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.
Ordovician Period (505-438 million years ago)
The Ordovician Period was first created (1879) to resolve an overlap between the upper Cambrian and lower Silurian as these two were first being explored and defined. It is named for an ancient Welsh tribe, the Ordovices. Its beginning and ending are marked by major extinctions, the latter perhaps the result of an ice age.
Life on Earth. Trilobites became much less dominant in the world, and brachiopods more abundant and diverse. Mollusks, including bivalves, diversified, and among their innovations were the nautiloid cephalopods (cephalopods are the squids, octopi and relatives). Some of these exceeded 6 meters in length and thus were the first large animals on Earth. Some new phyla also appeared, including the bryozoa (moss animals), which became codominants with brachiopods. The first corals, both solitary (rugose, or horn corals) and colonial (tabulate corals) also appeared in the Ordovician. These extinct groups are called corals, and are regarded as cnidarians [coelenterates] with similar supportive structures, but they are not direct ancestors of present-day corals. Ordovician reefs were formed mainly by bryozoans early in the period, with a transition to dominance by stromatoporoids (recently determined to be a form of sponge) and tabulate corals. Graptolites were another new group, widely distributed, probably planktonic, and colonial. Long mysterious, they now are regarded as allied to the chordates. The first jawless fishes, i.e., the first vertebrates, evolved from earlier chordates.
The earliest land plants are evidenced by mid-Ordovician spores, the worldwide distribution of which suggests a cosmopolitan distribution. These would have been similar to algae and mosses. Late Ordovician spores indicate that the first vascular plants had evolved by then (Science 324:353). A major ice age occurred toward the end of the Ordovician, and is a likely cause of the major extinctions that mark the end of the period.
Local landscape. Sand continued to be deposited here in the early Ordovician, producing the St. Peter formation of sandstone (at the surface in Starved Rock State Park). The nearest land became farther away, perhaps because all the land to the North had become covered by the sea, until by the late Ordovician shales were formed here. Ordovician deposits are on top of Cambrian ones, which in turn are on top of the Precambrian granite (the nearest Ordovician bedrock is in Kane County; most of that county’s bedrock is Ordovician). Our location continued to be just south of the equator. At this time the collision of continents east of us began to form the first incarnation of the Appalachians (Taconic orogeny), so that area became the closest significant land. Later episodes of that collision would lift our area out of the sea for good.
Local life. Sponges are abundant in Middle Ordovician deposits in Kentucky and Tennessee. In the Middle to Late Ordovician, the sponge-like Receptaculites (it may in fact have been a calcareous alga) was in northern Illinois. Horn corals were found in eastern North America in the Middle Ordovician, as were tabulate corals. Fossil bryozoa from the Ordovician are diverse in Ohio. Various brachiopods are found on all sides of us in the middle to late Ordovician. Ordovician crinoids are known in Ohio shales, and late Ordovician starfish also have been found in Ohio and Minnesota. Though jawless Ordovician fishes are known from North America, their fossils are rare and they apparently were unimportant ecologically.
Local Ordovician fossils according to Illinois Geological Survey monographs include Receptaculites, the horn coral Streptelasma, the snails Maclurites, Trochonema, and Hormotoma, the trilobite Isotelus, and brachiopods Refinesquina, Dalmanella, Platystrophia, Sowerbyella, Rhynchotrema, Paucicrura, Hesperorthis, Pionodema, Lepidocyclus, Hebertella, and Strophomena. Note that this should not be taken to mean that all these species occurred together: Isotelus occurs in the Scales shale, while Receptaculites and Hormotoma are in the earlier Wise Lake dolomite.