Chemistry Reference
In-Depth Information
The Ordovician radiation of heavily calcified skeletons
Although Cambrian evolution produced diverse body plans, species diversity within
major clades remained low as the Ordovician Period began. But all this was about to
change. During renewed Ordovician diversification, the global diversity of families and
genera preserved as fossils increased three- to four-fold (Figure 5; Sepkoski 1982; Miller
1997), crown group members of major invertebrate phyla and classes came to dominate
marine ecosystems, and the marine carbonate and silica cycles both came under
substantial biomineralogical control.
Animal taxa that radiated during the Ordovician Period include sclerosponges and
other new experiments in demosponge calcification; both tabulate and rugose corals;
cephalopods, bivalves and gastropods; bryozoans; calciate brachiopods; crinoids and
other echinoderms; and fish with dermal armor—establishing the fauna that would
dominate oceans for the rest of the Paleozoic Era (Sepkoski and Sheehan 1986).
Metazoan reefs returned to the oceans, skeletonized red and green algae became
widespread, and radiolarians expanded dramatically, firmly establishing biological
control over the marine silica cycle (Maliva et al. 1989; Racki and Cordey 2000).
Moreover, both maximum size (e.g., Runnegar 1987) and skeletal mass increased in
many groups.
Again, it is helpful to frame these observations in terms of evolutionary costs and
benefits. The physiological cost of precipitating such massive skeletons must have been
high, although, if, as noted above, Late Cambrian environmental conditions made the
costs of carbonate skeletons prohibitive, then relaxation of those conditions might have
contributed to Ordovician radiation. In any event, the phylogenetically broad evolution of
robust skeletons as part of an overall increase in biological diversity once again suggests
that an upward ratcheting of predation pressure contributed to the observed evolutionary
pattern; important new predators included nautiloid cephalopods and starfish. (Increase in
skeleton mass is a biomechanical requirement of increased body size but cannot explain
why groups like corals and bryozoans evolved robust carbonate skeletons
de novo
.)
Miller (1997) pointed out that Ordovician diversification took place in the context of
increasing tectonic activity and by implication, therefore, increased nutrient flux to the
oceans. Increasing nutrient status would certainly help to explain increased body size and
predation in Ordovician oceans (see Vermeij 1995 and Bambach 1999 for arguments why
this should be so).
Permo-Triassic extinction and its aftermath
The end of an era.
For most of the Permian Period, marine biology looked much as
it had in for the preceding two hundred million years. Large, well-skeletonized sponges,
rugose and tabulate corals, calciate brachiopods, mollusks, and stenolaemate bryozoans
characterized benthic communities, while radiolarians contributed a skeletal component
to the plankton. Calcified algae played important roles in carbonate build-ups on
continental shelves and platforms (Samankassou 1998; Wahlman 2002), and—in contrast
to most earlier Paleozoic reefs—so did non-skeletal cementstones, precipitated at least in
part under microbial influence in reef cavities and on the seafloor (Grotzinger and Knoll
1995; Weidlich 2002). Evidently, Permian seawater was highly oversaturated with
respect to calcium carbonate minerals, a circumstance related in part to low sea levels
and, consequently, limited shelf area for skeleton-forming benthos (Grotzinger and Knoll
1995). Despite the broad Paleozoic continuity of the marine biota, several important
innovations in skeletal evolution did occur after the Ordovician radiation. By the
Devonian Period, foraminiferans had evolved the capacity to precipitate calcitic tests,
allowing benthic forams, especially the large, symbiont-bearing fusulinids, to emerge as
