Stethacanthus: Difference between revisions
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{{Taxobox |
{{Taxobox |
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|image = |
|image = StethacanthusesDB 2.jpg |
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|image_caption = '' |
|image_caption = Restoration of ''S. altonensis'' and ''S. productus'' |
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|fossil_range = {{fossilrange|385|320}}<small>Late [[Devonian]] to Early [[Carboniferous]]</small> |
|fossil_range = {{fossilrange|385|320}}<small>Late [[Devonian]] to Early [[Carboniferous]]</small> |
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|regnum = [[Animal]]ia |
|regnum = [[Animal]]ia |
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|subdivision_ranks = [[Species]] |
|subdivision_ranks = [[Species]] |
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|subdivision = |
|subdivision = |
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''S. altonensis'' |
*''S. altonensis'' |
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''S. gansuensis'' |
*''S. gansuensis'' |
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''S. neilsoni'' |
*''S. neilsoni'' |
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''S. praecursor'' |
*''S. praecursor'' |
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''S. resistens'' |
*''S. resistens'' |
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''S. thomasi'' |
*''S. thomasi'' |
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''S. productus'' |
*''S. productus'' |
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|type species=''Stethacanthus altonensis'' |
|type species=''Stethacanthus altonensis'' |
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}} |
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''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.<ref name="Zangerl Reconstruction">{{cite journal|last1=Zangerl|first1=Rainer|title=On the Microscopic Anatomy and Possible Function of the Spine-"Brush" Complex of Stethacanthus (Elasmobranchii: Symmoriida)|journal=Journal of Vertebrate Paleontology|date=1984|volume=4|issue=3|pages=372–378}}</ref> |
''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.<ref name="Zangerl Reconstruction">{{cite journal|last1=Zangerl|first1=Rainer|title=On the Microscopic Anatomy and Possible Function of the Spine-"Brush" Complex of Stethacanthus (Elasmobranchii: Symmoriida)|journal=Journal of Vertebrate Paleontology|date=1984|volume=4|issue=3|pages=372–378}}</ref> |
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==Description |
==Description== |
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[[File:Stethacanthus productus size.png|thumb|left|Size of ''S. productus'']] |
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''Stethacanthus'' was around {{convert|70|cm|ft}} long, and in many respects, had a typical 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 denticle]]s commonly covering shark skin) covered this crest, and the shark's head as well.<ref name=EoDP>{{cite book |editor=Palmer, D.|year=1999 |title= The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals|publisher= Marshall Editions|location=London|page= 27|isbn= 1-84028-152-9}}</ref> The crest may have played a role in mating rituals, aided in clamping to the belly of larger marine animals, or used to frighten potential predators.<ref>[http://www.elasmo-research.org/education/evolution/golden_age.htm Elasmo-research]</ref> |
''Stethacanthus'' was around {{convert|70|cm|ft}} long, and in many respects, had a typical 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 denticle]]s commonly covering shark skin) covered this crest, and the shark's head as well.<ref name=EoDP>{{cite book |editor=Palmer, D.|year=1999 |title= The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals|publisher= Marshall Editions|location=London|page= 27|isbn= 1-84028-152-9}}</ref> The crest may have played a role in mating rituals, aided in clamping to the belly of larger marine animals, or used to frighten potential predators.<ref>[http://www.elasmo-research.org/education/evolution/golden_age.htm Elasmo-research]</ref> |
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=== The Spine-Brush Complex === |
=== The Spine-Brush Complex === |
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[[File:Steth pair1.jpg|thumb|Restoration of a ''S. altonensis'' pair]] |
