Orthacanthus

The two genera Orthacanthus and Pleuracanthus were erected by Louis Agassiz based 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 in the United Kingdom in Dudley, Leeds, North Wales, Carluke, and Edinburgh. Three additional species from the Carboniferous formation of Ohio were described by John Strong Newberry, but two of them were junior homonyms of another species, Orthacanthus gracilis (Giebel, 1848). Accordingly, these two species received replacement names, O. adamas Babcock 2024 and O. lintonensis Babcock, 2024. 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, and in a separate paper, the same specimen was described under the name Xenacanthus dechenii. One year later, in 1849, Dr. Jordan mistakenly identified this specimen as the remains of a fossil shark, Triodus sessilis. This mistake was corrected and the specimen was identified as Xenacanthus by Mr. Schnur. == Description ==

Teeth , Cumbria, England The larger teeth of Orthacanthus compressus and Orthacanthus texensis are differentiated by a more pronounced basal tubercle in O. compressus. 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. 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. == 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.}}}} }}}}}}}}|label1=Chondrichthyes}} ==Paleobiology==

of O. platypternus and O. texensis A 2013 analysis of oxygen and strontium isotope composition of the teeth and spines of Late Carboniferous and Early Permian shark taxa was performed to infer the hydrochemistry of their ambient water, thus contributing to the controversy between an obligate freshwater or euryhaline diadromous lifestyle. Facies interpretations in the Permian of North America suggested that salinity tolerances of xenacanthiforms were restricted to near marine environments whereas only Orthacanthus could tolerate brackish water environments. Predator-Prey relationship Orthacanthus and Triodus have a predator-prey relationship in which Orthacanthus preyed on Triodus. Cranial remains of specimens of both Orthacanthus and Triodus from the Upper Carboniferous in Puertollano basin, Spain, give evidence of this predator-prey relationship. Orthacanthus had a diet that consisted of actinopterygians, acanthodians, dipnoans, xenacanthids and tetrapods, based on analysis of coprolites and gut contents. There have also been suspicions of filial cannibalism due to the presence of juvenile Orthacanthus teeth inside an Orthacanthus coprolite. The feces of Orthacanthus has a spiral shape due to a corkscrew-shaped rectum. == Paleoecology ==

The paleobiogeographical distribution of O. platypternus suggests ontogenetic habitat partitioning. While smaller individuals likely lived in shallower waters such as in small ponds and stream channels of the coastal plain, larger individuals likely lived in deeper water such as the fluvio-lacustrine (rivers and lakes) and marginal marine areas. ==References==