What is Diplograptus?

Abstract

Diplograptus pristis (Hisinger, 1837) is the type species of the genus Diplograptus and name bearer for Suborder Diplograptina. This species is primarily kown from the Dalarne region of Sweden, where it occurs in the P. linearis Zone. D. pristis exhibits a suite of features (particularly the shape of the proximal end, the configuration of proximal spines, and the gradient in thecal form) that is uniquely shared with a number of other late Katian and earliest Hirnantian species including Glyptograptus nicholsoni Toghill, 1970; Glyptograptus posterus Koren' & Tzai (in Apollonov et al. 1980); Orthograptus maximus Mu, 1945; and Orthograptus truncatus rarithecatus Ross & Berry, 1963. The phylogenetic affinities of the Diplograptus clade are equivocal, however. Two isolated, three-dimensionally preserved specimens of D. pristis from the Paasvere 309 core in Estonia reveal that the rhabdosome is aseptate and has a simple proximal structure comparable with a Pattern G astogeny. The pattern G astogeny, long, fully-sclerotized sicula, aseptate rhabdosome with free nema all point to its being a member of the derived orthograptids. On the other hand, the rapidly enclosed sicula and lack of antivirgellar spines together with the apertural spines on the first pair suggest archiclimacograptid affinity. Cladistic analysis supports orthograptid affinities. If these relations are correct, the Orthograptidae Mitchell, 1987, must be regarded as a junior synonym of the Diplograptidae Lapworth, 1873.