Fish may have evolved from an animal similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case. Vertebrates, among them the first fishes, originated about 530 million years ago during the Cambrian explosion, which saw the rise in organism diversity. Outdated evolutionary view of continual gradation (click to animate) The early vertebrate Haikouichthys, from about 518 million years ago in China, may be the "ancestor to all vertebrates" and is one of the earliest known fish. The first ancestors of fish, or animals that were probably closely related to fish, were Pikaia, Haikouichthys and Myllokunmingia. These three genera all appeared around 530 Ma. Pikaia had a primitive notochord, a structure that could have developed into a vertebral column later. Other early candidates in the fossil record for the title of "first vertebrate" or "first fish" are the Myllokunmingia and the Haikouichthys ercaicunensis. 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. All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed. These were followed by indisputable fossil vertebrates in the form of heavily armoured fishes discovered in rocks from the Ordovician Period 500Ц430 Ma. The colonisation of new niches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm Dunkleosteus, which could grow 7 meters long. The Devonian Period (395 to 345 Ma) brought in the changes that allowed primitive air-breathing fish to remain on land as long as they wished, thus becoming the first terrestrial vertebrates, the amphibians. The first jawed vertebrates appeared in the late Ordovician and became common in the Devonian, often known as the "Age of Fishes". The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common. The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hag
ish, 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 labyrinthodonts, which was a transitional between fishes and amphibians. The reptiles appeared from labyrinthodonts in the subsequent Carboniferous period. The anapsid and synapsid reptiles were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. The dinosaurs gave rise to the birds in the Jurassic. The demise of the dinosaurs at the end of the Cretaceous promoted expansion of the mammals, which had evolved from the therapsids, a group of synapsid reptiles, during the late Triassic Period. The later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans. The first animals to venture onto dry land were arthropods. Some fish had lungs and strong, bony fins and could crawl onto the land also. Neoteny (pron.: /ni?t?ni/) /ni?tni/, or /ni?t?ni/ also called juvenilization, is one of the three ways by which pedomorphism can arise. Pedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological (or somatic) development of an organism (typically an animal) is slowed or delayed. In contrast, in progenesis, sexual development occurs faster. Both processes result in pedomorphism. Ultimately this process results in the retention, in the adults of a species, of juvenile physical characteristics well into maturity and pedogenesis (paedogenesis), the reproduction in a neotenized state. Neoteny is one of three dimensions of heterochrony, or the change in timing of developmental events: acceleration (faster) vs. neoteny (slower), hypermorphosis (further) vs. progenesis (not as far), and predisplacement (begins earlier) vs. postdisplacement (begins later). The word neoteny is borrowed from the German Neotenie, the latter constructed from the Greek (neos, young) and ? (teinein, tend to). The adjective form of the word is either "neotenous" or "neotenic". The opposite of neoteny is either called "gerontomorphic" or "peramorphic".