Acanthodes

The primitive fish named Acanthodes bronni was the common ancestor of all jawed vertebrates on Earth. A recent reanalysis of a braincase dating back 290 million years shows it was an early member of the modern gnathostomes (meaning 'jaw-mouths') that include tens of thousands of living vertebrates, ranging from fish to birds, reptiles, mammals and humans. Acanthodes, a Greek word for 'spiny', existed before the split between the earliest sharks and the first bony fishes—the lineage that would eventually include human beings. Compared with other spiny sharks it was relatively large, measuring a foot long. It had gills instead of teeth, large eyes and lived on plankton. Professor Michael Coates, a biologist at the University of Chicago, said: 'Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks. 'Our work is telling us the earliest bony fishes looked pretty much like sharks, and not vice versa. What we might think of as shark space is, in fact, general modern jawed vertebrate space.' Fossils have been found in Europe, North America and Australia.

Austrophyllolepis

Austrophyllolepis is an extinct genus of phyllolepid arthrodire placoderm from Middle to Late Devonian freshwater strata of Australia. The type species, A. ritchiei is found in Givetian to early Frasnian-aged freshwater strata near what is now Mount Howitt, Victoria. A second species, A. dulciensis, is found from Middle Devonian freshwater strata from the Dulcie Sandstone of Georgina Basin, Central Australia.

Buchanosteus

Buchanosteus confertituberculatus is an extinct, primitive coccosteinid arthrodire from the Emsian-epoch aged Taemas-Wee Jasper reef of Early Devonian south-eastern Australia.

Bkanodus

Bkanodus is a genus of prehistoric lobe-finned fish which lived during the Early Devonian period (Pragian stage, about 407 to 411 million years ago). Bkanodus have been found in Victoria, and described in 2007 by palaeontologist Zerina Johanson et al. in the Journal of Paleontology.

Coelacanthus

Coelacanthus ("hollow spine") is a genus of extinct coelacanths that first appearing during the Permian period. In fact, this was the first genus of coelacanths ever described, as the order Coelacanthiformes is named after it. Members of the related but extinct suborder Rhipidistia are considered to have been the ancestors of land vertebrates. In some systems of classification, the coelacanths and rhipidistians are considered separate orders, members of the subclass Crossopterygii. They bear a superficial similarity to the living Latimeria, though they were smaller, and had more elongated heads. Individuals grew up to 3 feet in length, and had small, lobed fins, suggesting that Coelacanthus were open-water predators. It was a long-lived genus with a worldwide distribution. Coelacanths appeared about 350 million years ago and were abundant over much of the world; the genus Coelacanthus has been found as fossils in rocks from the end of the Permian, 251 million years ago. They survived the Permian–Triassic extinction event, and eventually died out during the Late Jurassic, around 145 million years ago. Coelacanthus, like other coelacanths, showed a reduction in bone ossification and a general trend toward a marine mode of life.

Enigmatichthys

Enigmatichthys is a genus of extinct semionotiformid bony fish that lived during the Middle Triassic epoch of what is now Australia.

Helicoprion

Helicoprion is a long-lived genus of extinct, shark-like eugeneodontid holocephalid fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls." Helicoprion first arose in the oceans of the late Carboniferous 310 million years ago, survived the Permian–Triassic extinction event, and eventually became extinct during the Early Triassic, 250 million years ago. Until recently, the only known fossils of this animal are the teeth, which were arranged in a "tooth-whorl" strongly reminiscent of a circular saw. As the skeletons of chondrichthyid fish are made of cartilage, including those of Helicoprion and other eugeneodonts, the entire body disintegrates once it begins to decay, unless exceptional circumstances preserve it. It was not until the discovery of the skull of a related genus of shark, Ornithoprion, that it was realized that the tooth-whorl was in the lower jaw. The tooth-whorl represented all of the teeth produced by that individual in the lower jaw, in that as the individual grew, with the older, smaller teeth being moved into the center of the whorl by the appearance of larger, newer teeth. Helicoprion's true physical identity remains hidden in 280 million year old rocks. Comparisons with other eugenodontids suggest that Helicoprion may have grown up to 3-4 m long. For over a century, it was not certain where the tooth-whorl was located in the lower jaw. A 2013 study based on new data places the tooth-whorl at the back of the jaw, where the tooth-whorl occupied the entire mandibular arch. The teeth are symmetrically opposed to one another. Additionally, other extinct fish, such as onychodontiformes, have analogous tooth-whorls at the front of the jaw. While no complete skulls of Helicoprion have been officially described, the fact that related species of sharks had long, pointed snouts suggests that Helicoprion did as well. Fossils of Helicoprion species first appear in Upper Carboniferous marine strata, proliferate greatly during the Permian, and eventually disappear during the Early Triassic. Fossils have been found in the Ural Mountains, Wandagee Mountain of Western Australia, China, and Western North America. Due to the fossils' locations, it is speculated that the various species of Helicoprion lived off the south-western coast of Gondwana and, later, Pangaea.

