Page 235 - Introduction to Paleobiology and The Fossil Record

222 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.8 Acritarchs and the food chain

Groups such as the acritarchs formed a prominent base to the relatively short, suspension-feeding
Early Paleozoic food chains, yet it is virtually impossible to quantify the abundance of microfossils
in sediments because many factors such as cyst production, hydrodynamic sorting and taphonomy
come into play. Unfortunately, diversity cannot be used as a proxy for abundance, so there is no
direct evidence in the fossil record of just how densely packed the water column was with phyto-
plankton, say during the Ordovician. However, it may be possible to speculate that primary produc-
tion increased rapidly during the Ordovician: This period was marked by the appearance and
radiation of the graptolites, phyllocarids, some groups of echinoderms and the radiolarians. Huge
bursts in diversity are seen among the brachiopods, mollusks and trilobites, while there was increas-
ing complexity in benthic and reef communities. Yet little is known about the cause of this phenom-

enal diversification. Marco Vecoli and his colleagues (2005) have now suggested that these massive
metazoan radiations probably signal a cryptic explosion in primary production in the world’s oceans
(Servais et al. 2008) that may have been one of the main triggers for the great Ordovician biodiver-
sifi cation (see p. 253). The diversity curves of these protistan groups appear to match perfectly those
of the metazoans (Fig. 9.14).

Figure 9.14 Acritarch and invertebrate diversity through Ordovician Period. (Courtesy of
Thomas Servais.)