The Dinosaurs and Prehistoric Animals of Illinois

Illinois may be home to one of the world's first-class cities, Chicago, but you'll be sad to learn that no dinosaurs have ever been discovered here—for the simple reason that this state's geologic sediments were being eroded away, rather than actively deposited, during most of the Mesozoic Era. Still, the Prairie State can boast a significant number of amphibians and invertebrates dating to the Paleozoic Era, as well as a handful of Pleistocene pachyderms, as detailed in the following slides. These slides focus on Illinois, but dinosaurs have been discovered across the U.S.

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Stanton F. Fink/Wikimedia Commons/CC BY-SA 2.5

The official state fossil of Illinois, Tullimonstrum (the "Tully Monster") was a soft-bodied, foot-long, 300-million-year-old invertebrate vaguely reminiscent of a cuttlefish. This strange creature of the late Carboniferous period was equipped with a two-inch-long proboscis studded with eight tiny teeth, which it probably used to suck up small organisms from the sea floor. Paleontologists have yet to assign Tullimonstrum to an appropriate phylum, a fancy way of saying that they simply don't know what kind of animal it was!

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 Nobu Tamura/Wikimedia Commons/CC BY-SA 3.0

If the name Amphibamus ("equal legs") sounds similar to "amphibian," that's not a coincidence; clearly, the famous paleontologist Edward Drinker Cope wanted to emphasize this animal's place on the amphibian family tree when he named it in the late 19th century. The importance of the six-inch-long Amphibamus is that it may (or may not) mark the moment in evolutionary history when frogs and salamanders split off from the mainstream of amphibian evolution, about 300 million years ago.

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Wikimedia Commons/Public Domain

Greererpeton is better known from West Virginia—where over 50 specimens have been discovered—but fossils of this eel-like tetrapod have also been unearthed in Illinois. Greererpeton most likely "de-evolved" from the first amphibians about 330 million years ago, abandoning a terrestrial, or at least semi-aquatic, lifestyle in order to spend its entire life in the water (which explains why it was equipped with near-vestigial limbs and a long, slender body). 

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Smokeybjb/Wikimedia Commons/CC BY-SA 3.0

Yet another eel-like amphibian of the late Carboniferous period, Lysorophus lived around the same time as Greererpeton (see previous slide) and possessed a similarly eel-like body equipped with vestigial limbs. The fossil of this tiny creature was unearthed in Illinois' Modesto Formation, in the southwest corner of the state; it lived in freshwater ponds and lakes and, like many other "lepospondyl" amphibians of its time, burrowed itself in moist soil during extended dry spells.

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Mammoths and Mastodons


Dantheman9758/Wikimedia Commons/CC BY-SA 3.0

For much of the Mesozoic and Cenozoic Eras, from about 250 to two million years ago, Illinois was geologically unproductive—hence the lack of fossils dating from this vast expanse of time. However, conditions improved tremendously during the Pleistocene epoch, when herds of Woolly Mammoths and American Mastodons tramped across this state's endless plains (and left scattered fossil remains to be discovered, piecemeal, by 19th and 20th century paleontologists).

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Your Citation
Strauss, Bob. "The Dinosaurs and Prehistoric Animals of Illinois." ThoughtCo, Aug. 28, 2020, Strauss, Bob. (2020, August 28). The Dinosaurs and Prehistoric Animals of Illinois. Retrieved from Strauss, Bob. "The Dinosaurs and Prehistoric Animals of Illinois." ThoughtCo. (accessed March 27, 2023).