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.
Mesozoic Era, Triassic Period (251-208 million years ago)
The Mesozoic Era was named in 1841 for the fossil life that characterized it, literally translated “middle life.” It is divided into three periods, the first of which is the Triassic. The Triassic Period was named for the fact that, as then understood (1834), it consisted of 3 distinct rock layers. It ended with a mass extinction.
Life on Earth. The end of the Paleozoic Era marked a major divide in marine ecosystems. The major extinctions that ended the Permian Period may have acted to remove dominant species that had held others back by niche pre-emption. After the start of the Triassic, sessile filter feeders became less dominant, and mobile species and ones living on other species became more important. In particular, the superior ability of clams to metabolize toxins, coupled to their mobility, may have allowed them to survive when the attached brachiopods and bryozoans died en masse (Paleobiology 33:397-413; Science. 322:359). Complexity of marine ecosystems in general increased (Science 314:1289). Some data indicate that oxygen climbed to levels matching today’s, but then plummeted again, hitting a minimum of just over 10% coinciding with a mass extinction at the Triassic-Jurassic boundary (Science 316:557). Other results indicate that oxygen never dropped below 15% in the Mesozoic (Science 321:1197-1200).
Ammonoids diversified greatly during the Triassic, filling a broad ecological range. Most became extinct at the close of the Triassic, however, leaving a single family to diversify even more in the following Mesozoic periods. Clams also diversified in the Triassic. Rugose corals became extinct, and a new group of corals appeared. These were forming reefs by the middle Triassic, ending a long gap that began with the wiping out of reefs in the late Devonian.
On land, the first true flies (Diptera), thrips, aphids and possibly caddis flies and termites appeared. This period was a time of transition for plants. Plenty of Paleozoic groups such as ferns, seed ferns, cycads, club mosses and horsetails remained. Conifers and ginkgoes became increasingly important as the Triassic passed.
Among early Triassic vertebrates, therapsids, the so-called mammal-like reptiles, were important. They included large browsers, medium-sized root feeders, and large predators. Dicynodonts in large numbers but reduced diversity were among the survivors of the Permian extinctions that dominated the early Triassic. These squat, short-tailed herbivores included Lystrosaurus, mentioned in the Permian chapter. Cynodonts were another group of therapsids, consisting of a variety of herbivores and predators that were the closest relatives of the mammals. The first mammals, small and rodent-like, emerged from this group at about the same time the dinosaurs appeared, in the late Triassic.
Reptiles increasingly dominated the Triassic vertebrate fauna as time went by, both in numbers and species diversity. Most of these groups are now long extinct. Procolophonids were small chubby herbivores. Relatives of today’s lizards and turtles became more common. Reptiles known as archosaurs and their relatives were the dominant terrestrial vertebrates through most of the period. It has been speculated that a major reason for this was that the dry continental climate gave the advantage to animals that could conserve water by eliminating nitrogen waste in the form of the more solid uric acid rather than watery urea, like the therapsids and mammals. Some archosaurs, among them the aetosaurs, were on all fours. The herbivorous aetosaurs were the first heavily armored terrestrial vertebrates. Rhynchosaurs were large barrel-bodied herbivores with beaklike jaws on their round heads.
There also were a variety of predators including groups that resembled dinosaurs and gave rise to them, the first crocodiles, and the phytosaurs. The phytosaurs, which had appeared by middle Triassic times, were similar in appearance to today’s crocodiles, the most obvious difference being the location of their nostrils near their eyes rather than at the tips of their snouts. They spread around the world by the end of the Triassic. True crocodilians appeared by the late Triassic, but most of them were terrestrial predators that ran actively on all fours; today’s crocodile niche then was filled by the phytosaurs. Early ichthyosaurs (dolphin-like reptiles) appeared by the middle Triassic. Turtles appeared by the late Triassic. In the early Triassic there are fossils of nothosaurs, semi-aquatic reptiles which in post-Triassic times had plesiosaurs as descendents. The first pterosaurs appeared in the Triassic. They are not dinosaurs, though they are close relatives of them.
