planktonic communities, while a new form of gastropods, the entirely carnivorous neogastropods, joined a host of other shell-breaking predators to totally transform the benthos in what has been called the Marine Mesozoic Revolution. A response to this increase in predation was the evolution of infaunal siphonate clams (with heterodont dentition) and stalkless crinoids (the comatulids). All of these events are well known and documented and thus are not the targets of revisionist history. But the reasons behind the specific designs of some of these same organisms are another matter. The rising oxygen of the Jurassic and Cretaceous following the Triassic nadir caused an increase in the number of species, but, as we have seen, it was low oxygen that stimulated new kinds of body plan. Let’s look at four types of aquatic body plans that are related to oxygen levels in the seas and that also made the Mesozoic oceans very different places than our present-day oceans.
1. The evolution of low-oxygen-tolerant bivalves. The oxygen low of the early through middle Triassic produced a new and poorly habitable ocean. As we have seen, animals do very badly in low oxygen. Atmospheric oxygen levels affect oceanic oxygen levels and quite often even serve to magnify the effects. Many ocean bottoms of the Mesozoic were completely anoxic, and most were at least hypoxic. Rare was an ocean community of these times that had oxygen levels like those on the bottoms of modern-day oceans.
Just as in the Cambrian Explosion, where animals were stimulated to produce new kinds of body plans based around respiratory systems, so too did animals of the Triassic seas show a multitude of new adaptations. As we have seen, the land fauna experimented with a variety of lung types. The same kind of exploration took place in the oceans. The bivalved mollusks were one group that evolved a new kind of body plan, and even physiology, in response to the nearly endless expanse of nutrient-rich but low-oxygen bottoms.
The very lack of oxygen on the ocean bottoms made them, in one sense, wonderful places to live. Vast quantities of reduced carbon, in the form of dead planktonic and other organisms, fell to the seafloor and were buried there. On an oxygenated bottom this material would soon be consumed, by filter- or deposit-feeding organisms and scavengers, and used for food. But the low-oxygen conditions kept these or-