Dictyonema J. Hall, 1851

Rickards, R. B., Chapman, A. J., Wright, A. J. & Packham, G. H, 2003, Dendroid and Tuboid Graptolites from the Llandovery (Silurian) of the Four Mile Creek Area, New South Wales, Records of the Australian Museum 55 (3), pp. 305-330 : 310

Dictyonema J. Hall, 1851 and Callograptus J. Hall, 1865

The distinctions between these two genera are not very great. Bulman (1970) considered Callograptus to have no dissepiments or to have fewer than Dictyonema . Both genera originate in the Middle Cambrian (see Chapman et al., 1996, fig. 2) and range into the Carboniferous. Both genera have rhabdosomes with varying geometry, although each species seems to have a single mode of growth. Both genera have regular branching patterns, and long parallel stipes: and they both exhibit the same range of variations of autothecal and bithecal type. The sole difference seems to be the extent to which dissepiments are developed: Callograptus has none or few; Dictyonema has common dissepiments. Species of Callograptus with no dissepiments are not unlike some of the earliest species of Dendrograptus (see Chapman et al., 1996, p. 195) which have fairly ordered branching patterns and long, roughly parallel stipes in bushy, flabellate and possibly conical rhabdosomes. However, in Dendrograptus the autothecae remained simple denticulate whereas in Callograptus quite varied autothecae were quickly evolved.

The evolutionary scenario deduced by Chapman et al. (1996) was of ordered Dendrograptus species giving rise in the Middle Cambrian to Callograptus and Dictyonema by increased development of dissepimental connecting bars between adjacent stipes, with a concomitant restriction in the arrangement of stipes giving rise to flabellate and conical rhabdosomes. In the present work, Fig. 3 extends and revises the relationships deduced in the earlier paper, suggesting that the development of anastomosis and dissepiments were key stages in the evolution of the benthic Dendroidea . This reinforces the earlier suggestion that the method of stipe connection is of more significance taxonomically than is rhabdosome morphology.

Figures

Fig.3. Early evolution of some benthic Dendroidea,and modifying the suggestions of Chapman et al. (1996), and illustrating the rôle of anastomosis and dissepiments development. Full explanation is to be found in text. Asterisks indicate genera from which species are described in this paper. Boxes indicate approximate time of appearance of that character.