MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF THE LASIOGRAPTINAE (DIPLOGRAPTACEA): ITERATIVE AND DRIVEN TRENDS IN THE EVOLUTION OF “RETIOLITE-TYPE" GRAPTOLITE COLONIES

Diplograptacean graptolites exhibit a dramatically directional record of change in colony form.  The subclade Lasiograptinae, with their peculiar "retiolite-type," basket-work rhabdosome also have an apparently directional history.  Restudy of type material and new three-dimensionally preserved, isolated specimens forms the basis for a broadly-based reassessment of the phylogenetic history of the subclade.  We conducted a cladistic analysis of 107 phylogenetically informative, multistate characters for 56 species, including twelve species among the Lasiograptinae. Heuristic searches conducted in Paup4* with a range of settings produced variable reconstructions, but in all instances the Lasiograptinae emerged as a highly stable clade with consistent sister group relations and internal branching structure.  The group is united by several consistent features of the proximal structure, thecal architecture, and the extrathecal lacinia.  The present set of taxa included in the analysis are insufficient to fully constrain the sister group relations, but it is clear that the Lasiograptinae were derived from an hustedograptid ancestor. During the course of the subsequent evolutionary history of the subclade, early, well-sclerotized species were succeeded by species with very thin to incomplete fusellar periderm and more fully enveloping lacinia. The most highly derived forms (Archiretiolites, Phormograptus, Paraplegmatograptus, Yinograptus, etc.) almost entirely lack fusellar periderm except for the sicula, and posses highly modified rhabdosomes in which the thecae are nearly lost as major constructional units. By the Late Ordovician, such "fuzz balls" comprised the entire subclade, suggesting the group evolved in response to a driven trend.  Orthoretiograptus denticulatus and related species, on the other hand, appear not be lasiograptines but are sister to a derived amplexograptid, most likely an Anticostia.  The repeated evolution of very similar retiolite-type species in these and several other graptolite lineages suggests that the driving force behind the lasiograptine trend was adaptively progressive.  This progressive episode was entirely reset, however, during the terminal Ordovician mass extinction when the entire subclade became extinct.