Stethacanthus
|
Stethacanthus Temporal range: 382.7–323.2 Ma Late Devonian to Early Carboniferous |
|
|---|---|
 |
|
| Restoration of S. altonensis and S. productus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Holocephali |
| Order: | Symmoriida |
| Family: | Stethacanthidae |
| Genus: |
Stethacanthus Newberry, 1889 |
| Type species | |
|
Stethacanthus altonensis |
|
| Species | |
|
|
Stethacanthus is an extinct genus of ratfish which lived from the Late Devonian to Early Carboniferous epoch, dying out around 323.2 million years ago. Fossils have been found in Europe and North America.
Stethacanthus comes from the Greek “steth” meaning “chest” and “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 was around 70 centimetres (2.3 ft) long, and in many respects, 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, and cladodontic teeth that aligned precisely. In addition to these features, Stethacanthus also had male pelvic claspers with non-prismatic calcified cartilage at the distal ends.
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. The trabecular dentine contains patches of fibers suggesting attachments to the epaxial musculature. The way these muscles would have been positioned implies that the spine could have been moved in anterio-posterior direction. The so-called "brush" is not fibrous as was originally believed, but consists of a number of parallel, membranous tubules made of globular calcified cartilage. The brush base and basal plate are covered in a thin, acellular bone layer. Zangerl asserts that these tubules are similar to erectile tissues in humans, and thus the complex may have been inflatable. The complex itself is covered in up to nine rows of large denticles pointing anteriorly. The dorsal side of the head has its own collection of denticles which point posteriorly. The presence of these large denticles has led to theories that the spine-brush complex in combination with the denticles on the head was used to scare away predators by simulating the mouth of a larger fish. Interestingly, the complex has only been affirmed in males, and only in those males that have reached sexual maturity. Whether the complex was present in females of the species is still unknown. Another theory for the spine-brush complex is that it was involved either in attracting a mate or in the mating process itself.
...
Wikipedia