Prehistoric fish in the context of "Gnathostomata"

Gnathostomata (/ˌnæθoʊˈstɒmətə/; from Ancient Greek: γνάθος (gnathos) 'jaw' + στόμα (stoma) 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned and lobe-finned, including their terrestrial tetrapod relatives) and cartilaginous fishes, as well as extinct prehistoric fish such as placoderms and acanthodians. Most gnathostomes have retained ancestral traits like true teeth, a stomach, and paired appendages (pectoral and pelvic fins, limbs, wings, etc.). Other traits are elastin, horizontal semicircular canal of the inner ear, myelinated neurons, and an adaptive immune system which has discrete lymphoid organs (spleen and thymus) and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.

It is now assumed that Gnathostomata evolved from ancestors that already possessed two pairs of paired fins. Until recently these ancestors, known as antiarchs, were thought to have lacked pectoral or pelvic fins. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and pelvic basal plates in females — a pattern not seen in any other vertebrate group. The jawless Osteostraci are generally considered the closest sister taxon of Gnathostomata.

Placoderms (from Ancient Greek πλάξ [plax, plakos] 'plate' and δέρμα [derma] 'skin') are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates (hence the name), and the rest of the body was scaled or naked depending on the species.

Placoderms were among the first jawed fish (their jaws likely evolved from the first pair of gill arches), as well as the first vertebrates to have true teeth. They were also the first fish clade to develop pelvic fins, the second set of paired fins and the homologous precursor to hindlimbs in tetrapods. 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.

Otodus megalodon (/ˈmɛɡələdɒn/ MEG-əl-ə-don; meaning "big tooth"), commonly known as megalodon, is an extinct species of giant mackerel shark that lived approximately 23 to 3.6 million years ago (Mya), from the Early Miocene to the Early Pliocene epochs. This prehistoric fish was formerly thought to be a member of the family Lamnidae and a close relative of the great white shark (Carcharodon carcharias), but has been reclassified into the extinct family Otodontidae, which diverged from the great white shark during the Early Cretaceous.

While regarded as one of the largest and most powerful predators to have ever lived, megalodon is only known from fragmentary remains, and its appearance and maximum size are uncertain. Scientists have argued whether its body form was more stocky or elongated than the modern lamniform sharks. Maximum body length estimates between 14.2 and 24.3 metres (47 and 80 ft) based on various analyses have been proposed, though the modal lengths for individuals of all ontogenetic stages from juveniles to adults are estimated at 10.5 meters (34 ft). Their teeth were thick and robust, built for grabbing prey and breaking bone, and their large jaws could exert a bite force of up to 108,500 to 182,200 newtons (24,390 to 40,960 lbf).

Haikouichthys /ˌhaɪkuˈɪkθɪs/ is an extinct genus of craniate (animals with notochords and distinct heads) that lived 518 million years ago, during the Cambrian explosion of multicellular life. The type species, Haikouichthys ercaicunensis, was first described in 1999. Haikouichthys had a defined skull and other characteristics that have led paleontologists to label it a true craniate, and even to be popularly characterized as one of the earliest fishes. More than 500 specimens were referred to this taxon and phylogenetic analyses indicates that the animal is probably a basal stem-craniate. Some researchers have considered Haikouichthys to be synonymous with the other primitive chordate Myllokunmingia, but subsequent studies led by the British paleontologist Simon Conway Morris identified both genera to be distinct, separate taxa on the basis of different gill arrangement, the absence of branchial rays in Myllokunmingia and the myomeres having a more acute shape in Haikouichthys.