Stethacanthidae is an extinct family of prehistoric

holocephalians 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 ...

. It is estimated to have existed approximately between 380 and 300 million years ago. Members of this family are noted for their peculiar dorsal fin.

Introduction

The taxonomic history of the Family Stethacanthidae has been rather complicated because the findings of complete skeletons are very unusual, and as result early workers such as St. John &

Worthen Worthen is a village in Shropshire, England approximately 13 miles west of Shrewsbury. The village forms part of the Worthen with Shelve civil parish, which includes the hamlet of Little Worthen immediately to the north-east and the villages ...

, and Newberry were unable to recognise the association of the spine, dentition teeth, and

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 of these sharks. The genus ''Stethacanthus'' was established by Newberry (1889)NEWBERRY, J.S., 1889. The Paleozoic fishes of North America. ''United States Geological Survey Monograph'', 16, 340pp. for a series of large thin walled,

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 ...

-cored spines encountered in Mississippian (

Carboniferous Period The Carboniferous ( ) is a Period (geology), geologic period and System (stratigraphy), system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago (Myr, Mya), to the beginning of the Permian Period, ...

) rocks of the mid-continental United States. Decomposition of the internal cartilage and compression during burial resulted in distortion of the spines, leading Newberry to misinterpret them, he believed that the spines belonged to

pectoral Pectoral may refer to: * The chest region and anything relating to it. * Pectoral cross, a cross worn on the chest * a decorative, usually jeweled version of a gorget * Pectoral (Ancient Egypt), a type of jewelry worn in ancient Egypt * Pectorali ...

and

pelvic fins Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two e ...

of a new species of shark. The first associated skeletal remains, from the Mississippian of MontanaLUND, R., 1985a. Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana. ''American Museum Novitates'', 2828, 1-24 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,WILLIAMS, M.E., 1985. The «cladodont level» sharks of the Pennsylvanian black shales of central North America. ''Palaeontographica Band A, Stuttgart'', 190, 83-158ZANGERL, R., 1981. Chondrichthyes I: Paleozoic Elasmobranchii. H.P. Schultze (ed.), ''Handbook of Paleoichthyology, New York'', 115pp were not described until a century later. The Family Stethacanthidae was described by Richard Lund in 1974,LUND, R., 1974. ''Stethacanthus altonensis'' (Elasmobranchii) from the Bear Gulch Limestone of Montana. ''Annals of Carnegie museum'', 45(8), 161-178 he argued that "''Stethacanthus'' represents an experiment in

elasmobranch 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 ...

evolution that is significantly divergent enough to warrant family-level separation". This classification was later corroborated by another authors (e.g. Zangerl, 1990ZANGERL, R., 1990. Two new stethacanthid sharks (''Stethacanthidae, Symmoriida'') from the Pennsylvanian of Indiana, USA. ''Palaeontographica Band A'', 213, 115-141). Further reports of material attributed to ''Stethacanthus'' have extended its range to the Mississippian of Oklahoma, 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 Central Russia and the basal

Namurian The Namurian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 326 and 313 Ma (million years ago). It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Namurian is ...

Serpukhovian The Serpukhovian is in the ICS geologic timescale the uppermost stage or youngest age of the Mississippian, the lower subsystem of the Carboniferous. The Serpukhovian age lasted from Ma to Ma. It is preceded by the Visean and is followed b ...

of Scotland.

Description

''Stethacanthus altonensis'' is the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

of the Family Stethacanthidae, therefore, all Stethacanthids meet certain morphological characters best represented in this species. Stethacanthids are medium-sized

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 ...

shark-like holocephalians with a short

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

, broad supraorbital region, and short otic region. The teeth on jaws are of cladodont type, displaying 5

cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifurc ...

s. 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 ...

bears a large, thin walled compressed spine, displaying no ornamentation and concave anteriorly. This dorsal spine is fitted over a long basal plate and articulating at its base with the apex of a high triangular fin. The second dorsal fin is fitted over a very small, anterior basal plate apparently lacking a spine. The entire dorsal surface of head and first dorsal fin are covered with enlarged single cusped denticles. Secondary

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

is present, only mature males bear a first dorsal fin.

First dorsal fin and spine

The first dorsal fin is one of the strangest features of these fish. The fin itself is triangular and is composed of long, thin, calcified tubes radiating from the apex. The posterior dorsal surface of the first dorsal fin is covered with a belt of up to nine rows of enlarged dermal denticles. The spine, composed of

trabecular A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has ...

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 ...

,ZANGERL, R., 1984. On the microscopic anatomy and possible function of the spine-"brush" complex of ''Stehacanthus'' (Elasmobranchii: Symmoriida). ''Journal of Vertebrate Paleontology'', 4(3), 372-378 is roughly a right triangle in shape, with the hypothenuse concave anterodorsally.St. JOHN, O. & WORTHEN, A.H., 1875. Descriptions of fossil fishes. ''Geological Survey of Illinois'', 6(2), 245-488 The trabecular dentine contains a large number of fibres in the dorsal half of the spine. This suggests that, in live, a large portion of the spine was covered by

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

, probably anchoring the first dorsal fin.

