Greererpeton is better known from West Virginia--where over 50 fossils have been discovered--but specimens 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, living its entire life in the water (which explains why it was equipped with extremely vestigial limbs).
Yet another eel-like tetrapod of the Carboniferous
period, Lysorophus lived around the same time as Greererpeton
, and possessed a similarly eel-like body and vestigial limbs. This tiny creature was unearthed in Illinois' Modesto Formation, in the southwest corner of the state.
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 was equipped with a two-inch-long proboscis studded by eight tiny teeth, which it probably used to suck up small organisms from the sea floor.