both freshwater and saltwater. But this diversity ultimately suffered many casualties during the extinction event at the Frasnian–Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the end-Devonian extinction; not a single placoderm species has been confirmed to have survived into the Carboniferous. History of study The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms. Some naturalists even suggested that they were shelled invertebrates or even turtle-like vertebrates. In the late 1920s, Dr. Erik Stensiö, at the Swedish Museum of Natural History in Stockholm, established the details of placoderm anatomy and identified them as true jawed fishes related to sharks. He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface in wax. Once the specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with sharks; however, as more fossils were found, placoderms were accepted as a sister group of chondrichthyans. Much later, the exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with other gnathostome groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase. Placoderms also share certain anatomical features only with the jawless osteostracans; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of stem gnathostomes. Taxonomy and phylogeny Currently, Placodermi are divided into eight recognized orders. There are two further controversial orders: One is the monotypic Stensioellida, containing the enigmatic Stensioella; the other is the equally enigmatic Pseudopetalichthyida. These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten. Placoderm orders Arthrodira Arthrodira ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying