People, as curious primates, dote on concrete objects that can be seen and fondled. God dwells among the details, not in the realm of pure generality. We must tackle and grasp the larger, encompassing themes of our universe, but we make our best approach through small curiosities that rivet our attention–all those pretty pebbles on the shoreline of knowledge …
What are they? To geologists and paleontologists these fascinating beads are as common as dirt. They know them as fossils of crinoids, strange creatures that throve in the midwest 500 to 300 million years ago–long before the Indians, long before the lake, long before the glaciers came that formed the lake, long before the dinosaurs. Crinoids were plantlike marine animals that lived in vast meadows rooted to the bottom of the warm, shallow coral seas that washed over Chicago (and large portions of the earth) as long as half a billion years ago–roughly the last 10 percent, by the way, of the earth’s lifetime.
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Amazingly adaptive, crinoids are survivors. They haven’t lived in this area since it was the bottom of a saltwater sea, but of 5,000 identified crinoid species, some 500 thrive today from the Arctic Ocean to the South Pacific, from Greenland to the Caribbean. Crinoids survived the cataclysmic extinctions that mark major geologic eras, including the great Permian extinction of 250 million years ago, which wiped out practically everything (perhaps 96 percent of all species then living, according to a recent National Geographic), and the better known Cretaceous extinction of 65 million years ago, which did in the dinosaurs.
The buttonlike crinoid fossils that come ashore may seem extraordinary in a handful of common pebbles, but they are the least interesting-looking part of a full crinoid. The buttons are like vertebrae, pieces of the long stalks that held up the crinoids’ strange, magnificent heads, called calyxes. In some forms the calyxes looked like flowers, as suggested by the popular name “sea lilies.” Others had calyxes shaped like ragged-edged tulips or intricately carved goblets. Some smaller species look like miniature palm trees. Others suggest surreal, scaly chicken feet reaching out of the sea bottom. Some look truly ominous, like a crusty octopus when the arms (sometimes called pinnules) are open, or like an alien creature when shut, like the pods in the original Invasion of the Body Snatchers.
Among the most lurid of these prehistoric Chicagoans were trilobites and cephalopods. The trilobites, ancestors to insects, had three-part bodies, armored heads and tails, and three pairs of jointed legs on which they scurried around the seafloor. Judging from their fossils they looked like egg-sized beetles. Ron Vasile thinks the trilobite was the model for the hideous creature in the movie Alien. Though trilobites were once the dominant form of marine life, they did not make it through the Permian extinction.
“The earth is about five billion years old, but the earliest we know anything about this region is about two billion years ago, during Precambrian time. This was a mountain range about the height of Mount Everest. We know this because the steel mills have one-mile-deep disposal wells for acids produced during the steel-making process. At the bottom of these wells is granite, an igneous rock that is in the core of mountain ranges. Its presence this close to the surface of the earth indicates mountain-making activity.
The abundant meadows of crinoids that grew here then–unlike some later free-floating species–were rooted to the seafloor and swayed slightly with the currents, like underwater sunflowers facing the sun. Their arms, or pinnules, curled upstream from the calyx to trap their food–tiny waterborne particles, like microplankton–as it floated by in the current. The particles were caught on the tips of the arms by strings of tiny, sticky balloons called “tube feet.” The tube feet would move the food bucket-brigade style into groovelike canals in the arms, which in turn led to an internal mouth located in the center of the calyx. The animals defecated through an anus located close to the mouth but facing downstream, away from the pinnules and feeding grooves. Because of competition for food from other crinoids, different species grew to different heights.
