 |
| Stethacanthidae | |
|---|---|
 | |
| Stethacanthus altonensis | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Holocephali |
| Order: | † Symmoriiformes |
| Family: | † Stethacanthidae |
| Genera | |
Stethacanthidae is an extinct family of prehistoric holocephalians. [1] 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.
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, [3] and Newberry [4] were unable to recognise the association of the spine, dentition teeth, and dermal denticles of these sharks. The genus Stethacanthus was established by Newberry (1889) [4] for a series of large thin walled, cartilage-cored spines encountered in Mississippian (Carboniferous 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 and pelvic fins of a new species of shark. The first associated skeletal remains, from the Mississippian of Montana [5] [6] and the Devonian and Mississippian of Ohio, [7] [8] were not described until a century later. The Family Stethacanthidae was described by Richard Lund in 1974, [5] he argued that "Stethacanthus represents an experiment in elasmobranch evolution that is significantly divergent enough to warrant family-level separation". This classification was later corroborated by another authors (e.g. Zangerl, 1990 [9] ). Further reports of material attributed to Stethacanthus have extended its range to the Mississippian of Oklahoma, [10] the Lower Tournaisian of Central Russia [11] and the basal Namurian/Serpukhovian of Scotland. [2] [12]
Stethacanthus altonensis is the type species of the family Stethacanthidae, therefore, all stethacanthids meet certain morphological characters best represented in this species. Stethacanthids are medium-sized cladodont shark-like holocephalians with a short rostrum, broad supraorbital region, and short otic region. The teeth on jaws are of cladodont type, displaying 5 cusps (pentacuspids). [13] The first dorsal fin 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 is present, only mature males bear a first dorsal fin. [5]
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. [5] The spine, composed of trabecular dentine, [14] is roughly a right triangle in shape, with the hypothenuse concave anterodorsally. [3] [4] 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, probably anchoring the first dorsal fin. [14]
A shark with a structure on its back, such as a 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. [14] 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, [14] therefore, an effective defence mechanism. However, only males possessed the "armoured" first dorsal fin and spine, and this suggests that the function was merely sexual display.
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. 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 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. [5]
Pelvic girdles fall into two types. The first, thought to be the primitive condition, bears virtually the entire pelvic fin. [15] The second, consists of a prominent metapterygial plate. [16] The claspers, in the male, are separated from the body of the fin by about four blocks of calcified cartilage. [5]
The caudal fin is heterocercal, but approaches a functional homocercal condition. [17]
Most Stethacanthids found come from the Bear Gulch Limestone of Montana. The Bear Gulch is a lagerstätte that is unique in preserving virtually an entire small marine bay of extremely brief duration in the late Mississippian (Heath Formation, Palaeozoic; 323Ma). [18] During deposition, it was located approximately 12ºN latitude [19] and was part of an extensive sabkha environment [20] and subjected to monsoonal climatic regime. [21] The bay was subject to minimal fluvial input and was opened to the East. [18] It is likely that the diversity of the Bear Gulch fauna may be representative of upper Mississippian marine faunas, due to the accessibility to migratory forms and the bay likely provided breeding and nursery grounds for those not endemic to it. [22] The most complete skeleton ever found has been published recently [2] [12] from the Manse Burn Formation (Serpukhovian) in Bearsden near Glasgow, Scotland. As well as the Bear Gulch Limestone, the lithology of this formation indicates that it was deposited under variable conditions of salinity with seasonal periodicity.
Chondrichthyes is a monophyletic group divisible into two sister taxa, the Elasmobranchii and Holocephali, and the extant chondrichthyans are derivable from Mesozoic forms. Yet, the relationship of these with the Palaeozoic 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 with copulation. [22] 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 Elasmobranchii. On a lower taxonomical level, the classification of this group is very controversial. There are two main hypothesis:
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.
Holocephali, 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 primitive forms, and the only surviving group in the subclass is the order Chimaeriformes.
Cladoselache is an extinct genus of shark-like chondrichthyan from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks, but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers.
Orthacanthus is an extinct genus of fresh-water xenacanthiform elasmobranch, named by Louis Agassiz in 1843, ranging from the Upper Carboniferous into the Lower Permian. Orthacanthus had a nektobenthic life habitat, with a carnivorous diet. Multiple authors have also discovered evidence of cannibalism in the diet of Orthacanthus and of "filial cannibalism" where adult Orthacanthus preyed upon juvenile Orthacanthus. Synonyms of the genus Orthacanthus are Dittodus Owen, 1867, Didymodus Cope, 1883, Diplodus Agassiz, 1843, Chilodus Giebel, 1848.
Stethacanthus is an extinct genus of shark-like holocephalians which lived from the Late Devonian to Late Carboniferous epoch, dying out around 298.9 million years ago. Fossils have been found in Australia, Asia, Europe and North America.
Xenacanthida is an order or superorder of extinct shark-like chondrichthyans known from the Carboniferous to Triassic. They were native to freshwater, marginal marine and shallow marine habitats. Some xenacanths may have grown to lengths of 5 m (16 ft). Most xenacanths died out at the end of the Permian in the End-Permian Mass Extinction, with only a few forms surviving into the Triassic.
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 considered a component of the more widespread Heath Formation. The Bear Gulch Limestone reconstructs a diverse, though isolated, marine ecosystem which developed near the end of the Serpukhovian age. It is a lagerstätte, a particular type of rock unit with exceptional fossil preservation of both articulated skeletons and soft tissues. Bear Gulch fossils include a variety of fish, invertebrates, and algae occupying a number of different habitats within a preserved shallow bay.
