Stethacanthus

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.[7]

Size of S. productus

Stethacanthus had different sizes depending on species, S. altonensis had length about 1.5 metres (4.9 ft), while S. productus reached 3 metres (9.8 ft).[8] 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.[9] 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.[10]

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,[11] 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.[12]

Spine-brush complex

Restoration of a S. altonensis pair

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.[13] 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[7] made of globular calcified cartilage.[14] The brush base and basal plate are covered in a thin, acellular bone layer.[13][14] Zangerl asserts that these tubules are similar to erectile tissues in humans, and thus the complex may have been inflatable.[7] 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 [14] which point posteriorly.[7] 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.[7] The complex has been affirmed only in males, and only in those males that have reached sexual maturity.[15] Whether the complex was present in females of the species is still unknown.[14] Another theory for the spine-brush complex is that it was involved either in attracting a mate[15] or in the mating process itself.[16]

Restoration of S. productus with prey

It is certain that Stethacanthus was a carnivore, and considering its small size probably fed on small fish, brachiopods, and crinoid ossicles like other sharks of its time.[18] 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.[15] Considering that most of the Stethacanthus specimens were recovered in the Bear Gulch Limestone in Montana, it is possible that this area was not only a breeding ground for other sharks but also for Stethacanthus, suggesting that they were migratory.[19]

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.[7] Meanwhile, the first associated skeletal remains found in the Mississippian of Montana and the Devonian and Mississippian of Ohio remained undescribed for nearly a century.[12] Since complete skeletons were extremely rare, Stethacanthus classification was vague and based on few characteristics. It was not until 1974 that the family Stethacanthidae was defined by Richard Lund because Stethacanthus differed so greatly from other Elasmobranchs of the time.[12] Relative classifications of symmoriids compared to stethacanthids are still debated. More Stethacanthus specimens have been discovered, expanding their range from the Midwestern United States to the Lower Carboniferous of Bearsden, Scotland[12] and the Lower Tournaisian of the Tula Region of Central Russia[20] and China.[21] Stethacanthus teeth have been recovered from the Frasnian-Famennian Napier Formation and the Tournaisian Laurel Formation and Moogooree Limestone in Western Australia.[22] A partial palate and jawbone referred to a Stethacanthus sp. has also been recovered from the Bonaparte Basin, Western Australia.[23]

Restoration of the possible synonym Symmorium reniforme

The presence of globular calcified cartilage in both the spine-brush complex base plate and brush and in the claspers is interesting because it is the first record of such a large mass of globular calcified cartilage in chondrichthyan. The high presence of globular calcified cartilage raises several questions about the evolution of sharks. It is possible that prismatic cartilage, a defining feature of chondrichthyans, is an evolutionary derivative of globular calcified cartilage. If this were the case, primitive chondrichthyans would have appeared with shark-like scales based instead on globular calcified cartilage. Another feature of note is the thin, acellular bone layer coating the brush and baseplate of the spine-brush complex. It is possible that the coating on the spine-brush complex is the first record of endoskeletal bone in primitive chondrichthyans, and that these endoskeletal features were lost in extant chondrichthyans. It is also possible that the fin spine could be a unique distribution of dermal skeleton and thus derived from neural crest. Following this assumption, the brush would be a fin-baseplate extension. The endoskeletal location and absence of fin radials supports the latter hypothesis.[13][14]

Taxonomic relationships are hard to define for Stethacanthus as there is much variability in the characteristics of the discovered specimens.[15] Chondrichthyes is a monophyletic group characterized by the development of endoskeletal tesserae (mineralized blocks of cartilage) and internal fertilization.[24] Chondrichthyes is further divided into two subclasses: Elasmobranchii and Holocephali. Stethacanthids have been classified as a member of the group Paleoselachii, which is a subdivision of Elasmobranchii. Stethacanthus has been further classified as part of the order Symmoriida, a classification that has sparked a controversy. There are two main hypotheses regarding this classification. One hypothesis states that the Order Symmoriida consists of the families Symmoriidae, Stethacanthidae and Falcatidae and thus are a monophyletic group. Another is that symmoriids are actually the females of stethacanthids[11][14] or are derived from stethacanthids.[11] This hypothesis is due to the fact that stethacanthids and symmoriids are poorly defined; symmoriids are thought to lack a spine-brush complex but are otherwise identical to stethacanthidae. Stethacanthids are identified by the presence of a spine-brush complex, which is in some cases non-existent (e.g. juvenile males), making the certain classification of stethacanthids and symmoriids difficult.[11]

More recently, Symmoriida as a whole has been reclassified as part of Holocephali, meaning that Stethacanthus is more closely related to modern chimaeras than to sharks.[6]

• Stethacanthidae
• Symmoriiformes
• Akmonistion