Prehistoric Life 8

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.

Mississippian Period (360-320 million years ago)

Most of the world calls the period following the Devonian the Carboniferous Period. It was named for coal deposits in Europe in 1882, marking the first massive forest terrestrial communities. Its beginning (as is that for the Mississippian Period) is defined by the first appearance of the conodont Siphonodella sulcata. In North America the Carboniferous long has been subdivided into two parts, though this distinction is vanishing as worldwide communication highlights the idiosyncratic nature of the North American pattern. The Mississippian Period is distinguished by the relative stability of the sea. It was named in 1870 for the Mississippi valley.

Walking Fern. Ferns were abundant land plants in the Mississippian.

Life on Earth. Ferns became extremely abundant in the Carboniferous, and a distinction developed between the terrestrial floras of what are now the northern hemisphere continents (characterized by club mosses, some seed ferns, and primitive conifers), and the southern hemisphere (characterized by a genus of seed ferns, Glossopteris). In North America, however, these forests were limited in the Mississippian Period by the small area of dry land.

Graptolites vanished in the Mississippian. There were no huge leaps in vertebrate evolution in the Carboniferous, but amphibians diversified and evolved better functioning legs. For example, a common ancestor of frogs and salamanders, having characteristics intermediate to those groups, lived 340 million years ago (Science 318:1237). Crinoids reached their all-time greatest abundance around the Mississippian reefs. Protozoa also diversified, with Foraminifera particularly becoming common.

Local landscape. Illinois was mainly marine during the Mississippian, but the land was not too far east and northeast, and moved closer and farther away as the sea level fell and rose, so there were episodes of limestone deposits (mainly), some of muddy bottoms (closer to shore), and even sand (very close to shore). Late in the period, the limestone-deposition areas occasionally were exposed to air, and the sea retreated as our area became land.

Indeed, much of the North American continent was raised above the sea by growing tectonic activity in that time. Our area was still south of the equator.

The nearest Mississippian bedrock is south of us, in Iroquois County (the next county south of Kankakee County), with one interesting exception. There is an odd small area called the Des Plaines Disturbance, in Cook County just northeast of DuPage, where there is a complex of faults and displacements thought to have been caused by a meteorite strike. These have placed some small areas of Mississippian rock at the bedrock surface (i.e., the meteorite drove the younger rock layers down into the older Silurian strata, so that in that place they remained while being eroded away all around).

Local life. Mississippian fossils are abundant in the southern two-thirds of Illinois. The early Mississippian was a limestone time, when brachiopods as well as crinoids and other stemmed and starfish echinoderms were abundant and diverse. Bryozoans often were important, too. Mixed in (where the rock remains in western IL) are chert nodules indicative of siliceous sponges and other marine invertebrates. A crustacean, a crayfish-like malacostracan, also was here. Geode-forming species and the spiral bryozoan Archimedes were abundant later in the Mississippian. There were some trilobites.

Fossils listed in Illinois Geological Survey monographs for the Mississippian include shark teeth, the corals Cyathaxonia arcuata and Lithostrotionella, bryozoans Evactinopora sexradiata, Prismopora serrulata, Batostomella nitidula, Worthenopora, Archimedes and Rhombopora, crinoids and blastoids Platycrinites penicillus, Pugnoides ottumwa, Talarocrinus, Pterotocrinus menardensis, Onychocrinus, Batocrinus, Schizoblastus, and Pentremites spicatus, snails Platyceras and Straparollus, the pelecypods (clams) Sulcatopinna missouriensis and Euphemia randolphensis, and brachiopods Syrigothyris, Spirifer vernonensis, S. rowleyi, S. logani, S. increbescens, Dictyoclostus crawfordsvillensis, Orthotetes keokuk,  Rotaia subtrigonia, Athyris lamellosa, Leptaena rhomboidalis, Martinia contracta, Primospora serrulata, Composita subquadrata, Skelidorygma, and Ovatia. Note that this list lumps together species from different parts of the Mississippian.

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