Prehistoric Life 7

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.

Devonian Period (408-360 million years ago)

The Devonian Period was named in 1837 for Devonshire, where fossils were first found intermediate between those of Silurian and Carboniferous periods. Its beginning formally is defined by the first appearance of the graptolite Monograptus uniformis. For DuPage County, with a single exception, the geological deposits end with the Silurian, and do not resume until the relative eyeblink of the past 12,000 years.

Life on Earth. In some ways the Devonian period continued much as the Silurian had, with reefs and their associated life forms continuing to develop and diversify. Petoskey stone, the Michigan state rock, is a middle Devonian massive colonial tabulate coral, Hexagonaria percarinata. Brachiopods remained diverse in the Devonian, but then began their decline. Trilobites remained diverse. Crabs were a new marine arthropod group in the Devonian. A significant new group of cephalopod mollusks, the ammonites, appeared. There was a massive extinction of marine forms in the late Devonian, marking the beginning of a long period where there were no large reefs until the mid-Triassic. This mass extinction coincided with a big drop in atmospheric oxygen (Science 316:557).

Today’s bluegill are members of Osteichthyes, the first of which appeared in the Devonian.

Fishes diversified, and the Devonian has been described as the “Age of Fishes.” There were 4 groups of jawed fishes: placoderms (armored; almost entirely limited to the Devonian), acanthodians (“spiny sharks,” some of which were abundant, small school-forming fishes), Chondrichthyes, and Osteichthyes. The latter two groups, which dominate today’s fishes, both first appeared in the Devonian, including both the lobe-finned (coelacanth, lungfish) and ray-finned branches of Osteichthyes, and the first true sharks (Chondrichthyes). Lampreys likewise first showed in the Devonian fossil record. These fishes were diverse ecologically as well, with plankton feeders, bottom feeders, and mobile predators.

In 2006 paleontologists described a fossil lobe-finned fish from northern Canada, 375 million years old, that is the most clearly intermediate example to date between fish and terrestrial vertebrates. Tiktaalik roseae had “fins better engineered for standing than swimming.” Also, it had lost gill cover plates, and possessed a shoulder girdle largely separated from the skull (i.e., the fish had a neck). They probably were both poor crawlers and poor swimmers, but in their river delta environment they could escape aquatic predators (Science 312:33).

The diversification of fishes and sharks as well as shell-crushing arthropods in the “Middle Paleozoic Marine Revolution” of the Silurian and Devonian Periods had a measurable impact on some of their prey (Science 305:1453). Crinoid arm regeneration increased and antipredator morphologies became more common as this time passed. Crinoids “developed more spines, thicker calyx plates, and reduced viscera.” Regenerating crinoid arms in fossils increased from less than 5% in the Ordovician and Silurian to more than 10% in the Devonian to Pennsylvanian. Close study of another group of marine invertebrates, rugose corals, has shown that the Earth was spinning faster then, so that days were 21 hours long.

Land plants were similar all over the world, implying a uniform warm climate. Early vascular land plants were small, and mainly consisted of branching stems with spore producing structures. True leaves appeared in the Devonian. The first forests emerged by the late Devonian, the trees belonging to groups called progymnosperms and seed ferns. The seeds contain spores, and the leaves are fernlike fronds. They were the ancestors of more modern plants that appeared in the Mesozoic, the cycads and flowering plants or angiosperms. There were also giant club mosses, and the first true ferns. The Devonian also produced the first lichens.

Terrestrial invertebrates diversified, producing the first millipedes, centipedes, pseudoscorpions, spiders, harvestmen (daddy long-legs), mites, and insects (springtails being a group that has persisted to the present day).

The first, labyrinthodont, amphibians appeared in the late Devonian. At least some did come out on land, and were predators. They were sufficiently widespread that they occurred in both Greenland and Australia, which even then were well separated.

Local landscape. In the early Devonian, our area is thought to have become low land as the sea retreated, the result of crustal warping, and still was a little south of the equator. The area subsided later in the period to become a shallow sea in which limestone again was deposited. The second phase of the Appalachian mountain building occurred with the Acadian orogeny in the late Devonian, as continental plates collided to the east. Then, muds eroding from uplifted lands to the east produced the Devonian New Albany shale that forms the bed of Lake Michigan. Some of that shale was gouged out, ground up, transported and deposited by the latest continental glacier to form DuPage County’s clay soil. The nearest Devonian bedrock therefore is just east of our area. However, cracks in the upper surface of the Silurian dolomite in an Elmhurst quarry contained clay that was deposited during the Devonian (or, possibly, Mississippian).

Local life. The mud-bottomed sea was contiguous in the late Devonian from here to northern Iowa, where there were forms ranging from a lobe-finned lungfish to an ammonite along with various stromatoporoids, corals, clams, snails, brachiopods and bryozoans. Horn corals and tabulate forms were diverse and abundant.

This enormous arthrodire fossil can be seen at the Field Museum.

The fierce looking arthrodires, the most diverse placoderms, were widely distributed across North America in the middle and late Devonian. The shark-like Cladoselache, of the Chondrichthyes, is known from Ohio. Falls of the Ohio State Park at Jeffersonville, Indiana, also has middle Devonian fossiliferous limestone and shale deposits. The limestone has more than 600 species of corals, sponges, brachiopods, mollusks, echinoderms, and others.

The Devonian or Mississippian clay found in Elmhurst actually contained some fossil shark teeth. Central Illinois Devonian fossils include the branching coral Alveolites, tabulate corals Hexagonaria and Pachyphyllum, the horn coral Zaphrentis, brachiopods Schizophoria, Douvillina, Cyrtina, Atrypa, Spinocyrtia, Schuchertella, and Strophodonta, and trilobites Odontocephalus and Phacops.


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