Prehistoric Life 6

by Carl Strang

This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. I include some references, particularly to papers published in the journal Science which commonly is available at public libraries. Contact me if you need sources for other items. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.

Silurian Period (438-408 million years ago)

The Silurian Period was named for the Silures (1835), another ancient Welsh tribe. Its fossils were recognized as distinct from those of the preceding Ordovician and following Devonian periods. Its beginning formally is defined by the first appearance of the graptolite species Akidograptus ascensus.

Life on Earth. During the Silurian, the Earth generally showed a warming trend after the ice age that ended the Ordovician. The Silurian reefs, which continued to be based mainly on stromatoporoids and tabulates, were huge. Brachiopods and bryozoans (including a number of colonial, massive stony or sea-fan-like forms) dominated the species count, but corals and echinoderms increased in importance. Nautiloid cephalopods and graptolites largely disappeared by the beginning of the Silurian. Eurypterids (sea scorpions, a group of chelicerate arthropods) appeared in the Silurian, some reaching 10 feet long. Jawless, armored fishes (Agnatha) became more diverse.

A significant step was the appearance of the first vertebrate jaw by the late Silurian, developed from gill arches in the first placoderm fishes and early sharks.  Diversifying early land plants were joined by the first fungi. The first terrestrial chelicerate arthropods (scorpions and eurypterids) had appeared by the end of the Silurian. These were very similar to marine forms of both groups. For instance, Brontoscorpio was an earlier Silurian marine scorpion, very similar to the familiar scorpion shape, which reached 3 feet in length. Genetic studies tie insect origins to crustaceans like today’s fairy shrimp and water fleas, the split occurring near the end of the Silurian. Also, millipedes and centipedes (which evolved in the Devonian) appear to be connected to chelicerate arthropods.

One place where the Niagaran formation is at the surface is at the tip of the Bruce Peninsula in southern Lake Huron.

Local landscape. Our area was a shallow, clear, saltwater sea with abundant dome-shaped reefs. The reefs were part of a barrier reef system surrounding the Michigan Basin. The rock is composed of dolomite, which is a limestone (calcium carbonate) in which magnesium replaces part of the calcium, the replacement possibly having occurred after the limestone was deposited. DuPage County’s bedrock thus is a part of the Silurian layer called the Niagaran formation, which forms a bedrock ring including the western shore of Lake Michigan, the Door peninsula, the southern shore of the U.P., the islands dividing Lake Huron from Georgian Bay, and a zone connecting to Niagara Falls, with large areas curving through Indiana and parts of Ohio. Outliers can be found in eastern Iowa. Our area probably was between the equator and 20° south latitude. The big Thornton Quarry on either side of Interstate 80-294 is mining one of the larger known reefs in this formation.

This structure, referred to as a “flowerpot,” is an isolated pillar of the Niagaran dolomite. It is on an island just off the tip of the Bruce Peninsula.

Local life. Fossils of the Niagaran formation found locally include reef-forming corals (including antler forms and large lumps such as Cladopora reticulata, Halysites catenularia and Favosites niagarensis). Other attached forms include rugose (horn) corals (Asthenophyllum racinensis, Dalmanophyllum wisconsinensis, Pycnostylus guelphensis); bryozoans (Fenestrellina spp., Hallopora ellengantula, Pachydicta crassa); calcareous algae (e.g., the plum-shaped green lump Calathium egerodae); and stromatoporids. Chert nodules are thought to have been derived from the hard parts of sponges (great for arrowheads, lousy for stone crushing machines), but at least one identifiable fossil sponge is known, Calathium sp. The diverse brachiopods include Eospirifer(Spirifer) radiatus, Apopentamerus racinensis, Leptotaena (Leptaena)  rhomboidelis, Rhynchotreta cuneata, Atrypa reticularis niagarensis, Schucheretella subplana, Uncinulus stricklandi, Meristina maria, Conchidium laqueatum, Kikidium, and Wilsonella. There also are several species of crinoids (Crotalocrinites cora, Lampterocrinus infatus, Marsupiocrinus chicagoensis, Siphanocrinus nobilus, Eucalyptocrinus crassus, Periechocrinus infelix), and crinoid-like cystoid echinoderms (Caryocrinites ornatus, Holocystites alternatus)

This diorama is part of the Evolving Planet exhibit at the Field Museum of Natural History. It portrays a Silurian reef.

Mobile life forms include trilobites (Bumastus niagarensis, B. harrisi, B. insignis, Calymene celebra, C. niagarensis, Arctinurus chicagoensis, Dalmanites sp., [=?] Dalmanella platycordata), and mollusks (snails Euomphalopterus halei, Tremanotus alphaeus, T. chicagoensis, Straparollus magnus, Lophospira rotunda, Phanerotrema occidens; pelecypods Mytilarca denticostia, Matheria recta; straight-shelled cephalopods Amphicyrtoceras orcas, Dawsonoceras bridgeportensis, Kionoceras orus, K. cancellatum; and at least one coil-shelled species, Discoceras marshi).

This fossil trilobite is in one of the Silurian dolomite flagstones at Fullersburg Woods, mined from the Lemont Quarry and used by the Civilian Conservation Corps in its construction projects at that forest preserve.

The earlier Alexandrian series, also dolomite, has the brachiopods Platymerella manniensis, Microcardinalia pyriformis, and Pentamerus oblongus.

Vertebrates, i.e., fishes, continued to be relatively uncommon through at least most of the Silurian, and apparently fossils of them have not been found locally.

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