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The Geologic Record of Animal Fossils
The fossil record left by organisms reaches back more than 3.5 billion years and represents every single kingdom. This fossil record does not give a perfect representation even though about 250,000 animal species have been named from the marine fossil record. From this record it can be estimated that the first animal showed up about 600 million years ago. Some parts of the animal fossil record are more complete than others. For example, there is an almost complete sequence of animal fossils for the Miocene lake fish but there are quite a few incomplete and poor fossils for soft-bodied worms. The best and most complete fossil record for animal fossils can be seen with the invertebrates that are composed of calcium carbonate. Animals that have more complicated skeletal structures are easy to find in the fossil record but harder to study when their skeleton is not intact.
Evolution of Animal Fossils
About 60% of all marine animals can be found as fossils in the Venedian and Cambrian periods but only about 5% are found in the Permian an Pleistocene. The Venedian and Cambrian periods are known as the first half of time for living animals and the Permian and Pleistocene are the last half. Animals that were in smaller classes tended to go extinct before the last part of the Paleozoic era and animals that were successful during this time are usually found living in modern times. An example of one such successful class is the Trilobita. Many of the more soft-bodied animals were very successful even though there is a limited fossil record for their evolution. Examples of these classes are the Nemertina, Priapulida, and the Sipunculida. It is important to recognize that about 20% of the marine animal fossils all are found in the Cambrian Burgess Shale, the Carboniferous Francis Creek Shale in the Mazon Creek area and the Devonian Hunsruck Shale.
On land, there is a similar lack of certain animal fossil families. Over 50% of the 39 classes of animals have only 7 fossil families. Almost half of the 2500 families of animal fossils are in the class Insecta. The class Insecta has the most families all the way from the Carboniferous up to modern times, even more than all of the vertebrates together. The earliest animal fossils found on land come from the Silurian in contrast to the marine animal fossils which are found starting earlier in the Venedian. More than 2/3 of the animal classes show up before the last half of the Carboniferous.
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