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.
Pennsylvanian Period (320-286 million years ago)
The Pennsylvanian Period is named for the state of Pennsylvania (1891). This North American subdivision of the European Carboniferous Period is distinguished by many cyclic repeated advances and retreats of the sea, as indicated by alternating rock layers.
Life on Earth. This was the time of the coal forests, when the growing land area provided the home for forests of lycopsids (club mosses, the most abundant trees), sphenopsids (the group containing today’s scouring rush, horsetails and other members of genus Equisetum), ferns (including tree ferns), and seed ferns. There were early conifers as well. These were vascular but not flowering plants. Most coal was produced during this period because fungi, critical to decomposition, had not yet developed that ability to a significant extent. Dead plant tissue piled up without breaking down, ultimately was buried and fossilized into coal. As a result, oxygen built in the atmosphere to an all time high of 30% (Science 316:557).
You can see a life-sized reconstruction of a Pennsylvanian forest at the Field Museum in Chicago. Here are some model sphenopsids.
The earliest Amniota (the terrestrial egg-bearing group ultimately including reptiles, birds and mammals) appeared and diverged in the Pennsylvanian, producing the cotylosaurs (and other anapsid reptiles, a group represented by turtles and tortoises today; fossil cotylosaurs have been found as close to Illinois as Nova Scotia), synapsids (also known from N.S. and the group from which mammals ultimately evolved; the basal synapsids are referred to as pelycosaurs), and the diapsids, a reptile group that evolved into lizards, snakes, dinosaurs, birds and crocodiles.
Winged insects (including the first mayflies and enormous primitive dragonflies) first appeared in the Pennsylvanian, as did cockroaches, grasshoppers and crickets. The earliest beetle was reported from Illinois fossil material in 2009 (J. Paleont. 83:931). Some invertebrates, such as the dragonflies and certain millipedes, reached giant sizes (thanks at least in part to the elevated oxygen levels).
Local landscape. In Illinois, the sea continued its advance and retreat cycling, so that our area alternated between marine and land, often low and swampy. Our area remained just south of the equator, and the climate was warm and humid. It is thought that alternating periods of glaciers forming and thawing on the southern Gondwana supercontinent (at that time drifting over the South Pole) caused the rises and falls of sea level that produced the local advances and retreats of the sea. Over geologic time, glacial episodes typically are associated with a continental mass at one of the poles (Antarctica in recent times).
Tree ferns still exist today. This one in Tasmania had a thick stem more than 10 feet tall.
The North American continent was beginning to collide with Europe and Africa as the sea that had begun to appear between them closed, forming the northern supercontinent of Laurasia. This event is what lifted our part of the world above the sea for good.
The nearest Pennsylvanian bedrock to Chicago is the Mazon Creek area (much of Illinois’ bedrock is Pennsylvanian), except for some bits in the Des Plaines Disturbance.
Local life. Coal forests dominated Illinois during the Pennsylvanian. Not only was coal left (itself fossil plant material), but remains of a variety of plant and animal fossils can be found just a little south of us in the world-famous Mazon Creek deposits of Middle Pennsylvanian age, just a little southwest of Joliet. Seed-fern leaves such as Medullosa, Neuropteris inflata, N. scheuchzeri, N. ovata and N. rarinerus are especially abundant (note: names of these plants are confusing, because different names are given to different parts such as leaves, stems and reproductive parts). There also were the giant sphenopsid Calamites, the smaller weedy horsetail Sphenophyllum, the tree fern Psaronius, small ferns (Pecopteris, Sphenopteris, Alloiopteris), the conifer relative Cordaites, giant club moss relatives Cyperites, Lepidodendron, Lepidophloios, and Sigillaria (up to 6 feet in diameter!), and other, smaller club mosses (Lycopodites, Bothrodendron).
Here are some giant club mosses in the Field Museum exhibit.
Most bizarre among the diverse aquatic animals was the Tully monster (Tullimonstrum gregarium), first found by amateur fossil collector Francis Tully, Illinois’ state fossil, and only known from this area. There were horseshoe crabs (Palaeolimulus, Euproops), freshwater fish (Rhabdoderma oxiguum, Conchopoma edesi, Elonichthyes peltigerus, Platysomus circularis), and mollusks, as well as a lamprey-like fish, Actinopterygian fishes (Elonichthys pettigerus, Platysomus circularis), polychaete worms (Astreptoscolex anasillosus, Escorites zelus, and others), shrimps (Belotelson sp., Kallidecthes richardsoni, Acanthotelson stimsoni, and others), a sea cucumber (Achistrum sp.), a nematode (Nemavermes mackeei), a chiton (Glaphurochiton concinnus), ribbon or priapulid worms (Archisymplectes rhothon, Priapulites konecniorum), the arrow worm Paucijaculum samamithion, the spoonworm (phylum Echiura) Coprinoscolex ellogimus, jellyfish (Essexella asherae, Octomedusa pieckorum, Anthracomedusa turnbullii), cephalopods, brachiopods (Lingula sp.), the scallop Aviculopectin mazonensis, as well as several “mystery animals” of unknown affinities.
The Field Museum model forest includes a millipede you could put a saddle on!
Land animals included centipedes, millipedes (the giant millipede Arthropleura cristata was a flat species, 16” wide and more than 6 feet long), scorpions, cockroaches (Platymylacris paucineruis) and their relatives (Gerarus danielsi, G. vetus), and spider-like arachnids. There were amphibians (Amphibamus grandiceps, A. yelli).
The upland trees, less well known, were different from those in the swamps, and included the genera Megalopteris and Lesleya. An upland animal was the scorpion Labriscorpio alliedensis.