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<u>index fossil:</u> are commonly found, widely distributed fossils that are limited in time span. Index fossils also known as guide fossils or indicator fossils are fossils used to define and identify geologic periods . Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Index fossils used as guide fossils, indicator fossils or zone fossils , fossils areused to define and identify geologic periods (or faunal stages).
<u>Examples of index fossils include:</u> Ammonites were common during the Mesozoic Era (245 to 65 mya), They were not found after the Cretaceous period, as they went extinct during the K-T extinction (65 mya).
<u>Paradoxides:</u> is a characteristic Middle Cambrian trilobite(fossil of marine arthropode) of the 'Atlantic' (Avalonian) fauna.Avalonian rocks were deposited near a small continent called Avalonia in the Paleozoic Iapetus Ocean. Avalonian beds are now in a narrow strip along the East Coast of North America, and in Europe.
Paradoxides is a genus of very large trilobites found throughout the world during the Mid Cambrian period. One specimen of Paradoxides davidis is 37 cm (15 in). It has a semicircular head, free cheeks each ending with a long, narrow, recurved spine, and sickle-shaped eyes, providing almost 360° view, but only in the horizontal plane. Its elongated trunk was composed of 19-21 segments and was adorned with longish, recurved lateral spines. Its pygidium(caudal shield) was comparatively small.
