology

EDITOR'S NOTE: This article is reprinted from LORE magazine, a benefit of museum membership. ©1996 Milwaukee Public Museum, Inc.

Four hundred million years ago southeastern Wisconsin was covered by warm, shallow seas. Fossil-filled limestones deposited in these ancient seas have preserved massive coral reefs. These reefs were to become important in the history of geology because they were the first to be recognized in the geologic record. T.C. Chamberlin, the Chief Geologist of Wisconsin, demonstrated for the first time that coral reefs existed in ancient environments. When he discovered the reefs more than 100 years ago, modern coral reefs had been described only recently. In his work Chamberlin drew extensive analogies with the studies of modern reefs by James Dana and Charles Darwin.

Since Chamberlin's time reefs have been found in all geologic periods from the Cambrian to the present. In the early part of this century, porus reef-rock was found to contain vast reservoirs of oil and natural gas. The effort to understand reefs became frenetic.

Petroleum contained in reef structures was not actually formed in the reefs. Oil and gas ultimately was derived from plants and animals that died and fell to the sea floor. As the organic matter was buried in sediments, petroleum was generated. The petroleum migrated laterally and was concentrated in porous rocks such as reefs. Because reef reservoirs are so important to the petroleum industry, they have been studied intensively. By the middle of this century, the distribution of reefs in time and space had been well documented.

But one enigma had remained since Chamberlin's original discovery. Modern coral reefs are confined to the tropics because reefs die if water temperatures fall below 18 degrees C, but fossil reefs are found worldwide, even in polar regions. No modern reefs live as far north as Wisconsin, and in North America their northern limit is in Florida.

When the paradigm of plate tectonics revolutionized geology in the 1960s, the distribution of fossil reefs was explained. Reefs have always lived in the tropics, but as continents drift out of the tropics, they bring with them their tropical record. Wisconsin's 400 million year old reefs were formed when the North American Plate was on the equator. Millions of years later North America moved to its present position, bringing with it tropical fossils entombed in stone.

The Silurian reef diorama in the museum's exhibition hall, "Continents, Oceans, and Life in Motion -- A New View of the Third Planet," reconstructs a reef from southeastern Wisconsin. The organisms on the reef are models of fossil species from reefs that have been found in local limestone quarries such as the one County Stadium now occupies. The reef was constructed by Terry Chase Studios, of Cedar Creek, Missouri.

The diorama was conceived originally by the museum staff. Close cooperation between the author, Terry Chase and his staff, and Don Mikulic and Joanne Kluessendorf, both of the Illinois Geological Survey, assured accuracy of the reconstructions. The reef was fabricated in Missouri and transferred to Milwaukee for installation.

Exhibits on either side of the reef diorama display actual fossils of many marine organisms in the diorama. All models are full-scale reconstructions.

The reef mass was bound together by colonial corals, bryozoans and algae. Attached flower-like echinoderms dominate the surfaces. These plant-like animals, related to starfish and sea urchins, were filter feeders which obtained food by sieving microscopic organisms from sea water with their delicate arms. Other filter feeders were the bivalved brachiopods. Grazing and browsing animals included snails and the extinct group of arthropods called trilobites. The primary predators were conical shelled nautiloid cephalopods - relatives of modern coiled nautiloids, squid, and octopi.

Reefs, both ancient and modern, are delicate, intricate ecosystems with an amazing diversity of life. They are formed by the shells of organisms bound together into mounds which grow upward from the sea floor. Reef mounds may reach hundreds of meters in height. Framework organisms grow upward and binding organisms cement shells to the framework, giving the reef a firm structure. Many animals provide shells to the reef mass. The surface of the reef is very irregular, providing numerous habitats for organisms. Through geologic time reefs have been the most diverse communities in the oceans. More different kinds of organisms live in reefs than in any other marine setting.

The kinds of animals inhabiting reefs have changed many times during earth history. For example, the predators in the Silurian reef were conical nautiloids. In modern reefs the predators are primarily fishes, which had not yet evolved in the Silurian. The corals in the Silurian belonged to the extinct groups which are only distantly related to modern corals. The group of corals in the Silurian reef became extinct 200 million years ago, and modern corals evolved from soft bodied sea anemones. Stalked echinoderms and trilobites are no longer found in reefs.

Through geologic time there have been seven different reef- forming faunas. During the last 600 million years there has been an erratic increase in the kinds of organisms living in the oceans. Only a dozen families with skeletons were present 600 million years ago, while nearly 700 families are present in the modern ocean. Jack Sepkoski of the University of Chicago has documented this history by examining the geologic ranges of all families of skeletonized animals in the fossil record. His study was published by Milwaukee Public Museum as "Contributions in Biology and Geology No. 51."

Six extinction events exterminated vast numbers of animals, temporarily depressing the diversity curve. But in spite of the extinction of many families, evolution has produced progressively more numerous kinds of organisms and the general trend of the diversity curve has been upward.

Reefs are very sensitive to changing environments. The reef ecosystem is as fragile as it is complex and the six great extinctions completely decimated the reefs. Following extinction events reefs were absent from the fossil record for millions of years before evolution produced new reef biotas. Catastrophes which terminated reef intervals included glacial periods, intervals when the shallow seas were drained from the continents and at least on time when an asteroid struck the earth.

Silurian coral reefs are well known in the geologic community. The reef display brings to public attention the beauty and diversity of life that was part of Wisconsin's geologic past.