Kolata, Dennis R.

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An early inhabitant of Illinois - the trilobite

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r~i~, ~~ c...J. C C ILLINOIS STATE GEOLOGICAL SURVEY Urbana, Illinois 61801 AN EARLY INHABITANT OF ILLINOIS-THE TRILOBITE Dennis R. Kolata GEOGRAM 8 November 19 7 6 Many strange creatures have inhabited Illinois in the past and have left their fossil remains entombed in the rocks that underlie our prairie lands. One such animal is the trilobite (figs. 1 and 2), an extinct marine arthropod that is distantly related to the living crabs, lobsters, and crayfish. Trilobites were among the earliest inhabitants of Illinois. The oldest specimens have been found in Cambrian age rocks formed approximately 500 million years ago (fig. 3). After the Ordovician Period the trilobites slowly declined in abundance and diversity, finally becoming extinct at the close of the Permian Pe-riod, about 200 million years ago. They swam in the warm, shallow seas that covered all of Illinois and most of North America and crawled on and burrowed ;i.n the muddy sea bottoms. As the seas advanced and retreated over a span of about 350 million years (Paleozoic Era, fig. 3), the trilobites slowly evolved-that is, changed struc-turally and functionally through time-into a great variety of forms. They were variously adapted as scavengers, predators, and filter feeders that occupied niches in the level sea bottom and in reef communities. Trilobites lived with sponges, corals, clams, snails, brachiopods, crinoids, and other marine animals. Trilobites are so named because the segments on their upper (dorsal) sur--face usually possess longitudinal furrows that form a three (tri-) lobed division of the body (fig. 4). The central lobe is called the axial lobe, and the two lateral counterparts are called pleural lobes. The cephalon (head) is located at the ante-rior (front) end of the body, a segmented thorax in the middle, and a pygidium (tail) at the posterior end. The dorsal surface consisted of a hard, mineralized protec-tive shield (carapace); it is this part of the shell, or exoskeleton, that is most connn.only preserved in the fossil record. The lower (ventral) surface bore a pair of antennae and numerous pairs of jointed appendages that served as walking, swimming, feeding, and respiratory organs. The ventral surface, however, consisted Fig. 1 (left) -Ceraurus cf. pleurexanthemus Green (xl). Found in Ordo-vician age rocks of the Grand Detour Forffiation, Platteville Group, near Rockford, Winne-bago County, Illinois. Fig. 2 (right) -Calymene celebra Raymond (xl). Dorsal (upper picture) and side (lower picture) views of a specimen found in the Silu-rian age Racine Formation near Grafton, Jersey County, Illi-nois. of relatively soft tissue and rarely is preserved. A typical trilobite is about 2 inches long, but some are less than half an inch in length and giants of the group measure fully 2 feet. T,ike the living crustaceans (crabs, lobsters, crayfish, etc.), trilobites shed their shells periodically in order to grow. In some species a single trilobite produced 27 shells or more. In fact, it is very likely that most trilobite fossils are the discarded shells. Although trilo-bite fragments are rather abundant in some rocks, complete specimens are rare. It was only under the most exceptional condi-tions, such as burial by sediment before or immediately after death, that complete trilobites were preserved relatively un-changed. Because of their unusual and inter-esting appearance, trilobites are among the fossils most sought after for collec-tion and study. Avid collectors continu-ally comb the countryside searching for new trilobite localities. The most produc-tive efforts are made at outcrops of shale, limestone, and dolomite in quar-ries, roadcuts, and natural exposures. The Paleozoic rocks of Illinois have long been known for their abundant and well-pre-0 iil 80 million Cambrian 80 million PROTEROZOIC and ARCHEOZOIC ERAS __>l 41/z billion years @~ Fig. 3 - Geologic time chart. served trilobite fossils. Cambrian age trilobites have been found in a few small outcrops in north-central Illinois and in several cores drilled from deeply buried Cambrian rocks at various localities throughout the state. Some of the better tri-z 0 ...J <( J: a. w (J X <( a: 0 J: I-~ ::> 0 Cl > a. Fig. 4 - Dorsal view of Phacops, a connnon Devonian tri-lobite, -showing various structures (xl). lobite collections have been made from the more accessible Ordovician and Silurian rocks that are exposed in the northern and southwestern parts of the state. Trilobites have been found in some post-Silurian Paleozoic rocks in Illinois, but they are much less abundant and diverse than those in the older rocks. The study of trilobites is not just an aca-demic exercise, because these fossils are useful in determining the relative age of some sedimen-tary rocks. Knowing the relative age is important for economic reasons, particularly where it is necessary to locate and identify strata containing oil, natural gas, coal, and ore deposits. The study of such index fossils and their relationship to the strata in which they are found is called biostratigraphy. c-L 

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An early inhabitant of Illinois - the trilobite

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