Belantsea is a genus of extinct petalodontid cartilaginous fish that lived during the Lower Carboniferous, about 350 million years ago. Its fossils are found in the Bear Gulch Limestone lagerstätte. Its body was leaf-shaped, with muscular fins and a small tail. Such a body plan would allow for great maneuverability, but at the cost of speedy cruising. Its few, large, triangular teeth formed a beak-like arrangement that allowed it to graze bryozoans, sponges, crinoids, and other encrusting animals. The genus contains two species, B. montana and B. occidentalis.
Symmoriiformes is an extinct order of stem-group 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 name to Symmoriiformes in order to prevent it from being mistaken for a family. The symmoriiform fossils record begins during the late Devonian. Most of them died out at the start of the Permian, but Dwykaselachus is known from the Artinskian-Kungurian of South Africa. Teeth described from the Valanginian of France and Austria indicate that members of the family Falcatidae might have survived until the Early Cretaceous; however, these teeth were also argued to be more likely neoselachian teeth.
Falcatus is an extinct genus of falcatid chondrichthyan which lived during the early Carboniferous Period in Bear Gulch bay in what is now Montana.
Harpagofututor is an extinct genus of cartilaginous fish from the Mississippian of North America.
Netsepoye is an extinct genus of cartilaginous fish distantly related to the modern order Chimaeriformes, containing the single species Netspoye hawesi. It lived more than 320 million years ago during the Late Mississippian.
Symmorium is a dubious genus of extinct stethacanthid holocephalian from the Devonian and Carboniferous of the United States (Illinois) and Russia. The type species, Symmorium reniforme, was named by Edward Drinker Cope in 1893 and several other species were originally classed under this genus, but they have since been classified into other genera such as Petalodus. Symmorium bears close similarity in size and appearance to Stethacanthus but the former is missing the "spine and brush" on its back. Some paleontologists think that the two forms are simply the males and females of related species, while other scientists think they were distinct genera.
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 that they survived well into the early Cretaceous, though its putative Cretaceous members were also argued to be more likely neoselachians.
Echinochimaera is an extinct genus of chimaeriform fish, known from the Lower Carboniferous Bear Gulch Limestone in Montana, United States. It is one of the earliest Chimaeriformes known.
Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays (Neoselachii) as part of the clade Euselachii. They are distinguished from other chondrichthyans by their distinctive fin spines and cephalic spines present on the heads of males. An ecologically diverse group, they were abundant in marine and freshwater environments during the late Paleozoic and early Mesozoic, but were rare in open marine environments by the end of the Jurassic, having been largely replaced by modern sharks, though they were still common in freshwater and marginal marine habitats. They survived until the end of the Cretaceous, before going extinct.
Iniopterygiformes is an extinct order of chimaera-like cartilaginous fish that lived from the Devonian to Carboniferous periods. Fossils of them have been found in Montana, Indiana, Illinois, and Nebraska. The Iniopterygians are characterized by large pectoral fins, wing-like projections on their backs, mounted high on the body and denticulated bony plates on the head and jaws. Iniopterygian sharks were small, and their average length was about 18 inches (46 cm). The elongated pectoral fins had denticles along the leading edge which may have had a role in mating. They are thought to have been able to move their pectorals in a vertical plane, ”flying” through the water much like modern-day flying fish.
Gregoriidae is an extinct family of early chondrichthyans from the Carboniferous period. It currently includes three described genera: Gregorius, Srianta, and Bealbonn. This family includes remains formerly ascribed to "Desmiodus," which is now considered a nomen vanum. The relationships between the included genera are not entirely clear. Fossils are known from Serpukhovian-aged formations including the Bear Gulch Limestone and Surprise Canyon Formation.
Ctenacanthiformes is an extinct order of elasmobranch fish. They possessed ornamented fin spines at the front of their dorsal fins and cladodont-type dentition, that is typically of a grasping morphology, though some taxa developed cutting and gouging tooth morphologies. Some ctenacanths are thought to have reached sizes comparable to the great white shark, with body lengths of up to 7 metres (23 ft) and weights of 1,500–2,500 kilograms (3,300–5,500 lb). Ctenacanths are typically thought to have existed from the Devonian to the Late Permian, becoming extinct in the Permian-Triassic extinction event. Members of the family Ctenacanthidae may have survived into the Cretaceous based on teeth found in deep water deposits of Valanginian age in France and Austria, however, other authors contend that the similarity of these teeth to Paleozoic ctenacanths is only superficial, and they likely belong to neoselachians instead. The monophyly of the group has been questioned, with some studies recovering the group as a whole as paraphyletic or polyphyletic.
Cretacladoides is a genus of chondrichthyan, possibly a falcatid, found in France and Austria. Known solely from teeth, mainly found in the Klausrieglerbach locality of Austria, it consists of two species, C. ogiveformis and C. noricum. Assuming a falcatid identity, it is the most recent member of the family, which otherwise became extinct at the end of the Carboniferous.
Squatinactis is a genus of extinct elasmobranch chondrichthyan known from the Carboniferous aged Bear Gulch Limestone in Montana. This fish was discovered in 1974 by Richard Lund. The type specimen, named CMNH 46133, consists of a brain case, poorly preserved jaws and gills, a pectoral fin, and a partial vertebral axis. This creatures most startling feature were its broad pectoral fins which resembled those of stingrays and angel sharks (Squatina). The holotype specimen has about 15 teeth in its jaw. This creature is named after the angel shark. Remains found in the South Urals of Russia and the Eyam Limestone of Derbyshire, England, have been tentatively identified as those belonging to S. caudispinatus.
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