Life in our oceans has changed greatly since the Paleozoic. There have been many attempts in describing the major trends and innovations that led to the evolution of the modern benthic marine ecosystem. According to Bush et al. (2007), there are three variables that led to evolution of benthic marine ecosystems: 1) vertical tiering, the position of an organism relative to the sediment /water interface; 2) motility, the capability of an organism to move under its own power; and 3) complex feeding mechanism. These three variables led to the transformation of the modern benthic marine ecosystem through evolutionary innovations.
Trend 1: Infaunalization
Bush et al. revealed that the marine ecosystem transitioned from one that was dominated by surficial and erected organisms during the Paleozoic to one that was dominated by infauna during the late Cenozoic era. Before this transition, over 90% of mid-Paleozoic fossil assemblages—brachiopods, bryozoans, stalked crinoids, and rugose and tabulate corals—lived on the surface of the seafloor—dubbed “surficial.”
However, as the Cenozoic era approached, the number of semi-infauna and infauna—organisms that live buried beneath the sediment—increased, mostly due to the diversification of siphonate bivalves and even some gastropods and echinoids. Consequently, several surficial organisms such as brachiopods were out competed and removed from the soft-bottom marine environment. For instance, brachiopods and stalked crinoids are mostly found in the deap-sea (Vermeij).
In addition, the increase in infauna resulted in the increase of bioturbation. Therefore, what was anoxic sediment became oxygenated, and thus resulted in an expansion of the benthic environment and its subsequent niches, which were now available for organisms to fill.
Trend 2: Increased Motility
Motility also increased dramatically from the Paleozoic to the Cenozoic. Bush et al. demonstrated that 70-90% of Paleozoic organisms were non-motile, mostly of which were brachiopods and stalked crinoids. However, by the late Cenozoic, only 4-9% of the sample was non-motile. Consequently, non-motile organisms such as stalked crinoids no longer dominated benthic communities due to their inability reattach and reestablish themselves.
Furthermore, the number of organisms with facultative motility increased to 50-60% of Cenozoic benthic communities due to the diversification of bivalves. In addition, the number of fully motile but benthic organisms, such as gastropods, increased as well. This increase in motility led to increased interactions amongst the organisms and between the organisms and the environment as well.
Trend 3: Increased Predation
The increase in predation methods and intensity is the third trend, in what is dubbed the Mesozoic Marine Revolution. During this arms race between predator and prey, predators developed innovations such as drilling (gastropods and octopods) and more advanced jaws, teeth, and claws (fish, arthropods) for ripping through prey; as a result, more specialized predators evolved.
Similarly, prey also developed defensive innovations such as thicker skeleton/shells and spines (gastropods and bivalves). Furthermore, the increase in grazers—such as sea urchins—have profound influence on the composition and architecture of benthic communities by grazing on the algae and even reefs. Sea urchins began to dominate intertidal environments in the later half of the Mesozoic.
References
Bush, A. M., Bambach, R. K., and Daley, G. M. 2007. Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Paleozoic and late Cenozoic. Paleobiology 33: 76-97.
Vermeij, G. J. 1977. The Mesozoic marine revolution: evidence from snails, predators and grazers. Paleobiology 3: 245-258.
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