SECOND-ORDER SEA-LEVEL FLUCTUATIONS TRIGGER GRAPTOLITE MIGRATION INTO DEEP WATER

Planktic graptolites originated from a benthic ancestor in the shelf areas of the early Paleozoic oceans and quickly adapted to the fully planktic marine life style. From the shallow marine, proximal shelves they conquered the wider distal shelf regions and differentiated into epipelagic and mesopelagic faunas. The migration and adaptation of these planktic graptolite faunas to deeper water environments during the Ordovician parallels the flooding of ample shelf regions worldwide caused by second-order sea-level rises. Whereas the graptolite habitat was relatively limited during overall sea-level lowstands, the highstands opened up large areas where deep-water graptolites now flourished.

During the Ordovician, graptolites conquered the deep shelf habitat twice. From the earliest Tremadoc on, the cone- to umbrella- shaped Rhabdinopora successively migrated towards deeper waters while developing larger rhabdosomes and leading to various new rhabdosome shapes. The tranquil deep shelf environment must have favored the evolution of large multiramous species and initiated their radiation during the Early Ordovician, leading to the evolution of up to magnificent, mega-sized forms such as Clonograptus, Hunnegraptus, Holograptus and Araneograptus. These dominate the deeper water regions, while smaller forms of a more varied rhabdosome shape frequented the shallower regions. With the extinction of the clonograptids and sigmagraptines during the early Darriwilian, the abundance of deep-water species worldwide decreased considerably.

The second invasion of large, multiramous graptolites into the deep-water habitat took place in the earliest Upper Ordovician. Again, multiramous forms developed such as Nemagraptus gracilis and multiramous Dicellograptus derivatives, reflecting a significant increase in size and diversity compared to their earlier late Middle Ordovician counterparts. With the onset of the Hirnatian glaciation, the Late Ordovician deep-water graptolites disappeared again.