Fish Evolution

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Evolution of Fishes

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Science does not consider fish to comprise a monophyletic group (meaning a clade or group that contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group), and therefore fish evolution should be taken as comprising parallel events. The vast proliferation of fish was possibly due to the acquisition of a hinged jaw, because the jawless fish left very few descendants. Addtionally, lampreys possibly approximate pre-jawed fish. The first jaws are found

in Placodermi fossils. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of these characteristics. Fish may have evolved from a creature similar to a coral-like Sea squirt, (Phylum Chordata, Subphylum Tunicata, also called urochordates) that have larvae that resemble those of primitive fish.

Jawless Fishes

The first unequivocal fishes and the first vertebrates were the ostracoderms that appeared during the Cambrian Explosion some 510 million years ago, and became extinct near the end of the Devonian, some 130 million years later. Ostracoderms were jawless fishes found mainly in fresh water. They were covered with a bony armor or scales and were normally less that a foot in length. The ostracoderms are placed in Class Agnatha together with forms of extant jawless fishes, the lampreys and hagfishes that are believed to be direct descendents of ostracoderms, as are all jawed fishes, or gnathostomes. Paired fins, or limbs, are believed to have evolved within this group.

The putative Agnathan Haikouichthys from the Chengjiang Maotianshan Shales had a defined skull and other characteristics that have led paleontologists to label it a true craniatemay be the earliest known fish in the fossil record.

The placoderms, a group of jawed fishes, appeared by the beginning of the Devonian, about 416 million years ago. They because extinct at somewhere around the end of the Devonian to the beginning of the Mississippian (Carboniferous) some 360 million years ago. Recent studies suggest that the placoderms are a possibly paraphyletic group of basal gnathostomes, and the closest relatives of all living jawed vertebrates. Some Placoderms were small bottom-dwellers, such as antiarchs, while others, the arthrodires, were active predators. Dunkleosteus was the largest and most famous of these. The upper jaw was firmly fused to the skull, but there was a hinge joint between the skull and the bony plating of the trunk region. This allowed the upper part of the head to be thrown back and, in arthrodires, this allowed them to take larger bites.

The acanthodians (also called spiny sharks) look like but were not sharks, and appeared by the late Silurian some 420 million years ago. They became extinct before the end of the Permian some 250 million years ago. Despite being called "spiny sharks," acanthodians predate sharks. However, scales and teeth attributed to this group, as well as more derived gnathostomes such as Chondrichthyes and Osteichthyes, date from the Ordovician (~460 million years ago). Acanthodians were generally small fishes varying from toothless filter-feeders to toothed predators. They were once often classified as an order of the class Placodermi, but more recent evidence places acanthodians nearer to or within the living gnathostomes.

Cartilaginous fishes, class Chondrichthyes, consisting of sharks, rays and chimaeras, appear in the middle Devonian fossil record before about 400 million years ago. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Both the Osteichthyes and Chondrichthyes may have arisen from either the acanthodians or placodermi. A subclass of the Osteichthyes, the ray-finned fishes (Actinopterygii), have become the dominant group of fishes in the post-Paleozoic and modern world, with some 30,000 living species. However, another subclass of Osteichthyes, the Sarcopterygii, including lobe-finned fishes including coelacanths and lungfish) and tetrapods, was the most diverse group of bony fishes in the Devonian. Sarcopterygians are basally characterized by internal nostrils, lobe fins containing a robust internal skeleton, and cosmoid scales.