Prehistoric Life 10

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.

Permian Period (286-251 million years ago)

The Permian Period was named for the Perm District of Russia (1841), added because the fossils found there, though distinctive, were recognizably between those of the Carboniferous and Triassic periods. It began with a glacial period, and the fusion of the continents into the supercontinent of Pangaea. Its ending (and that of the Paleozoic Era) was a massive extinction of as many as 95% of species, now correlated with the largest known volcanic eruption in the history of the Earth, in Siberia.

Life on Earth. Trilobites became rare during the Permian, and the last of them vanished when it closed. This was the last time in which brachiopods were important, continuing to diversify during the Permian, but having many extinctions, and becoming relatively unimportant after the Permian. Snails and clams were relatively unimportant, but ammonoids (octopus relatives living in coiled shells) became widespread and diverse.

The drier climate of the Permian led to the rapid decline of the giant club moss and horsetail forests. Conifers, cycads, ginkgoes and seed ferns, which better protected their seeds from drying out, became dominant.

Ostracods and insects became more abundant. The proto-dragonflies still were around, e.g. in Kansas, and the modern Odonata, which may have evolved from them, appeared early in the Permian, as did the true bugs, scorpion flies, caddis flies and Neuroptera.

Here is part of an exhibit on Permian animals at the Field Museum of Natural History. The predator Dimetrodon is in the upper left corner.

There were two important groups of synapsids in the Permian. The pelycosaurs (e.g., carnivorous Dimetrodon and herbivorous Edaphosaurus, both known from Texas) often had long, sometimes branched spines down their backs, usually depicted as supporting a sail-like structure that they possibly used in thermoregulation. Pelycosaurs became extinct in the last half of the Permian. The other synapsids were the therapsids, often informally called mammal-like reptiles, the mammalian ancestors. The therapsids are thought by most to have descended out of the pelycosaurs; they appeared at about the time the pelycosaurs vanished. Cynodonts are the therapsid group ancestral to mammals, emerging in the late Permian.

There were reptiles other than synapsids in the Permian. This pareiasaur is an example of another group, the anapsids (again, part of the Field Museum exhibit).

Amphibians declined, though a number of large ones are known from Texas, including Eryops. There was also the bizarre, wide-skulled Diplocaulus of North America. The dry climate resulting from the formation of Pangaea boosted terrestrial vertebrates with shelled eggs; amphibians were at a disadvantage.

The Permian ended with massive extinctions, apparently the most severe the world has seen, with possibly only 5-10% of marine species and 10-35% of terrestrial vertebrates surviving. The cause is unknown, but various climatic and geological upheavals would have accompanied the collision and fusion of continents that was happening then. Two recent studies suggest that a sudden major release of methane gas may have been the direct cause of the extinctions (Science 301:1168). Other studies closely tie the extinctions to the time of the most massive volcanic eruption in Earth’s history, in Siberia (Science 305:1705, 320:434-435). Toxic hydrogen sulfide, another gas that could have been released in large amounts by volcanic eruptions, also spiked in the atmosphere at that time (Science 307: 706). The Permian was a time of stress also because atmospheric oxygen levels appear to have dropped from 30% in the early Permian to only 12% in the late Permian. This would have limited terrestrial animals to isolated pockets of habitat at low altitudes. In addition to measures of oxygen, the fact that one of the few surviving therapsids was a tunneling species (Lystrosaurus), presumably pre-adapted to low oxygen, provides support for low oxygen as a contributing factor (Science 308:398; see also Science 309: 2202). This is underlined by the fact that at least 90% of surviving terrestrial vertebrates in South Africa and Antarctica were Lystrosaurus, which also is known from South America, China, and Russia.

Local landscape. The Permian is the only Paleozoic period with no remaining deposits anywhere in Illinois (the closest areas with Permian bedrock are in the southeastern corner of Nebraska and in southeastern Ohio). Our area was land then, the seas well east and west of us. There was geological activity, though, as a magma body moved into Mississippian limestones during the Permian, creating Illinois’ fluorspar deposits and giving us our state mineral. This event probably was the result of the collision between the northern supercontinent of Laurasia with southern Gondwana to form Pangaea. Much as today’s Himalayas are building from India’s push into south Asia, a Pangaean mountain range (persisting in part as the Appalachians) extended from what is now the Gulf of Mexico to New England and Canada’s Maritime Provinces and beyond. This mountain range had an east-west orientation and was just north of the equator, with the result that our part of North America was a desert. North America was united with South America (the northern bulge of which fitted the Gulf coast) and Africa (Africa’s west coast fitted against South America and North America together).

The ice age described for the Pennsylvanian Period continued into the early Permian, but was limited to what are now the southern continents. Later in the period, atmospheric carbon dioxide rose to the point where its greenhouse function prevented glacial periods from occurring, and this inhibition continued until the Pleistocene. The warming selected for increasingly drought-tolerant plant communities as the Permian progressed, for example favoring conifers at the expense of tree ferns and seed ferns (moisture-loving plants persisted in limited low, moist refuges).

Local life. The best guess may be that we had local savannas to desert woodlands of the plant groups mentioned above, with a variety of insects, and some therapsids and pelycosaurs. Local streams would have had fishes and amphibians. But to the extent that this area was elevated, and the oxygen depletion mentioned above was a factor, there may have been a biological depletion of our central part of the supercontinent.

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Prehistoric Life 9

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.

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