Illinois State Museum

cretaceous plant fossil
Cretaceous plant fossil

Missing Pieces - What's Here—What's Not?

The fossil record of neighboring states gives us an idea of what was going on in Illinois. Trilobites, rugose and tabulate corals, and blastoids all disappeared in the Permian extinction. Early ancestors of mammals dominated at the beginning of the period. Dinosaurs appeared around 225 million years ago and then went extinct 160 million years later — leaving their descendants, the birds.

Flowering plants appeared around 130 million years ago. Grasses appeared by 65 million years ago. Ancestral forms of rhinoceros, horse, and camel arrived 55 million years ago. They were joined by ancestral elephants around 15 million years ago. The smaller animals were preyed upon by carnivores, such as creodonts and giant birds. These predators were eventually replaced by modern carnivore families.

Where did Mammals Come from?
Pelycosaurs appear in the fossil record around 300 million years ago. They were the dominant land vertebrates by 290 million years ago until their extinction at the end of the Permian. The first true mammals descended from pelycosaurs and smaller, more agile therapsids. Mammals appeared around 230 million years ago, but they looked very different than their ancestors. They were tiny, hairy, and most likely active only at night.