Жоғалған Stethacanthus – тікенді акула тұқымдасы

Extinct genus of cartilaginous fishes

Stethacanthus is an extinct genus of shark like holocephalians which lived from the Late Devonian to Late Carboniferous epoch, dying out around 298.9 million years ago. Fossils have been found in Australia, Asia, Europe and North America.

Stethacanthus comes from the Greek στῆθος (stēthos), meaning "chest", and ἄκανθος (akanthos), meaning "spine" or "thorn". The name refers to the distinctive anvil shaped first dorsal fin and spine displayed by mature males of the genus.

Stethacanthus had different sizes depending on species, S. altonensis had length about , while S. productus reached In many respects, it had a shark like appearance. However, it is best known for its unusually shaped dorsal fin, which resembled an anvil or ironing board. Small spikes (enlarged versions of the dermal denticles commonly covering shark skin) covered this crest, and the ratfish's head as well. The crest may have played a role in mating rituals, aided in clamping to the belly of larger marine animals, or been used to frighten potential predators. Like other members of Stethacanthidae, Stethacanthus had unique pelvic girdles, single crowned and non growing scales, a pectoral fin composed of metapterygium with an accompanying ‘whip’ attached and a distinctive first dorsal fin and spine, termed the spine brush complex. The neurocranium had a narrow suborbital shelf, a broad supraorbital shelf, a short otico occipital division, large orbits,

The spine brush complex occupies the same site as the first dorsal fin on other ratfish and contains a basal plate extending inside a usually posterior pointing dorsal spine composed of trabecular dentine. The spines resemble those of modern sharks and rays but curiously lack any enamel like surface tissue. which point posteriorly. Whether the complex was present in females of the species is still unknown.

The pectoral fins of Stethacanthus were composed of the triangular shaped metapterygium observed in modern day sharks, but had an additional long, metapterygial structure called a fin whip. These fin whips contain at least 22 axial cartilages and extended past the pelvic fins. The three most anterior axials are shorter than the more posterior axials. The teeth themselves are of the cladodont variety; the bases of the teeth are broadest on the lingual side, and each support a single large cusp and two pairs of smaller accessory cusps for a total of five cusps. Additionally, as the spine brush complex is rather a large structure, it seems likely that, in combination with the forward facing denticles on the structure, it would have produced a drag force during fast locomotion. Therefore, Stethacanthus was probably a slow moving shark. The fins of Stethacanthus were also smaller than in other sharks of the same size, and their teeth were also on the small side relative to other small Paleozoic sharks, suggesting that Stethacanthus may have been a bottom dweller.

The several species of Stethacanthus discovered in the late 1800s were established based solely upon isolated spines, which initially confused paleontologist John Strong Newberry into thinking the spines constituted a new kind of fin. He originally believed that the spines were part of the pectoral fins and that they were not bilaterally symmetrical. and China. Stethacanthus teeth have been recovered from the Frasnian Famennian Napier Formation and the Tournaisian Laurel Formation and Moogooree Limestone in Western Australia. A partial palate and jawbone referred to a Stethacanthus sp. has also been recovered from the Bonaparte Basin, Western Australia.