Animal life was abundant during the Ordovician (450-500 million years ago) when the limestone/dolomite of the cave was deposited. We find relatively few fossils here because the sediment was reworked by various bottom feeders present at that time. In essence, the bugs and worms pretty much ate everything organic. In other areas close to the cave (Punkin Center, Ellsworth, River Falls, Eau Galle Lake) fossils are abundant and give us a picture of life at that time.
Common Fossil Types
A. Trilobite. The trilobite was the most abundant creature on earth during the Ordovician. There were literally hundreds of species, found throughout the world, ranging in size from a few millimeters to 1/2 to 1 meter in length. Trilobite fossils are so common in Wisconsin, the legislature made it our state fossil. They looked like a common-day pill bug and were able to roll into a ball to protect themselves. Trilobites had excellent eyesight and very complex eye structure for that time. Trilobites were bottom feeders and crawled along the floor of the sea eating organic material.
B. Crinoids. Crinoids were a common filter feeder. They resembled a lily with a stem and flower-like head. Crinoids were anchored to the seafloor and lived close to the shore but in water deep enough so intense wave action would not break them off. They filtered water through the “flower head” or calyx eating microscopic organisms. Whole crinoid fossils are rare but the small Cheerio-like segments of the stem are very abundant.
C. Cephalopod. Cephalopods swam through the oceans and seas much like present-day squids. Their shells were long and cone-shaped with one end open. This is the end where the animal’s body could be seen. Cephalopods grew to a length of 20-30 feet. The name means “head-foot”, cephal=head, pod-foot. In other words, the head and feet were on the same end. Cephalopods were carnivores.
D. Gastropod. Gastropods were also bottom dwellers. They looked like today’s snails. They also fed off decaying organic matter or were filter feeders. Their name means “stomach-foot”, gastro=stomach, pod=foot. Their stomach was located in their foot.
E. Trace fossils. Throughout the cave are found trace fossils. These are not fossils of the animals themselves but are traces that the critters left behind, sort of like a footprint. The trace fossils found in the cave were made by small worms living just below the surface of the seafloor. They were analogous to today’s earthworms in that they burrowed into the seafloor filtering out organic material. Because the bodies were soft and without shells, fossils of the worms are extremely rare. The burrows though are very common. Most of the burrows found in Crystal Cave are found in the New Richmond Sandstone.
F. Stromatolites. Stromatolites are essentially piles of algae. It is a very ancient form of life and has been found in rocks over a billion years old. Algae will grow in layers, one on top of the other, building mounds several feet high. The fossils show how the algae would drape over the older material as it grew. Stromatolites can be seen growing along the northern coast of Australia even today. Just imagine acres of green “toadstool-like” mounds and you have imagined stromatolites.Return to Cave Geology