Subclass-Elasmobranchii and Holocephali
Status: EXTINCT. Stethacanthus lived from the Late Devonian to Early Carboniferous epoch, dying out around 323.2 million years ago.
Average Size and Length: Stethacanthus was about 2.3 feet long.
Teeth and Jaw: Stethacanthus had small, sharp teeth that aligned precisely.
Head: The neurocranium had a narrow suborbital shelf, a broad supraorbital shelf, a short otico-occipital division, large orbits.
Denticles: Small spikes (enlarged versions of the dermal denticles commonly covering today’s shark skin) covered this crest, and the ratfish’s head as well there are also a number of buccopharyngeal denticles lining the oropharynx. The denticles lining the top of the head and the top of the spine-brush complex are larger than the dentition teeth, and they appear as elongate monocuspid denticles.
Tail: There was some caudal fin variety among Stethacanthus species; while some had low angle heterocercal tails, some had tails approaching homocercal. The broad hypochordal lobe was supported by long, splayed fin radials.
Demographic, Distribution, Habitat, Environment and Range: Fossils have been found in Asia, Europe and North America. It is believed to be migratory animals that traveled to strategic points to mate and have their offspring.
Diet: Stethacanthus was a carnivore, and considering its small size probably fed on small fish, brachiopods, and crinoid ossicles like other sharks of its time.
Aesthetic Identification: Stethacanthus had a shark-like appearance. However, it is best known for its unusually shaped dorsal fin, which resembled an anvil or ironing board.
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 lack any enamel-like surface tissue. The trabecular dentine contains patches of fibers suggesting attachments to the epaxial musculature. Research suggests that the way these muscles would have been positioned suggests that the spine could have been moved in anterio-posterior direction.
The spine-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. The complex has been affirmed only 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.
The pectoral fins of Stethacanthus were composed of the triangular-shaped metapterygium observed in today’s 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 purpose of the fin whips is unknown but research suggests that they were used during mating.
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.
Biology and Reproduction: Stethacanthus had male pelvic claspers with non-prismatic calcified cartilage at the distal ends.
Behavioral Traits, Sensing and Intelligence: Research suggests that the crest of Stethacanthus 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.
Speed: The spine-brush complex would have produced a drag force during fast locomotion. Therefore, Stethacanthus was probably a slow-moving shark.