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.
Phanerozoic Eon, Paleozoic Era, Cambrian Period (542-505 million years ago)
This begins the remaining 10% of Earth’s history, when more complex life forms dominated. The Cambrian Period was named in 1835 for Cambria, the ancient Roman name for Wales. Its beginning (and that of the Paleozoic Era and Phanerozoic Eon) is marked by the first occurrence (in Newfoundland) of a type of fossilized burrow referred to as Treptichnus (or Phycodes) pedum. This was some sort of worm, and is the first bilaterally symmetrical animal in the fossil record. The Cambrian Period ended with a significant mass extinction, but also contained 3 others before that final one.
Life on Earth. Though the Ediacara long were thought to have vanished before the Cambrian Period began, scientists in 2006 described (Science 312:731) a frond-like fossil, Stromatoveris psygmoglena, from the lower (early) Cambrian that resembles Ediacara. Detail shows apparently ciliated branches that could be precursors of ctenophore comb rows. The authors proposed that this in fact should be regarded as a ctenophore (comb jelly, common in today’s oceans), and believe it links that group to the Ediacara. Another of the Ediacara may have been an early urochordate (Science 319:1618-1619).
A famous source of fossil information from the Cambrian Period is the Burgess Shale formation of British Columbia. The Field Museum’s Evolving Planet exhibit contains an impressive, wall-sized animated reconstruction of the Burgess Shale community. Among its marine forms were early chordates, coelenterates, mollusks, sponges, trilobites and crustaceans and other arthropods, polychaete worms, echinoderms, and a number of other, often weird phyla that quickly vanished.
In other words, most of today’s phyla, or major groups of animals, appeared in the fossil record during the Cambrian period. Recent genetic studies have revealed significant amounts of shared genetic material through the broad range of present day animals and certain sponges (Science 316:1893) and sea anemones (Science 317:86), demonstrating the common ancestry of the animals. A similar level of plant radiation did not occur until much later (plants were essential, though, the primitive algae and other forms providing the energy and nutrient economy supporting all those animals).
Life still was limited to the sea. Land animals and plants will not appear until the Silurian period. In the Cambrian, more than half of the fauna was composed of trilobites, with the next largest group (30%) brachiopods, and mollusks making 5%. Stromatolites continued to be abundant. The first reefs appeared in the Cambrian, formed by vase-like archaeocyathids supplemented by algae and others. Trilobites and some other groups showed more structural variability among individuals in the early Cambrian than was the case in later times. This may have provided a foundation for the significant diversification that was to follow (Science 317:459). Most trilobite groups died out at the end of the Cambrian in an episode of mass extinctions that define the end of the period.
Local landscape. During the Cambrian, our part of the Earth’s surface was a little south of the equator. In the late Cambrian and Ordovician there was a net gradual advance of the ocean from the east and south, as the sea rose or the continent sank. The advance was slow, a few miles per million years, and consisted of a series of 5 advances and retreats of the sea. Each of these began by producing sandstone and progressed to limestone or dolomite as the shore continued to advance north and our area thus became farther out to sea. The sandstone formed from sand that eroded from exposed land to the north (when granite erodes, the tiny quartz crystals that form sand are the most resistant mineral within it). The sandstone of the Wisconsin Dells represents the start of the second cycle (The Dells were carved in a relatively brief and recent episode of major streamwater flow when Glacial Lake Wisconsin, blocked by the last glacier, was released). The Ordovician St. Peter sandstone of Starved Rock State Park formed at the start of the last of these cycles. Ultimately North America became largely a shallow sea, increasing perhaps a thousand times the environment available for diversification of marine life. Later marine-origin sedimentary rock was deposited over the Precambrian granite and Cambrian rock layers in all of Illinois, except for one spot to our west.
Local life. The Cambrian sands of northern Illinois generally do not contain much in the way of fossils, in comparison to the richer shales and limestones elsewhere. Exceptions include some trilobites and brachiopods. Soft bodied forms such as worms in various phyla may well have been here, but wouldn’t have left fossils. In Ogle County, west of us in north central Illinois, trilobites include Dikelocephalus, Illaenurus, and Saukiella. Brachiopods, snails, stromatolites and unspecified worm borings also are known.