The oceanic non-sulfidic oxygen minimum zone: a habitat for graptolites?

Abstract

The denitrified low oxygen zone in Early Paleozoic oceans is proposed as a potential habitat of planktic graptolites. Modern analogs of this zone are found in the eastern tropical Pacific (ETP) and in the north­ern Arabian Sea as shallow regions, up to a 100 meters thick, at the top of the pycnocline. There, oxygen is low or undetected and hydrogen sulfide has not been found. In modem oceans, denitrification regions are limited vertically and horizontally as oxygen is replenished from below by ventilated deep waters. In the Early Paleozoic ocean, the denitrification layer would be global due to poor deep ventilation. It would be transitional between oxygenated surface waters and toxic sulfide-rich water. Many branched graptolites could have evolved when the denitrified waters were in or close to the photic zone, feeding on the abun­dant phytoplankton attracted to both light and nutrients. As this zone sank below the photic zone, grapto­lites who developed planktic mode of life could have migrated daily toward the food supply, similar to euphausiids in the modern ETP. Thus the changes in graptolite rhabdosomes from pendent to scandent and from many branched to biserial and uniserial are suggested as adaptations to assist vertical migration and feeding. With the continued ventilation of the oceans and the shrinking of the denitrified layer, grap­tolite extinction could have resulted as a combination of reduction in food supply and living space, in­creased predation from the then evolving fish and ammonites, and competition in the zoop!ankton niche from smaller (less visible) and more motile forms.