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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