Study of the thecal form, proximal development, and internal structure of some late Hirnantian and Rhuddanian biserial graptolites has led to some new insights into the phylogenetic relationships among the major groups of Silurian diplograptids. Cladistic analyses suggest that Glyptograptus s.s. with a Pattern I (tamariscus) proximal development form a sister group with Pattern I species of Sudburigraptus. These taxa are probably ancestral to the later Llandovery petalolithids. The earliest known Pattern I taxa appear to belong to a group of species related to Paraclimacograptus? kiliani, which first appear at the beginning of the Rhuddanian. Preliminary evidence suggest that these latter forms were ancestral to all subsequent petalolithid (including Glyptograptus) and retiolitid taxa.

Preliminary evidence also indicates that the akidograptids, dimorphograptids, and early monograptids appear to share a common ancestry, from a nomalograptid ancestor like Normalograptus avitus at the beginning of the Rhuddanian. This group appears to form a clade distinct from other Rhuddanian taxa.

At the present time, the relationships among the remaining groups of Rhuddanian normalograptines remain to be studied.

It is recognized that biserial species with a full median septum clearly show two separately growing series of thecae (two stipes) and that those that are aseptate normally have a single series of thecae (one stipe) alternately arranged in two "series" along the rhabdosome. However, many taxa that possess a partial median septum throughout the length of the rhabdosome also appear to have only a single series of alternately arranged thecae. In addition, at least one species, Normalograptus indivisus, appears to possess two stipes but no preserved median septum. Thus, the terms "septate" and "aseptate" do not adequately characterize the range of variation of biserial taxa with one vs. two stipes. In addition, the terms "biserial" and "uniserial" are firmly entrenched as referring to rhabodosomes with theca with two sides and one side of the rhabdosome, respectively. We, therefore, proposed that the term "bistipular" be used to refer to any taxa with two separately growing stipes (i.e., those with a dicalycal theca in the proximal end) and "unistipular" to taxa with a single stipe (i.e., no dicalyc). Thus, monograptids are both uniserial and unistipular, septate diplograptids are biserial and bistipular, but most aseptate diplograptids and some taxa with a partial median septum are biserial and unistipular.

Accepted: June 15, 200