The first undisputed dinosaurs appeared in the late Triassic, evolving from bipedal archosaur predators, probably in South America. Fossil footprints found in Poland and reported in 2010 suggest that the line that led to the dinosaurs had split from other archosaurs as early as 2 million years after the end-Permian extinction. The first dinosaurs were all in the order Saurischia. Saurischians were abundant in the eastern U.S., and left many footprints in New England. Many of the first dinosaurs were theropods (carnivorous, bipedal, long-necked and long-tailed). Body sizes ranged from abundant 3- to 4-footers to the largest 16-20-footers.
Freshwater ecosystems included small freshwater sharks, coelacanths, lungfishes and a variety of large and small amphibians, though these declined toward the end of the Triassic.
The Four Corners area has deposits that give a good idea of late Triassic life. Uplands had forests of araucarian “pines” well known from Petrified Forest National Monument, and true ferns in the understory. There were large amphibians, mussels, lungfish, freshwater sharks, coelacanths, aetosaurs, running crocodiles, the predatory dinosaur-like Postosuchus which was the top carnivore, phytosaurs in swamps, saurischian dinosaurs including the small predator Coelophysis, and a few primitive mammals. In many parts of the world the prosauropods, the first herbivorous dinosaurs, with their long necks and ability to reach high vegetation, became dominant in numbers or biomass. Early members of the other dinosaur order, Ornithischia, appeared by the Late Triassic. But there was a long period in the Late Triassic when dinosaurs coexisted with other archosaurs similar to their ancestors, and dinosaurs were not as common in the Late Triassic, so their rise to dominance was gradual (Science 317:358).
The end of the Triassic is marked by the extinction of all archosaurs other than dinosaurs, pterosaurs and crocodylomorphs. The possible drop in atmospheric oxygen is regarded as significant here. The dinosaurs had evolved a new air-sac system (retained in modified form in modern birds) which may have permitted their survival and also set the stage for giant body sizes in the Jurassic and Cretaceous (Science 316:557). However, a variety of possibilities remain under discussion for explaining the end-Triassic extinctions. A 2008 study finds that, just as was the case at the end of the Permian, the end-Triassic extinctions occurred at the same time as a massive volcanic eruption, this one in the central Atlantic (Science 320:434-435). Massive lava flows at that time are associated with the continental split that ultimately produced the Atlantic Ocean.
Local landscape. There are no Triassic or Jurassic rocks in Illinois. The nearest Triassic bedrock is in south central Pennsylvania. Reconstructions of North America, which still was united with the other continents, indicate that in the early Triassic, the Appalachian chain was high and mountainous, sloping down west and north into our area, with a red sandy alluvial plain beginning just west of us and extending west.
Bedrock topography suggests that throughout most of the Mesozoic a major river, the Teays, drained westward from the Appalachians, cutting across the center of Illinois, and it is likely that some local stream or streams flowed south or west as Teays tributaries.
Our area was very close to, perhaps just north of, the equator. By the late Triassic, Illinois had drifted to about 10° N latitude. New volcanoes and faulting and uplift were occurring to the East, and our area was even more elevated, with the beginning of the alluvial plain farther to the west. The warmest climate in all of the time of Earth’s life began in the middle Triassic and lasted through the Cretaceous.
Local life. Our area was dry land, in a warm equatorial climate, but the kind of vegetation is difficult to speculate about. We have to base our understanding on fossils in New England and the Four Corners area, distant but on either side of us. Likely there were a variety of insects and some therapsids and early archosaurs, as well as saurischian dinosaurs (Coelophysis, Ammosaurus) and early mammals. Both eastern (North Carolina to Nova Scotia) and southwestern U.S. formations from the late Triassic share many groups, including phytosaurs, amphibians, ornithiscian and prosauropod dinosaurs. Worldwide examinations suggest that late Triassic-early Jurassic dinosaur faunas typically included “one or two sauropodomorphs (such as prosauropods), one or two theropods (two-legged predators), and possibly one ornithischian.”