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The Spine-Brush Complex occupies the same site as the first [[dorsal fin]] on other [[elasmobranchs]] 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 [[ray (fish)|rays]] but curiously lack any enamel-like surface tissue.<ref name="Spines and tissues of ancient sharks" /> 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<ref name="Zangerl Reconstruction" /> made of globular calcified cartilage.<ref name=SBC1 /> The brush base and basal plate are covered in a thin, acellular bone layer.<ref name="Spines and tissues of ancient sharks" /><ref name=SBC1 /> Zangerl asserts that these tubules are similar to erectile tissues in man, and thus the complex may have been inflatable.<ref name="Zangerl Reconstruction"/> 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 <ref name="SBC1">{{cite journal|last1=Maisey|first1=John G.|title=The Spine-Brush Complex in Symmoriiform Sharks (Chondrichthyes: Symmoriiformes), with comments on dorsal fin modularity|journal=Journal of Vertebrate Paleontology|date=2009|volume=29|issue=1|pages=14–24|url=http://www.tandfonline.com/doi/pdf/10.1671/039.029.0130|accessdate=2 March 2015}}</ref> which point posteriorly.<ref name="Zangerl Reconstruction"/> 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.<ref name="Zangerl Reconstruction"/> Interestingly, the complex has only been affirmed in males, and only in those males that have reached sexual maturity.<ref name=Lund1985>{{cite journal|last1=Lund|first1=Richard|title=Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana|journal=American Museum Noviates|date=1985|volume=2828|pages=1–24}}</ref> Whether the complex was present in females of the species is still unknown.<ref name="SBC1"/> Another theory for the spine-brush complex is that it was involved either in attracting a mate<ref name=Lund1985 /> or in the mating process itself.<ref name="Wood">{{cite journal|last1=Wood|first1=S.P.|title=New basal Namurian (Upper Carboniferous) fishes and crustaceans found near Glasgow|journal=Nature|date=1982|volume=297|pages=574–7}}</ref> |
The Spine-Brush Complex occupies the same site as the first [[dorsal fin]] on other [[elasmobranchs]] 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 [[ray (fish)|rays]] but curiously lack any enamel-like surface tissue.<ref name="Spines and tissues of ancient sharks" /> 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<ref name="Zangerl Reconstruction" /> made of globular calcified cartilage.<ref name=SBC1 /> The brush base and basal plate are covered in a thin, acellular bone layer.<ref name="Spines and tissues of ancient sharks" /><ref name=SBC1 /> Zangerl asserts that these tubules are similar to erectile tissues in man, and thus the complex may have been inflatable.<ref name="Zangerl Reconstruction"/> 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 <ref name="SBC1">{{cite journal|last1=Maisey|first1=John G.|title=The Spine-Brush Complex in Symmoriiform Sharks (Chondrichthyes: Symmoriiformes), with comments on dorsal fin modularity|journal=Journal of Vertebrate Paleontology|date=2009|volume=29|issue=1|pages=14–24|url=http://www.tandfonline.com/doi/pdf/10.1671/039.029.0130|accessdate=2 March 2015}}</ref> which point posteriorly.<ref name="Zangerl Reconstruction"/> 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.<ref name="Zangerl Reconstruction"/> Interestingly, the complex has only been affirmed in males, and only in those males that have reached sexual maturity.<ref name=Lund1985>{{cite journal|last1=Lund|first1=Richard|title=Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana|journal=American Museum Noviates|date=1985|volume=2828|pages=1–24}}</ref> Whether the complex was present in females of the species is still unknown.<ref name="SBC1"/> Another theory for the spine-brush complex is that it was involved either in attracting a mate<ref name=Lund1985 /> or in the mating process itself.<ref name="Wood">{{cite journal|last1=Wood|first1=S.P.|title=New basal Namurian (Upper Carboniferous) fishes and crustaceans found near Glasgow|journal=Nature|date=1982|volume=297|pages=574–7}}</ref> |
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There was some caudal fin variety among ''Stethacanthus'' species; while some had low angle [[heterocercal]] tails, some had tails approaching [[homocercal]].<ref name=Lund1985 /> The broad hypochordal lobe was supported by long, splayed fin radials.<ref name=Bearsden /> |
There was some caudal fin variety among ''Stethacanthus'' species; while some had low angle [[heterocercal]] tails, some had tails approaching [[homocercal]].<ref name=Lund1985 /> The broad hypochordal lobe was supported by long, splayed fin radials.<ref name=Bearsden /> |
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==Paleobiology== |
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[[File:Stethacanthus1DB.jpg|thumb|left|Restoration of ''S. productus' with prey']] |
