CompuServe Messages

Hi, Simon– The oldest fossils I know of are sea life, microbes too small to see without a microscope. In the Cambrian period at the start of the Paleozoic era, many different sorts of sea life that left larger fossils appeared. Brachiopods (like lopsided clams), crinoids (sea lilies, an animal that could easily be mistaken for a plant), ammonites (sort of like a squid in a spiral seashell), trilobites (a little like a giant sea-going sowbug), and some early fishes were among the sea animals of the Paleozoic (which means "old life") era. Many of these became extinct at the end of the Paleozoic; the trilobites were gone before the time of the dinosaurs (the Mesozoic or "middle life" era). During the Mesozoic, descendants of animals that had appeared on land during the Paleozoic became sea animals again, like the ichthyosaurs and plesiosaurs. The ammonites and fish were still common in the seas. At the end of the Mesozoic, the dinosaurs and many other creatures (including the ammonites) became extinct. The mammals rose in the modern era, the Cenozoic age. Some of them also returned to the sea, including the whales and dolphins. Fishes, of course, are still around, as are nautiluses, relatives of the ammonites. Whatever the era, when these creatures die and fall to the sea bottom, they may be covered with sediments (mud washed into the sea by rivers, other dead creatures, and so on that also fall to the bottom). The sediments often pile so deep over time that the sediments underneath are squeezed so they become rocks. The dead creatures in those sediments that became rocks are the fossils. These kinds of rocks are called sedimentary rocks. Look for layering in the rock as a clue that it might be a sedimentary rock. Shale and limestone are two kinds of sedimentary rock in which one may find sea fossils. There are many Paleozoic fossils of sea life to be found in Pennsylvania. People here who collect fossils in Pennsylvania may be able to tell you where to look for them. –Doug