Orthacanthus is an extinct genus of fresh-water xenacanthiform cartilaginous fish, named by Louis Agassiz in 1843, ranging from the Upper Carboniferous 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,
Discovery and history
thumb|left|Orthacanthus teeth from the [[Halgaito shale]]
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.
Dorsal spine, dentine, and denticles
thumb|left|Skull in metal body reconstruction, AMNH
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.
Paleobiology
thumb|right|[[Life restoration 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. 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.