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.