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The coelacanth was long considered a "living fossil" because scientists thought it was the sole remaining member of a taxon otherwise known only from fossils, with no close relatives alive, [8] and that it evolved into roughly its current form approximately 400 million years ago. [1]
The Devonian (/ d ə ˈ v oʊ n i. ən, d ɛ-/ də-VOH-nee-ən, deh-) [9] [10] is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at 419.62 million years ago (), to the beginning of the succeeding Carboniferous period at 358.86 Ma.
End Ordovician: 440 million years ago, 86% of all species lost, including graptolites; Late Devonian: 375 million years ago, 75% of species lost, including most trilobites; End Permian, The Great Dying: 251 million years ago, 96% of species lost, including tabulate corals, and most trees and synapsids
Fish began evolving about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys.
[398] [399] These remains were later named Ichthyotitan and it has been estimated to reach up to 25 m (82 ft), which makes it the largest ichthyosaur and the largest marine reptile ever. [400] Another large ichthyosaur was the Late Triassic Shastasaurus sikanniensis at 21 m (69 ft) in length [401] [402] and 81.5 t (180,000 lb) in weight. [403]
They appeared in the Cambrian, about 510 million years ago, and became extinct towards the end of the Devonian, about 377 million years ago. Initially Ostracoderms had poorly formed fins, and paired fins, or limbs, first evolved within this group. They were covered with a bony armour or scales and were often less than 30 cm (12 in) long.
The order Arthrodira belongs to the class Placodermi, the large group of extinct prehistoric armored fish that is thought to have diverged over 400 million years ago from all sharks and bony fishes (and thus also all subsequent tetrapods, including mammals, birds, reptiles and amphibians).
The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.