ganisms out, and not even the usual bacteria that decompose dead creatures on the sea bottom were around. As we have seen, this is one reason that oxygen levels plummeted in the Triassic. But the clams figured a way out of this. A few kinds, living on the seafloor of the ocean bottoms that had at least some oxygen, fed not on the falling organics but on methane-containing compounds coming up from some fraction of the organic-rich sediment. Methanogens are a group of bacteria that thrive in low- or no-oxygen conditions, and even several inches down into the sediment on a sea bottom with some oxygen, they would have penetrated into an oxygen-free zone—thus it was an ideal environment for methanogens.

As methanogens metabolize, they release methane as a by-product. The Mesozoic clams may have had other bacteria in their gills that could exploit the methane and other dissolved organic material, or they may simply have fed on the bacteria. A somewhat similar mechanism is found today in the deep-sea vent faunas, where giant tubeworms and clams use these chemicals as food. But the difference is that the modern vent faunas are oxygenated. The animals down there do not even need gills. The clams of the Triassic and Jurassic were not so lucky.

These kinds of clams are found in huge numbers in Triassic and Jurassic sediments. In the latest part of the Triassic, when oxygen reached its lowest levels, the number of these clams was so great that they formed rocks themselves with their shells—a kind of rock known as a coquina. Two Triassic taxa that did this were Halobia and, especially, Monotis. Both lay on the surface of the sediment and were immobile. There was no burrowing (like the majority of today’s clams) or movement on top of the sediment (like modern-day cockles). They were more akin to mussels—they just sat there. And like mussel beds, they were often abundant. In recent Triassic work, many scientists have now sampled and seen these kinds of beds from all over the world in rocks of this age, and the shock of seeing so much life packed into rocks is always striking. For tens of feet of stacked strata there is nothing but an endless packing of shells. There must have been billions of these clams lining the bottoms of the sea, presenting a spectacle that has no parallel today. But the most unusual aspect of the clam beds is

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