Function

A shark with a structure on its back, such as an stethacanthid, could not have possibly been a fast swimmer. The first dorsal fin and spine could have produced a considerable amount of drag during fast locomotion. This suggests that Stethacanthids may have been rather sluggish bottom dwellers. The crowns of the dermal denticles on the first dorsal fin point forward and those on the head point backward, however it is unlikely that these were used for biting or tearing food. If the animal was disturbed by a potential predator while resting or feeding near the bottom it may have raised the head and tilted forward the first dorsal fin and spine, simulating a toothed open mouth of a much larger fish, therefore, an effective

defence mechanism In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an unconscious psychological operation that functions to protect a person from anxiety-producing thoughts and feelings related to internal conflicts and ou ...

. However, only males possessed the "armoured" first dorsal fin and spine, and this suggests that the function was merely

sexual display A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate Mate choice, exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ( ...

Teeth and denticles

Teeth are typical cladodont in form. They are composed of 5 cusps, the central being the largest, the two extreme lateral smaller, and the intermediate very small. Cusps are rounded in cross section, slightly curved inwards and strongly striated vertically. Modified denticles cover the dorsal surface of the head and upper edge of dorsal fin. Both sets of denticles are smooth, monocuspid and curved

posteriorly Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

. The denticles on the head are relatively uniform in size (about as high as the central cusps in the teeth), and rounded in cross section. The denticles on the edge of the dorsal fin have

polygonal In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two tog ...

bases and are fitted together in a mosaic pattern. In the male, the denticles increase in height and decrease in curvature toward the midline of the fin, so that the central denticles reach 2 cm in height.

Pelvic girdles and claspers

Pelvic girdles 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 ...

fall into two types. The first, thought to be the primitive condition, bears virtually the entire pelvic fin. The second, consists of a prominent metapterygial plate. 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 ...

, in the male, are separated from the body of the fin by about four blocks of calcified cartilage.

Caudal fin

The

caudal fin 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 ...

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 ...

, but approaches a functional

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 ...

condition.

Environment

Most Stethacanthids found come from the

Bear Gulch Limestone The Bear Gulch Limestone is a limestone-rich geological lens in central Montana, renowned for the quality of its late Mississippian-aged fossils. It is exposed over a number of outcrops northeast of the Big Snowy Mountains, and is often considere ...

of Montana. The Bear Gulch is a

lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

that is unique in preserving virtually an entire small marine bay of extremely brief duration in the late Mississippian (Heath Formation, Palaeozoic; 323Ma).LUND, R., GREENFEST-ALLEN, E. & GROGAN, E.D., 2012. Habitat and diversity of the Bear Gulch fish: Life in a 318 million year old marine Mississippian bay. ''Palaeogeography, Palaeoclimatology, Palaeoecology'', 342-343, 1-16 During deposition, it was located approximately 12ºN latitude and was part of an extensive

sabkha A sabkha ( ar, سبخة) is a coastal, supratidal mudflat or sandflat in which evaporite-saline minerals accumulate as the result of semiarid to arid climate. Sabkhas are gradational between land and intertidal zone within restricted coastal p ...

environment and subjected to

monsoonal A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscill ...

climatic regime.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 The bay was subject to minimal

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

input and was opened to the East. It is likely that the diversity of the Bear Gulch

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

may be representative of

upper Mississippian The Upper Mississippian cultures were located in the Upper Mississippi River, Mississippi basin and Great Lakes region of the American Midwest. They were in existence from approximately A.D. 1000 until the Protohistory, Protohistoric and early H ...

marine faunas, due to the accessibility to migratory forms and the bay likely provided breeding and nursery grounds for those not

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

to it.GROGAN, E.D., LUND, R. & GREENFEST-ALLEN, E., 2012. The Origin and Relationships of Early Chondrichthyans. In CARRIER, J.C., MUSICK, J.A. & HEITHAUS, M.R. (eds.), Biology of Sharks and their Relatives. ''CRC press, USA'', 1, 3-29 The most complete skeleton ever found has been published recentlyWOOD, S.P., 1982. New basal Namurian (Upper Carboniferous) fishes and crustaceans found near Glasgow. ''Nature'', 291, 574-577 from the Manse Burn Formation (Serpukhovian) in Bearsden near Glasgow, Scotland. As well as the Bear Gulch Limestone, the

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

of this formation indicates that it was deposited under variable conditions of salinity with seasonal periodicity.

Taxonomic relationships

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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

group divisible into two sister taxa, the

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 ...

, and the extant chondrichthyans are derivable from

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

forms. Yet, the relationship of these with the

Palaeozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and '' ...

forms is still poorly understood. Chondrichthyes are distinguished based on a two unique autapomorphous character sets: the development of tesserae endoskeletal mineralisation and

internal fertilisation Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For inter ...

with copulation. During the Carboniferous chondrichthyans radiated rapidly and expansively in all available aquatic regimes and some of the most bizarre forms originated during this period. Stethacanthids are classified within the division Paleoselachii, of the Subclass

. On a lower taxonomical level, the classification of this group is very controversial. There are two main hypothesis: * 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 ...

are included within the Order

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 ...

and form a monophyletic group.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 * Stethacanthids are a very derived group of Paleoselachii sharks, defined by a highly modified first dorsal fin (only in adult males) and strong sexual dimorphism. This condition is also seen in symmoriids and is unique among known elasmobranchs of any time. So, either symmoriids are the females of some stethacanthids, or they are derived from this group. The classification of symoriiform sharks (including the Families Stethacanthidae, Symmoriidae and Falcatidae) will remain a controversy until other complete specimens are found outside of the Bear Gulch lens.

References

{{Taxonbar, from=Q1339753 Late Devonian first appearances Late Devonian animals Pennsylvanian extinctions Symmoriiformes