Treatise Online no. 108: Part V, Second Revision, Chapter 20: Suborder Dichograptina: Introduction, morphology, and systematic descriptions

Abstract

The Suborder Dichograptina includes multiramous to two-stiped graptoloids with biradiate isograptid proximal development and maeandrograptid proximal symmetry; colony shape scandent to reclined, horizontal and pendent; prosicula small, with widening and much larger metasicula; thecae simple widening tubes with or without rutellum; a sicular bitheca may be present in early taxa; branching dichotomous or cladial. Lower Ordovician (upper Tremadocian, Hunnegraptus copiosus Biozone)-Middle Ordovician (upper Darriwiliam, Pterograptus elegans Biozone) worldwide distribution. Dichograptina is one of the two major groups originating of the early plancktic graptolites of the family Anisograptidae. According to Maletz, Carlucci and Mitchell (2009) the Dichograptina and Sinograpta (Sinograptina herein) can be regarded as sister groups. However, Fortey and Cooper (1986) and Lindholm (1991) suggested a polyphyletic origin of derived graptoloids (non bithecate graptoloids) from the Anisograptidae. Thus, the sister group relationship between Anisograptidae and the supercohort Eugraptoloidea (the Graptoloidea of Bulman, 1970) as shown by Maletz, Carlucci and Mitchell (2009) is unlikely to be correct and more research in the transition is needed. Lapworth (1873) understood the Dichograptidae as the group of genera that includes two stiped to multiramous taxa with simple thecae and dichotomous bifurcation or lateral branching. He listed the genera Didymograptus, Tetragraptus, Dichograptus, Loganograptus and Clonograptus. Obut (1957) erected the Suborder Dichograptina for the families Dichograptidae, Tetragraptidae, Bryograptidae and Didymograptidae, essentially confirming the classification of Lapworth (1873) for multiramous or pauciramous Lower to Middle Ordovician graptoloids. However, the name Dichograptina must be credited to Lapworth under the provisions of the ICZN, 1999, Article 36. Bulman (1970) used the term Didymograptina Lapworth, 1880 but included the Nemagraptidae, Dicranograptidae, and Phyllograptidae taxa that are not closely related. The Didymograptina of Bulman (1970) thus is a polyphyletic taxon and not be used. Fortey and Cooper (1986) used the Didymograptina Lapworth, 1873 for the non-virguelated graptoloids and introduced the new term Virgellina forall virgellated graptoloids. They defined the Dichograptina as ?graptoloids lacking bithecae and virgella? (Fortey & Cooper, 1986, p. 640) and referred the two superfamilies Dichograptacea and Glossograptacea to the taxon. Maletz, Carlucci and Mitchell (2009) introduced the supercohort Eugraptoloida as a monophyletic taxon defining all non-bithecate graptoloids. Of these, the stem reclinatids and the Pan-Tetragrapta would be included in the Dichograptina. Maletz (2014) referred four families to the Dichograptina: the Dichograptidae, Didymograptidae, Tetragraptidae, and the virgella-bearing Pterograptidae. He discussed the taxon as a paraphylletic unit with poor internal resolution.