MORPHOLOGY AND PHYLOGENY OF THE DICRANOGRAPTIDAE: SOLVING A LONG STANDING PROBLEM IN GRAPTOLITE SYSTEMATICS

One of the longest standing problems in graptolite systematics is the phylogenetic relationships of the Dicranograptidae (dicranograptines and nemagraptines). The early astogenetic development of Nemagraptus was thought to differ from dicranograptines and all other diplograptids by virtue of its right-handed origin of th1-2. However, Nemagraptus also shares a unique configuration of the sicula aperture with the Dicranograptinae that is not shared by other diplograptids - deep dorsal and ventral notches that form broad lappets on the lateral apertural margins. Several phylogenetic hypotheses have been suggested to explain the character distribution of these features. First, Nemagraptines may be the primitive sister group to the diplograptid - dicranograptid clade. This hypothesis implies that non-dicranograptid diplograptids have lost the unique form of the sicula aperture. However, this branching sequence conflicts sharply with the stratigraphic order of appearance of the taxa. A second hypothesis has the entire family Dicranograptidae as a sister to the other diplograptids. This requires that the similarities in proximal end structure (left-handed origin of th1-2) between dicranograptines and other diplograptids are independently derived and does not solve the problems posed by the stratigraphic record of these taxa. Finally, the Dicranograptidae may be a sister group only to the Orthograptidae based on these shared proximal structures or archiclimacograptids based on thecal structure. We have carefully re-evaluated the proximal end development and rhabdosome morphology of 56 species of diplograptids, including new isolated material of Nemagraptus, Dicranograptus, and Dicellograptus, and conducted a cladistic analysis of 107 phylogenetically informative, multistate characters. Heuristic searches conducted in Paup4* with a range of settings produced variable reconstructions, but re-weighting the characters using a rescaled consistency index consistently produced a single most parsimonious tree with comparable structure across repeated searches. In this tree, Nemagraptus is the primitive sister taxon to the Dicranograptinae, and the Dicranograptidae as a whole is the primitive sister group to the Orthograptidae. This set of taxa forms a sister group to archiclimacograptids.