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. 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,[13] 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.[13]