This episode of EarthParts shows how to identify fossils of ancient marine, invertebrate life forms from their shells of CaCO3 found in sedimentary rock from the Cambrian Period forward.

Fossils of ancient animals with mineral hard parts, like calcium carbonate, silica and calcium phosphate, preserve well in the geologic record. Shells don't rot, unlike flesh, so they tend to last in sedimentary rock over geologic time.

Most life forms don't end up as fossils, they mostly rot after death or get ripped apart and eaten by predators, scavengers and carrion-eaters. If a cadaver is buried by sediment faster than it can rot or be destroyed, it has a chance of becoming entombed in sediment and preserved as a fossil for geologic time.

Fossils became widespread in the rock record only after animals evolved mineralized hard parts, roughly around the beginning of the Cambrian Period (541 to 485 million years ago). Since then fossils have been commonplace in sedimentary rock, especially limestone and shale. Sandstone can carry fossils but is poor at preserving them most of the time because the grain size is usually too coarse. Limestone is essentially made of planktonic and animal hard parts of calcium carbonate, so limestone tends to preserve fossils especially well.

In this episode we'll look at groups of fossil animals that are representative of their time as dominant members of their contemporary ecology, including: Trilobites, brachiopods, crinoid echinoderms, mollusks including bivalve, gastropod and cephalopod mollusks such as fossil ammonites and belemnites, stromatolite algal nodules, and reef-building organisms such as sponges like the archaeocyathids and stromatoporoids, and cnidarian polyp organisms such as rugose corals and tabulate corals.

After the end-Permian extinction 252 million years ago all rugose corals, all tabulate corals, all archaeocyathids and stromatoporoids and all trilobites went extinct, in the worst mass extinction in Earth's history. After the Great Dying, as it's called now, only a few crinoid and brachiopod species survived, and today they survive in our modern oceans but with little biodiversity. In contrast, after the Great Dying the bivalve, gastropod and cephalopod mollusks expanded in diversity and ecological dominance. With most reef-builders gone, after the extinction new coral-builders evolved, such as scleractinian corals that dominate reefs today.

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