The Elopomorpha eels and tarpons appear to be the most basal teleosts. Earlier definition of Chondrostei is now known to be paraphyletic meaning that this subclass does not contain all the descendants of their common ancestor. There were 52 species divided among two orders, the Acipenseriformes sturgeons and paddlefishes and the Polypteriformes reedfishes and bichirs. Reedfish and birchirs are now separated from the Chondrostei into their own sister lineage, the Cladistia. It is thought that the chondrosteans evolved from bony fish but lost the bony hardening of their cartilaginous skeletons, resulting in a lightening of the frame.
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Evolution of lobe-finned fishes Spindle diagram for the evolution of lobe-finned fishes, tetrapods and other vertebrate classes.
Tooth from the sarcopterygian Onychodus from the Devonian of Wisconsin See also: Evolution of fish Lobe-finned fishes sarcopterygians and their relatives the ray-finned fishes actinopterygians comprise the superclass of bony fishes Osteichthyes characterized by their bony skeleton rather than cartilage.
There are otherwise vast differences in fin, respiratory, and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmoid layers in the scales of sarcopterygians. The earliest fossils of sarcopterygians, found in the uppermost Silurian ca Ma , closely resembled the acanthodians the "spiny fish", a taxon that became extinct at the end of the Paleozoic.
In the early—middle Devonian — Ma , while the predatory placoderms dominated the seas, some sarcopterygians came into freshwater habitats.
In the Early Devonian — Mya , the sarcopterygians split into two main lineages: the coelacanths and the rhipidistians. Coelacanths never left the oceans and their heyday was the late Devonian and Carboniferous , from to Ma, as they were more common during those periods than in any other period in the Phanerozoic; coelacanths genus Latimeria still live today in the open pelagic oceans.
The Rhipidistians, whose ancestors probably lived in the oceans near the river mouths estuaries , left the ocean world and migrated into freshwater habitats.
In turn, they split into two major groups: lungfish and the tetrapodomorphs. Lungfish radiated into their greatest diversity during the Triassic period; today fewer than a dozen genera remain. They evolved the first proto-lungs and proto-limbs, adapting to living outside a submerged water environment by the middle Devonian — Ma.
There are three major hypotheses as to how lungfish evolved their stubby fins proto-limbs. The traditional explanation is the "shrinking waterhole hypothesis", or "desert hypothesis", posited by the American paleontologist Alfred Romer , who believed that limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.
They argued that sarcopterygians may have first emerged unto land from intertidal zones rather than inland bodies of water. He argues that limbs may have developed in shallow bodies of water in woodlands as a means of navigating in environments filled with roots and vegetation. He based his conclusions on the evidence that transitional tetrapod fossils are consistently found in habitats that were formerly humid and wooded floodplains.
However, studies have found that sarcopterygians developed tetrapod-like limbs suitable for walking well before venturing onto land King , as cited by Pierce et al. The first tetrapodomorphs, which included the gigantic rhizodonts , had the same general anatomy as the lungfish, who were their closest kin, but they appear not to have left their water habitat until the late Devonian epoch — Ma , with the appearance of tetrapods four-legged vertebrates.
Tetrapods are the only tetrapodomorphs which survived after the Devonian. Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during the Permian—Triassic extinction event Ma.