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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.<ref name=Diet>{{cite journal|last1=Walker|first1=S.E.|last2=Brett|first2=C.E.|title=PREDATORS AND PREDATION IN PALEOZOIC MARINE ENVIRONMENTS|journal=Paleontological Society Papers|date=2002|volume=8|pages=93–118}}</ref> 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 dentition teeth were also on the small side relative to other small Paleozoic sharks, suggesting that ''Stethacanthus'' may have been a bottom-dweller.<ref name=Lund1985 /> 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.<ref name="Bear Gulch">{{cite journal|last1=Grogan|first1=E.D.|last2=Lund|first2=R.|title=The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition|journal=Geodiversitas|date=2002|volume=24|pages=295–315}}</ref> |
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.<ref name=Diet>{{cite journal|last1=Walker|first1=S.E.|last2=Brett|first2=C.E.|title=PREDATORS AND PREDATION IN PALEOZOIC MARINE ENVIRONMENTS|journal=Paleontological Society Papers|date=2002|volume=8|pages=93–118}}</ref> 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 dentition teeth were also on the small side relative to other small Paleozoic sharks, suggesting that ''Stethacanthus'' may have been a bottom-dweller.<ref name=Lund1985 /> 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.<ref name="Bear Gulch">{{cite journal|last1=Grogan|first1=E.D.|last2=Lund|first2=R.|title=The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition|journal=Geodiversitas|date=2002|volume=24|pages=295–315}}</ref> |
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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 part of the pectoral fins and that they were not bilaterally symmetrical.<ref name="Zangerl Reconstruction"/> Meanwhile the first associated skeletal remains found in the [[Mississippian (geology)|Mississippian]] of [[Montana]] and the [[Devonian]] and Mississippian of [[Ohio]] remained undescribed for nearly a century.<ref name="Bearsden"/> 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.<ref name=Bearsden /> 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<ref name=Bearsden /> and the Lower [[Tournaisian]] of the Tula Region of Central Russia.<ref name=Russia>{{cite journal|last1=Lebedev|first1=O.A.|title=Fish assemblages in the Tournaisian-Visean environments of the East European Platform|journal=Geological Society, London, Special Publications|date=1996|volume=107|issue=1|pages=387–415}}</ref> |
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 part of the pectoral fins and that they were not bilaterally symmetrical.<ref name="Zangerl Reconstruction"/> Meanwhile the first associated skeletal remains found in the [[Mississippian (geology)|Mississippian]] of [[Montana]] and the [[Devonian]] and Mississippian of [[Ohio]] remained undescribed for nearly a century.<ref name="Bearsden"/> 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.<ref name=Bearsden /> 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<ref name=Bearsden /> and the Lower [[Tournaisian]] of the Tula Region of Central Russia.<ref name=Russia>{{cite journal|last1=Lebedev|first1=O.A.|title=Fish assemblages in the Tournaisian-Visean environments of the East European Platform|journal=Geological Society, London, Special Publications|date=1996|volume=107|issue=1|pages=387–415}}</ref> |
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==Classification== |
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==Evolutionary Relationships== |
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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.<ref name="Spines and tissues of ancient sharks" /><ref name=SBC1 /> |
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.<ref name="Spines and tissues of ancient sharks" /><ref name=SBC1 /> |
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==Taxonomic Relationships== |
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Taxonomic relationships are hard to define for ''Stethacanthus'' as there is much variability in the characteristics of the discovered specimens.<ref name=Lund1985 /> |
Taxonomic relationships are hard to define for ''Stethacanthus'' as there is much variability in the characteristics of the discovered specimens.<ref name=Lund1985 /> |
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[[Chondrichthyes]] is a monophyletic group characterized by the development of endoskeletal [[tesserae]] (mineralized blocks of cartilage) and internal fertilization.<ref name=Greenfest>{{cite journal|last1=Grogan|first1=E.D.|last2=Lund|first2=Richard|last3=Greenfest-Allen|first3=E.|title=The Origin and Relationships of Early Chondrichthyans|journal=CRC press, USA|date=2012|volume=1|pages=3–29}}</ref> 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<ref name="Brain case" /><ref name=SBC1 /> or are derived from stethacanthids.<ref name="Brain case">{{cite journal|last1=Maisey|first1=J.G.|title=THE BRAINCASE IN PALEOZOIC SYMMORIIFORM AND CLADOSELACHIAN SHARKS|journal=American Museum of Natural History|date=2007|volume=Bulletin|pages=1–122}}</ref> 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.<ref name="Brain case" /> |
