| Orthacanthus Fossil range: Lower Carboniferous - Permian | |
|---|---|
 O. seckenbergianus | |
| Scientific classification
| |
|
Orthacanthus | |
|
See text. | |
Orthocanthus specimen in a German museum.
A tricuspid Orthacanthus tooth from Bolsovian shale at Whitehaven, Cumbria, England.
Orthacanthus fin spine from the Salem Formation in Indiana.
Orthacanthus is an extinct genus of fresh-water shark from a family of prehistoric sharks known as Xenacanths. Fragmented Orthacanthus teeth from the Halgaito shale of Arizona, USA, showing typical penis crescent-shaped bicuspids.
Members of the genus had a long spine growing from the back of their skull and a very long dorsal fin, which ran all along its back giving it an eel-like appearance.
About 260 million years ago, Orthacanthus was the terror of freshwater swamps and bayous in Europe and North America. Its body reached nearly 10 feet in length and its powerful jaws were lined with double-fanged teeth. They first appeared almost 400 million years ago in the Devonian, and became extinct just before the Mesozoic, about 225 million years ago.
Species[]
- Orthacanthus arcuatus(Newberry, 1856)
- Orthacanthus cylindricus(Agassiz, 1843)
- Orthacanthus senckenbergianus
Description[]
Teeth[]
The larger teeth of Orthacanthus compressus and Orthacanthus texensis are differentiated by a more pronounced basal tubercle in O. compressus. The basal tubercle of a typical tooth file is on the apical button of the underlying tooth. The larger adult teeth of O. compressus have a wider rather than longer base, similar to O. texensis, and tend to have serrations on both carinae of each cusp, while the medial carinae of smaller adult teeth are not serrated. The juvenile teeth of O. compressus are longer than wide, have a thinner base, and lack serrations, similar to O. platypternus teeth.
Orthacanthus platypternus from the Craddock Bonebed shark layer in Texas, USA, shows evidence of resorption, and the equivalent of an "enamel pearl." Some of the teeth specimens found at this location show evidence of resorption, which has not been previously observed in other faunas at the same location. Where the superjacent basal tubercle is expected to be resorbed if the teeth were to undergo resorption, the apical button is resorbed instead.
Sexual Dimorphism[]
The difference in characteristics between the large and small O. compressus adult teeth might indicate sexual dimorphism.
The spines of O. platypternus showing 3 to 4 dentine layers are interpreted to be subadults or young adults, and are separated into two size classes where females have the largest spines in comparison to males, indicating sexual dimorphism.
Dorsal Spine, Dentine, and Denticles[]
The dorsal spines of Orthacanthus platypternus from the Craddock Bone Bed in Texas, USA, preserve a highly vascularized wall mainly composed of centrifugally growing dentine (the outer layer of the wall of the spine) in a succession of inwardly growing dentine layers that line the pulp cavity. These dentine layers are likely deposited periodically in accordance with seasonal variations in water temperature and food availability. More specifically, the periodic nature of the dentine layer deposits could be due to variation in calcium phosphate deposition following the changes in water temperature. Spines of individuals with 1-2 dentine layers are likely juveniles and result in the smallest sizes, whereas individuals showing at least 3-4 dentine layers result in two separate size classes. The cross section is oval near the opening of the pulp cavity and circular/subtriangular in the distal part of the non-denticulated region and circular in the denticulated region. The pulp cavity of the spine is filled with calcite, quartz, and opaque minerals.
Occipital spine and denticles[]
The spine is superficially inserted in the skin, where it grows and moves from a deep position in the dermis where trabecular dentine forms, to a superficial location where centrifugally growing lamellar dentine forms. The number of denticles per annual cycle vary with growth rate, and are independent dermal elements formed by the dermal papilla and secondarily attached by dentine to the spine proper. The density of denticulation also varies with the growth rate of the occipital spine. The ratio of length of denticulated region to total length of the spine changes throughout ontogeny.
Historical information and discovery[]
Genera Orthacanthus and Pleuracanthus were founded by Agassiz in 1837 on isolated "ichthyodorulites" from the British Carboniferous System, and at the time were mistakenly thought of as the first indicators of Skates. They were initially found throughout the United Kingdom in Dudley, Leeds, North Wales, Carluke, and Edinburh. Three additional species from the Carboniferous formation of Ohio were described by Dr. Newberry. Teeth associated with Diplodus, a genus of sharks, was found in the Carboniferous slates of England in Stafford, Carluke, and Burdiehouse, and in Nova Scotia. A well preserved impression from Ruppelsdorf, Bohemia was described by Goldfuss, while a separate paper published the same specimen under the name Xenacanthus Dechenii. One year later in 1849, Dr. Jordan mistakenly identified the same specimen as the remains of a fossil shark Triodus sessilis. This mistake was corrected and identified as Xenacanthus by Mr. Schnur.
Classification[]
The teeth of Orthacanthus texensis and Orthacanthus platypternus from bonebeds from the Lower Permian of Texas, and the teeth of Orthacanthus compressus from the Upper Pennsylvanian of Nebraska and Dunkard Basin of the central Appalachians were used to determine the origin of O. texensis and O. platypternus. It has been proposed that both O. texensis and O. platypternus could be derived from O. compressus, where juvenile features of O. compressus are retained in the adult teeth of O. platypternus via paedomorphosis, and the juvenile features of O. compressus teeth are observed in the adult teeth of O. texensis.
Taxon Mimia sits outside of a clade that contains two monophyletic sister groups. The first monophyletic sister group defines a clade that includes the stem-group chondrichthyans which is visualized as a sister group of Doliodus and a large clade that comprises the cladodont sharks and Orthacanthus. The “cladodont sharks” plus Orthacanthus comprise two monophyletic sister groups: Orthacanthus and the Cladodoides and Tamiobatis on one side, and the Symmoriiformes on the other side. The second monophyletic sister group characterizes crown chondrichthyans which contains two monophyletic sister groups (Euchondrocephali on one side and Euselachii on the other side).
Cladistic Analysis[]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||