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The first ancestors of fish, or animals that were probably closely related to fish, were ''[[Pikaia]]'', ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]''.{{r|Shu1999}}{{sfn|Dawkins|2004|p=357}} These three [[Genus (biology)|genera]] all appeared around 530 [[mya (unit)|Ma]]. ''Pikaia'' had a primitive [[notochord]], a structure that could have developed into a [[vertebral column]] later. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.{{r|Waggoner2011}} All of these early vertebrates lacked [[jaw]]s in the common sense and relied on filter feeding close to the seabed.{{sfn|Haines|Chambers|2005}}
These were followed by indisputable fossil vertebrates in the form of heavily armoured fishes discovered in rocks from the [[Ordovician]] Period 500–430 [[mya (unit)|Ma]]. The colonisation of new [[Ecological niche|niche]]s resulted in
The first [[Gnathostomata|jawed vertebrates]] appeared in the late [[Ordovician]] and became common in the [[Devonian]], often known as the "Age of Fishes".{{sfn|Encyclopædia Britannica|1954|p=107}} The two groups of [[bony fishes]], the [[actinopterygii]] and [[sarcopterygii]], evolved and became common.{{sfn|Berg|2004|p=599}} The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the [[Placodermi]], a group of armoured fish that dominated much of the late [[Silurian]]. The Devonian also saw the rise of the first [[Labyrinthodontia|labyrinthodonts]], which was a transitional between fishes and [[amphibians]].
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