[[Chondrichthyes]] is a monophyletic group characterized by the development of endoskeletal [[tesserae]] (mineralized blocks of cartilage) and internal fertilization.<ref name=Greenfest>{{cite journal|last1=Grogan|first1=E.D.|last2=Lund|first2=Richard|last3=Greenfest-Allen|first3=E.|title=The Origin and Relationships of Early Chondrichthyans|journal=CRC press, USA|date=2012|volume=1|pages=3–29}}</ref> 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<ref name="Brain case" /><ref name=SBC1 /> or are derived from stethacanthids.<ref name="Brain case">{{cite journal|last1=Maisey|first1=J.G.|title=THE BRAINCASE IN PALEOZOIC SYMMORIIFORM AND CLADOSELACHIAN SHARKS|journal=American Museum of Natural History|date=2007|volume=Bulletin|pages=1–122}}</ref> 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.<ref name="Brain case" /> |
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==In popular culture== |
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* In the 2003 TV [[documentary film|documentary]] ''[[Sea Monsters – A Walking with Dinosaurs Trilogy|Sea Monsters]]'', a spin-off from the 1999 series ''[[Walking with Dinosaurs]]'', a ''Stethacanthus'' was briefly shown circling a large [[shark cage]] before being scared away by a much larger ''[[Dunkleosteus]]''. It reappeared in the 2005 prequel series ''[[Walking With Monsters]]'', chasing a ''[[Hynerpeton]]'', only to be eaten by a ''[[Hyneria]]''. |
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* ''Stethacanthus'' was featured in ''[[Paleoworld]]'', Episode 5 as the first hunting shark, yet prey for predators like the ''[[Rhizodus]]''. |
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==Gallery== |
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<gallery> |
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image:Steth pair1.jpg|A pair of ''Stethacanthus''. |
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</gallery> |
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==See also== |
==See also== |
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Revision as of 22:46, 28 August 2015
| Stethacanthus Temporal range: Late Devonian to Early Carboniferous
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| Restoration of S. altonensis and S. productus | |
| Scientific classification | |
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| Genus: | Stethacanthus Newberry, 1889
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| Type species | |
| Stethacanthus altonensis | |
| Species | |
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Stethacanthus is an extinct genus of shark which lived from the Late Devonian to Early Carboniferous epoch, dying out around 320 million years ago. Fossils have been found in Europe and North America.
Etymology
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.[1]
Description
Stethacanthus was around 70 centimetres (2.3 ft) long, and in many respects, had a typical 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 shark's head as well.[2] The crest may have played a role in mating rituals, aided in clamping to the belly of larger marine animals, or used to frighten potential predators.[3]
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,[4] 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.[5]
The Spine-Brush Complex
The Spine-Brush Complex occupies the same site as the first dorsal fin on other elasmobranchs 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.[6] 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[1] made of globular calcified cartilage.[7] The brush base and basal plate are covered in a thin, acellular bone layer.[6][7] Zangerl asserts that these tubules are similar to erectile tissues in man, and thus the complex may have been inflatable.[1] 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 [7] which point posteriorly.[1] 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.[1] Interestingly, the complex has only been affirmed in males, and only in those males that have reached sexual maturity.[8] Whether the complex was present in females of the species is still unknown.[7] Another theory for the spine-brush complex is that it was involved either in attracting a mate[8] or in the mating process itself.[9]
Pectoral Fin Whip
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.[5] The purpose of the fin whips is unknown but it has been suggested that they were used during mating.[9]
Dentition Teeth and Denticles
