PREHISTORIC LAKE COUNTY

Most of Lake County's rocks were created during the Silurian Period, around 420 million years ago. They formed as the mud of an ancient sea bottom slowly built up over time. In this sea, corals grew, bug-like trilobites scampered about, and sea scorpions swam overhead. Every once in a while, the skeletons of these creatures were buried in the mud. Over time, the skeletons turned to stone and became fossils. Fossils are the clues that scientists use to understand the past. Look closely at the fossils here, you're looking at the ancient residents of Lake County.

The fossil rock in the Dunn Museum collection was found in Lindenhurst in 1957 during excavation for a new home site. On the rock are fossils of the Silurian Period, including brachiopods, cephalopods and other ancient sea creatures.

Tullimonstrum gregarium, commonly known as the Tully monster, swam the shallow seas 310 million years ago. Their fossil remains were found in the late 1950s right here in Illinois. Growing to about a foot in length, the cryptic creature had a torpedo-shaped body, eyes at the ends of a long bar, a trunk-like snout that ended in a toothy claw-like structure, and—at the other end of the body—large fins. A swimming sausage with a bite—unlike anything ever discovered!

Little Coelophysis lived during the Triassic Period, around 215 million years ago. It was one of the very first dinosaurs. Eight-foot-long Coelophysis had a mouth full of sharp teeth and fingers ending in long claws. It was a fierce meat eater. With skeletons found in New Mexico and footprints discoved in New York and Connecticut, it likely also lived here in Illinois.

Dryptosaurus lived about 67 million years ago. The name means "tearing lizard", which actually refers not to the teeth of the dinosaur, but to the eight inch claws on the hands. Fossils for this dinosaur were first discovered in 1866. Watch the video to learn more about the Dryptosaurus!

Hadrosaurus lived alongside Dryptosaurus during the Cretaceous Period. The name Hadrosaurus means "heavy or bulky lizard." It was about 10 feet tall at the hips and about 25 feet long. This herbivore was part of a group of dinosaurs called Hadrosaurs, which include those you might know better as the "duck-billed" dinosaurs. This dinosaur was the first fairly complete dinosaur skeleton ever discovered! It was discovered in 1858 in New Jersey, and became the first mounted dinosaur skeleton displayed anywhere in the world in 1868.

For 150 million years, the dinosaurs ruled. But 65 million years ago, a mass extinction wiped out half of all life, including all dinosaurs except birds. The cause of the extinction is not certain, but it was probably the result of a massive asteroid impact and volcanic activity. Among the survivors were mammals. Soon, they evolved to fill the spaces left empty by the dinosaurs. They got bigger, and the mammals we know today began to appear.

Massive mastodons and mammoths munched on trees and grasses, while saber-toothed cats and dire wolves stalked their prey. Sloths were the size of elephants, and beavers were the size of bears.

Around 12,000 years ago, nomadic bands of hunter-gatherers arrived in this area. These people, the Paleoindians, traveled in small family groups along a seasonal route that extended hundreds of miles. Evidence from a site in Barrington Hills suggests that a Paleoindian camp once existed there. The work of archaeologists revealed that the site’s inhabitants made weapons for hunting.

At first glance, mastodons and mammoths may look very similar based on their long dark hair over a fine layer of wool. But they are actually quite different.

Mastodons have larger skulls with a flatter forehead.

The tusks of mastodons are straighter than those of mammoths.

The teeth of mastodons have big, bulbous cusps, whereas those of mammoths have many small, low ridges. Scientists think this difference is related to diet: mastodons ate from shrubs and trees, while mammoths ate grasses.

Mastodons have shorter legs and longer bodies.

The Earth’s climate fluctuates between periods of warmer and cooler climates. Changes in the circulation of the ocean’s water, as well as changes in the amount of the sun’s energy reaching Earth through its atmosphere, can lead to cooler climates. These cooler temperatures allow glaciers to form.