POPULATION-STRUCTURE OF GRAPTOLITE ASSEMBLAGES

Graptolite rhabdosomes display a diverse suite of morphologies. The ra nge of morphotypes present within most moderate-to high-diversity asse mblages from the Ordovician and Silurian is similar, despite the diffe rent taxonomic composition of the faunas at different rimes. Survivors hip analyses of graptolite faunas from the Ordovician and Silurian dem onstrate strong similarities in the mortality rates of unrelated grapt olites of similar functional morphology. They also show a strong corre lation between decreasing mortality rates among more mature colonies w ith increasing rhabdosome complexity. This similarity in both function al morphology and life history of graptolites suggests that they lived within a very stable planktic community structure.