The tooth files are whorl-shaped and the palatoquadrate 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.[10] 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 5 cusps.[10] 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.[8] In addition to the dentition teeth, there are also a number of buccopharyngeal denticles lining the oropharynx.[5][8] 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.[7]
Pelvic Girdles and Claspers
In Stethacanthus, the pelvic girdles consist of sheets of prismatic cartilage, 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.[5] 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 in order to accommodate the increased number of radials is a characteristic of Stethacanthus and other symmorriids.[8] The males had claspers that were club-shaped at the distal ends and composed of non-prismatic globular calcified cartilage.[6][8]
Caudal Fin
There was some caudal fin variety among Stethacanthus species; while some had low angle heterocercal tails, some had tails approaching homocercal.[8] The broad hypochordal lobe was supported by long, splayed fin radials.[5]
Paleobiology
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.[11] 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 dentition teeth were also on the small side relative to other small Paleozoic sharks, suggesting that Stethacanthus may have been a bottom-dweller.[8] 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.[12]
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 into thinking the spines constituted a new kind of fin. He originally believed that the spines part of the pectoral fins and that they were not bilaterally symmetrical.[1] 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.[5] 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.[5] 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[5] and the Lower Tournaisian of the Tula Region of Central Russia.[13]
Classification
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.[6][7]
Taxonomic relationships are hard to define for Stethacanthus as there is much variability in the characteristics of the discovered specimens.[8] Chondrichthyes is a monophyletic group characterized by the development of endoskeletal tesserae (mineralized blocks of cartilage) and internal fertilization.[14] 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[4][7] or are derived from stethacanthids.[4] 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.[4]
See also
References
- ^ a b c d e f Zangerl, Rainer (1984). "On the Microscopic Anatomy and Possible Function of the Spine-"Brush" Complex of Stethacanthus (Elasmobranchii: Symmoriida)". Journal of Vertebrate Paleontology. 4 (3): 372–378.
- ^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 27. ISBN 1-84028-152-9.
- ^ Elasmo-research
- ^ a b c d Maisey, J.G. (2007). "THE BRAINCASE IN PALEOZOIC SYMMORIIFORM AND CLADOSELACHIAN SHARKS". American Museum of Natural History. Bulletin: 1–122.
- ^ a b c d e f g h Coates S.E.K., M.I.; Sequeira, S.E.K. (2001). "A new stethacanthid chondrichthyan from the lower Carboniferous of Bearsden, Scotland". Journal of Vertebrate Paleontology. 21 (3): 438–459.
- ^ a b c d Coates, M. I.; Sequeira, S.E.K.; Sansom, I.J.; Smith, M.M. (December 1998). "Spines and tissues of ancient sharks". Nature. 396: 729–730. doi:10.1038/25467.
- ^ a b c d e f g Maisey, John G. (2009). "The Spine-Brush Complex in Symmoriiform Sharks (Chondrichthyes: Symmoriiformes), with comments on dorsal fin modularity". Journal of Vertebrate Paleontology. 29 (1): 14–24. Retrieved 2 March 2015.
- ^ a b c d e f g h i Lund, Richard (1985). "Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana". American Museum Noviates. 2828: 1–24.
- ^ a b Wood, S.P. (1982). "New basal Namurian (Upper Carboniferous) fishes and crustaceans found near Glasgow". Nature. 297: 574–7.
- ^ a b Smith, M.M.; Coates, M.I. (2001). The evolution of vertebrate dentitions: phylogenetic pattern and developmental models. pp. 223–240.
- ^ Walker, S.E.; Brett, C.E. (2002). "PREDATORS AND PREDATION IN PALEOZOIC MARINE ENVIRONMENTS". Paleontological Society Papers. 8: 93–118.
- ^ Grogan, E.D.; Lund, R. (2002). "The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition". Geodiversitas. 24: 295–315.
- ^ Lebedev, O.A. (1996). "Fish assemblages in the Tournaisian-Visean environments of the East European Platform". Geological Society, London, Special Publications. 107 (1): 387–415.
- ^ Grogan, E.D.; Lund, Richard; Greenfest-Allen, E. (2012). "The Origin and Relationships of Early Chondrichthyans". CRC press, USA. 1: 3–29.
External links
- ^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 27. ISBN 1-84028-152-9.
- ^ Elasmo-research
- ^ LUND, R., 1985a. Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana. American Museum Noviates, 2828, 1-24.
- ^ MAISEY, J.G., 2009. The spine-brush complex in Symmoriiform sharks (Chondrichthyes: Symmoriiformes), with comments on dorsal fin modularity. Journal of Vertebrate Paleontology, 29(1), 14-24.