Incisoscutum

Incisoscutum is an extinct genus of placoderm. Well preserved fossil embryos in the body cavities of female I. ritchiei, as well as in those of the ptyctodonts Australoptyctodus and Materpiscis, showed that, as a rule, placoderms, close to the common origin of all jawed vertebrates, gave birth to live young in a manner similar to modern sharks. Live birth requires internal fertilisation. In a study of fossil remains, comparison of the ontogeny of fourteen dermal plates from Compagopiscis croucheri and the more derived species Incisoscutum ritchiei suggested that lengthwise growth occurs earlier in the ontogeny than growth in width, and that dissociated allometric heterochrony has been an important mechanism in the evolution of the arthrodires, which include placoderms. These same fossil specimens also show that I. ritchiei was a predator, as muscle fibers from the tails of other placoderms have been found in the stomach regions.

Metaxygnathus

Metaxygnathus is an extinct genus of ichthyostegalian amphibian found in Late Devonian deposits of New South Wales. It is known only from a lower jawbone. Previously thought to be a lobe-finned fish, it has now been reassigned to the earliest group of tetrapods.

Mithakaspis

Mithakaspis is an extinct genus of arthrodire placoderm. Its fossils have been found in Craven Peak Beds, Queensland.

Phyllolepida

The order Phyllolepida ("leaf scales") was an order of flattened placoderms found throughout the world, with fossils being found in Devonian strata. Like other flattened placoderms, the phyllolepids were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, the phyllolepids were inhabitants of freshwater environments. Unlike the Rhenanida, the armour of the phyllolepids were made of whole plates, rather than numerous tubercles and scales, and unlike the Petalichthyida, the components of the extraordinarily wide mouth are known. The phyllolepids were considered to be blind, as the eyes are extremely small, so as to suggest that they were vestigial, and that they were placed on the sides of the head, as opposed to visual bottom-dwelling predators, like, say stargazers or flatfish, which have the eyes placed high on top of the head. Despite having a relatively clear idea of the phyllolepids' lifestyle and anatomy, most fossils consist of fragments of their thoracic armor, and only two genera, Phyllolepis and Austrophyllolepis have been thoroughly studied. From the articulation of the thoracic and head plates, it has been suggested that they are either the sister group of order Arthrodira, or are in fact a group of highly derived arthrodires.

Protopteraspis

Protopteraspis is an extinct genus of agnathan known from fossil finds in North America, Europe, and Australia. It lived from the Late Silurian to the Early Devonian. The animal's somewhat flat build has led some to believe that it was a bottom dweller, living in freshwater areas. It has been described as "somewhat unspecialized". Its snout was round and narrow, shorter than that of its immediate relatives. Protopteraspis also did not possess the cornua (horns) found on the headshields of other pterapsids; rather, it had "small points" behind the gill opening. It did possess a medium-sized dorsal spine. Plates covering the headshield had ridges of dentine. It is theorized that the headshield formed in youth and that it grew via growth of the plates, which fused in adulthood. The scales of the animal were small and diamond shaped.

