More Stethacanthus specimens have been discovered, expanding their range from the Midwestern United States to the Lower Carboniferous of Bearsden, Scotland and the Lower Tournaisian of the Tula Region of Central Russia and China. Stethacanthus teeth have been recovered from the Frasnian-Famennian Napier Formation and the Tournaisian Laurel Formation and Moogooree Limestone in Western Australia. A partial palate and jawbone referred to a Stethacanthus sp. has also been recovered from the Bonaparte Basin, Western Australia. Classification The presence of globular calcified cartilage in both the spine-brush complex base plate and brush and in the claspers is interesting because it is the first record of such a large mass of globular calcified cartilage in chondrichthyan. The high presence of globular calcified cartilage raises several questions about the evolution of sharks. It is possible that prismatic cartilage, a defining feature of chondrichthyans, is an evolutionary derivative of globular calcified cartilage. If this were the case, primitive chondrichthyans would have appeared with shark-like scales based instead on globular calcified cartilage. Another feature of note is the thin, acellular bone layer coating the brush and baseplate of the spine-brush complex. It is possible that the coating on the spine-brush complex is the first record of endoskeletal bone in primitive chondrichthyans, and that these endoskeletal features were lost in extant chondrichthyans. It is also possible that the fin spine could be a unique distribution of dermal skeleton and thus derived from neural crest. Following this assumption, the brush would be a fin-baseplate extension. The endoskeletal location and absence of fin radials supports the latter hypothesis. Taxonomic relationships are hard to define for Stethacanthus as there is much variability in the characteristics of the discovered specimens. Chondrichthyes is a monophyletic group characterized by the development of endoskeletal tesserae (mineralized blocks of cartilage) and internal fertilization. Chondrichthyes is further divided into two subclasses: Elasmobranchii and Holocephali. Stethacanthids have been classified as a member of the group Paleoselachii, which is a subdivision of Elasmobranchii. Stethacanthus has been further classified as part of the order Symmoriida, a classification that has sparked a controversy. There are two main hypotheses regarding this classification. One hypothesis states that the Order Symmoriida consists of the families Symmoriidae, Stethacanthidae and Falcatidae and thus are a monophyletic group. Another is that symmoriids are actually the females of stethacanthids or are derived from stethacanthids. This hypothesis is due to the fact that stethacanthids and symmoriids are