Taxa are recognized as non-cornuate (e.g., Ateleaspis, Hirella, and Hemicyclaspis), or part of a monophyletic clade of cornuate taxa, the Cornuata. Of the latter, several families might be recognized—for example, Benneviaspidae, Cephalaspidae, Dartmuthiidae, Kiaeraspidae, Sclerodidae (= Sclerodontidae), Thyestiidae, Tremataspidae, and Zenaspidae (e.g., Berg, 1940; Janvier, 1985, 1996). Afanassieva (1995) discussed the taxonomy of the Tremataspis-like forms and recognized five suborders. As with many groups, there is disagreement on the orthography of the family name (e.g., whether the ending should be -ididae or -idae, as used above). I have made no attempt here to determine which is grammatically correct or which is the traditional usage.
fOrder GALEASPIDIFORMES. The cephalic shield, though variable in shape, resembles that of the cephalaspidiforms. Instead of having a minute dorsal nasohypophyseal opening like cephalaspidiforms, galeaspidiforms have a large median dorsal opening in front of the eyes that connects with the paired nasal cavities and with the pharynx. Galeaspidiforms possessed up to 45 pairs of gill compartments, the greatest number among vertebrates (Janvier, 2004), possessed acellular perichondral bone associated with globular calcified cartilage (Zhu and Janvier, 1998), lacked a dorsal and paired fins, and may have had a hypocercal tail. Lower Silurian (Komoceraspis) to Upper Devonian of China and northern Vietnam (Jiang, 1992).
Wang (1991, 1995) reviewed various taxa. Jiang (1992) recognized 10 families in a cladogram in a revision of the group.
Many genera have been described (e.g., Duyunolepis, Eugaleaspis, Hanyangaspis, Huananaspis, Macrothyraspis, Pentathyraspis, and Polybranchiaspis).
fOrder PITURIASPIDIFORMES (Pituriaspida). Two species from the Lower Devonian of Australia (Young, 1991).
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