Uarbryichthys

Uarbryichthys (Uarbry Fish) is a genus of primitive teleost ray-finned fish from fossil beds near the Talbragar River Bed, NSW. It was a genus of lake-dwelling fish that lived during the Upper Jurassic. When alive, it would have had a superficial resemblance to a very small porgie, or sea bream, but with a heterocercal tail fin.

Wuttagoonaspidae

Wuttagoonaspidae is a family of primitive arthrodire placoderms from Early Devonian China and Middle Devonian Australia.

Wuttagoonaspis

Wuttagoonaspis is a genus of primitive arthrodire placoderms from Middle Devonian Australia. The box-like skull is up to 18cm in length, and the median dorsal plate averages in length about 10cm.

Xenacanthus

Xenacanthus is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, some 360 million years ago, and they survived until the end of the Triassic, 202 million years ago. Fossils of various species have been found worldwide. Xenacanthus had a number of features that distinguished it from modern sharks. This freshwater shark was about one meter in length. The dorsal fin was ribbon-like and ran the entire length of the back and round the tail, where it joined with the anal fin. This arrangement resembles that of modern conger eels, and Xenacanthus probably swam in a similar manner. From the back of the skull a long, sharp, movable spine projected. This spine was made of bone. Though bone manufacture is not commonly associated with cartilaginous fishes, it is possible in certain tissues, such as the vertebrae, of certain shark species. The teeth had an unusual V-shape, and it probably fed on small crustaceans and heavily scaled palaeoniscid fishes. As in all fossil sharks, Xenacanthus is mainly known because of fossilised teeth and spines.

Xeradipterus

A new holodontid lungfish, Xeradipterus hatcheri, is described from the Late Devonian (Frasnian) Gogo Formation, Western Australia. The material comprises a single specimen preserved in a large concretion, represented by elements of the skull roof, cheek, lower jaw, palate, shoulder girdle, and parts of the postcranial skeleton. Xeradipterus differs from other holodontids by its broad, crushing dentition, prearticular tooth plates with pronounced heel, lack of prominent teeth, and ordered tooth rows, and by its parasphenoid morphology. Results of PAUP analyses of 82 characters for 33 dipnoan taxa revealed that the new genus is a member of the Holodontidae, suggesting a possible endemic radiation of holodontids within the Gogo fauna. In addition, new “Holodipterus” elderae material examined shows the shape and extent of the nasal capsules, and the presence of an anterior median callus for the first time.

Xiphactinus

Xiphactinus (from Latin and Greek for "sword-ray") is an extinct genus of large (4.5-6m long) predatory marine bony fish that lived during the Late Cretaceous. When alive, the fish would have resembled a gargantuan, fanged tarpon (to which it was, however, not related). Skeletal remains of Xiphactinus have come from Kansas (where the first Xiphactinus fossil was discovered during the 1850s), Alabama, and Georgia in the United States, as well as Europe, Australia, Canada and Venezuela. Species of Xiphactinus were voracious predatory fish. At least a dozen specimens of X. audax have been collected with the remains of large, undigested or partially digested prey in their stomachs. In particular, one 4m fossil specimen was collected by George F. Sternberg with another, nearly perfectly preserved 1.8m-long ichthyodectid Gillicus arcuatus, inside of it. The larger fish apparently died soon after eating its prey, most likely due to the smaller fish prey struggling and rupturing an organ as it was being swallowed. Like many other species in the Late Cretaceous oceans, a dead or injured individual was likely to be scavenged by sharks (Cretoxyrhina and Squalicorax). Virtually nothing is known about the larval or juvenile stages. The smallest fossil specimen of X. audax consists of a tooth-bearing premaxilla and lower jaws of an individual estimated to be about 30cm long The species and all other ichthyodectids went extinct near the end of the Late Cretaceous. An incomplete skull of what may be a new species of Xiphactinus was found in 2002 in the Czech Republic, in a small town called Sachov next Borohradek city, by student Michal Matejka.

In July 2010 the bones of a Xiphactinus were discovered near Morden, Manitoba, Canada. The specimen is about 6m long, and was found with the flipper of a mosasaur between its jaws