''Stethacanthus'' is an
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...
-like
holocephalian
Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primit ...
s
[Coates, M., Gess, R., Finarelli, J., Criswell, K., Tietjen, K. 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. doi: 10.1038/nature20806] which lived from the
Late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...
to
Late Carboniferous
Late may refer to:
* LATE, an acronym which could stand for:
** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia
** Local-authority trading enterprise, a New Zealand business law
** Local average treatment effect, ...
epoch, dying out around 298.9 million years ago. Fossils have been found in
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
,
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
and
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
.
Etymology
''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.
Description
''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
A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...
, which resembled an
anvil
An anvil is a metalworking tool consisting of a large block of metal (usually forged or cast steel), with a flattened top surface, upon which another object is struck (or "worked").
Anvils are as massive as practical, because the higher th ...
or ironing board. Small spikes (enlarged versions of the
dermal denticle
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...
s commonly covering shark skin) covered this crest, and the
ratfish
Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At o ...
'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
Scale or scales may refer to:
Mathematics
* Scale (descriptive set theory), an object defined on a set of points
* Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original
* Scale factor, a number w ...
, a pectoral fin composed of metapterygium with an accompanying ‘whip’ attached and a distinctive first
dorsal fin
A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...
and spine, termed the spine-brush complex. The
neurocranium
In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria (skull), ...
had a narrow suborbital shelf, a broad supraorbital shelf, a short otico-occipital division, large orbits,
and
cladodont This is a typical Cladodont tooth, of a shark called ''Glikmanius''">Glikmanius.html" ;"title="shark called ''Glikmanius">shark called ''Glikmanius''
Cladodont (from Latin cladus, meaning branch and Greek Odon, meaning tooth) is the term for a com ...
ic teeth that aligned precisely. In addition to these features, ''Stethacanthus'' also had male pelvic claspers with non-prismatic calcified
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
at the distal ends.
Spine-brush complex
The spine-brush complex occupies the same site as the first
dorsal fin
A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...
on other
ratfish
Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At o ...
and contains a basal plate extending inside a usually posterior-pointing dorsal spine composed of trabecular dentine. The spines resemble those of modern
sharks
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorp ...
and
rays
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gra ...
but curiously lack any enamel-like surface tissue.
The trabecular
dentine
Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...
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
denticle
A denticle is any small tooth-like or bristle-like structure. "Denticle" may refer to:
* Denticle (tooth feature), serrations on the teeth of dinosaurs, lizards, sharks, and mammals
* Dermal denticles or placoid scales, in cartilaginous fishes
* ...
s 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.
Pectoral fin whip
The
pectoral fins
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
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 purpose of the fin whips is unknown but it has been suggested that they were used during mating.
Teeth and denticles
The tooth files are whorl-shaped and the
palatoquadrate In some fishes, the palatoquadrate is the dorsal component of the mandibular arch, the ventral one being Meckel's cartilage. The palatoquadrate forms from splanchnocranium in various chordates including placoderms and acanthodians.
See also
* ...
is scalloped with 6-7 recesses to allow for the tooth families. The individual teeth are widely separated from each other in the tooth whorls.
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.
The medial and most lateral cusps are the most fluted. The teeth appear to be mostly orthodentine, but when viewed in cross-section, change abruptly to osteodentine. The enameloid is single-layered, overlaying the thick mantle of orthodentine.
In addition to the dentition teeth, 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.
Pelvic girdles and claspers
In ''Stethacanthus'', the pelvic girdles consist of sheets of prismatic
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
, each in the shape of a subtriangular, rounded plate. The anterior edge of each girdle is slightly concave while the posterior is convex. There appears to be no union of the two plates.
There are two types of pelvic girdles found in stethacanthids: the primitive condition and the derived condition. In the primitive condition, the pelvic girdles have a metapterygial element supporting only one or two radials and most of the fin radials are attached directly to the pelvic plate. The derived condition differs in that there is a much higher number of radials supported by the pelvic plate. This feature, accompanied with a broadening of the
pelvic girdle
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The p ...
in order to accommodate the increased number of radials is a characteristic of ''Stethacanthus'' and other symmorriids.
The males had claspers that were club-shaped at the distal ends and composed of non-prismatic globular calcified cartilage.
Caudal fin
There was some caudal fin variety among ''Stethacanthus'' species; while some had low angle
heterocercal
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as s ...
tails, some had tails approaching
homocercal
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...
.
The broad hypochordal lobe was supported by long, splayed fin radials.
Paleobiology
It is certain that ''Stethacanthus'' was a
carnivore
A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...
, and considering its small size probably fed on small fish,
brachiopods
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, whi ...
, and crinoid ossicles like other sharks of its time.
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.
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.
History
The several species of ''Stethacanthus'' discovered in the late 1800s were established based solely upon isolated spines, which initially confused paleontologist
John Strong Newberry
John Strong Newberry (December 22, 1822 – December 7, 1892) was an American physician, geologist and paleontologist. He participated as a naturalist and surgeon on three expeditions to explore and survey the western United States. During the C ...
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.
Meanwhile, the first associated skeletal remains found in the
Mississippian of
Montana
Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...
and the
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
and Mississippian of
Ohio
Ohio () is a state in the Midwestern region of the United States. Of the fifty U.S. states, it is the 34th-largest by area, and with a population of nearly 11.8 million, is the seventh-most populous and tenth-most densely populated. The sta ...
remained undescribed for nearly a century.
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.
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
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
of Bearsden, Scotland
and the Lower
Tournaisian
The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from Ma to Ma. It is preceded by the Famennian (the uppermost stage ...
of the Tula Region of Central Russia
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.
Classification
The presence of globular calcified cartilage in both the spine-brush complex base plate and brush and in the
claspers
In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating.
Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's ...
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
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
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.
Taxonomic relationships are hard to define for ''Stethacanthus'' as there is much variability in the characteristics of the discovered specimens.
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
is a monophyletic group characterized by the development of endoskeletal
tesserae
A tessera (plural: tesserae, diminutive ''tessella'') is an individual tile, usually formed in the shape of a square, used in creating a mosaic. It is also known as an abaciscus or abaculus.
Historical tesserae
The oldest known tesserae ...
(mineralized blocks of cartilage) and internal fertilization.
Chondrichthyes is further divided into two subclasses:
Elasmobranchii
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gil ...
and
Holocephali
Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primiti ...
. 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
Falcatidae is a family of Paleozoic holocephalians. Members of this family include ''Falcatus'', a small fish from the Bear Gulch Limestone of Montana. The family first appeared around the start of the Carboniferous, and there is some evidence ...
and thus are a monophyletic group. Another is that symmoriids are actually the females of stethacanthids
or are derived from stethacanthids.
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.
More recently,
Symmoriida as a whole has been reclassified as part of
Holocephali
Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primiti ...
, meaning that ''Stethacanthus'' is more closely related to modern
chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At ...
s than to sharks.
See also
*
Stethacanthidae
*
Symmoriiformes
Symmoriiformes is an extinct order of holocephalians. Originally named Symmoriida by Zangerl (1981), it has subsequently been known by several other names. Lund (1986) synonymized the group with Cladodontida, while Maisey (2008) corrected the nam ...
* ''
Akmonistion
''Akmonistion'' is an extinct genus of holocephalian that lived in the Early Carboniferous. The genus contains a single species, ''A. zangerli'', discovered by Stan Wood in 1982. It is distinguished by an unusual enlarged formation of the dorsal ...
''
References
External links
{{Taxonbar, from=Q133370
Symmoriiformes
Paleozoic fish of Europe
Devonian fish
Carboniferous fish
Paleozoic fish of North America
Fossil taxa described in 1889