Graptolite are very common, particularly those which were deposited in mud rocks and shales, as a result are often used as "index fossils" - that is to say that rocks can be dated. Additionally, The particular type of graptolite fossil present in a rock can sometimes used by scientists to estimate the temperature or depth of a region at the time of the Graptolite's lifetime.
Although Graptolite fossils tend to be quite common, they are not always easy to see. Even if seen, many casual observers mistake them for fossil plants, and even Carl Linnaeus (the Swedish botanist who was the principle founder of modern taxonomy) regarded them as "pictures resembling fossils rather than true fossils"
Finally, the presence of two distinct types of Grapolites in the rocks of Europe and North America, just like the two distinct types of Trilobites found in the very same rocks, has been regarded as evidence of an ancient ocean (known as the "Iapetus Ocean") that once lay between the two continents. The Iapetus Ocean is thought to have closed-up again when all the continents joined together in the super-continent of Pangaea - and when a new ocean (the Atlantic Ocean), separating Europe and North America, did eventually appear, it did so along a somewhat different line of separation.
Graptolites were marine colonial animals that lived from the Cambrian period to the Carboniferous periods
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