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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
that there is virtually nothing else in there with them. There may be the occasional ammonite or other mollusk, but these are rare indeed. The clam beds are essentially monotypic—composed of a single species. It is very unusual to see monotypic assemblages in the modern-day marine sea bottom, especially those in the tropics. But the Triassic world was so warm that virtually all the world was tropical, and yet there is never much diversity to these beds, which are found worldwide.
The flat clams began to diminish in the shallows first in the mid-to late Triassic but held on in deeper water until the end of the Cretaceous. The abundant Cretaceous clam Inoceramus lived in many environments, but giants of this kind are found in deep-water deposits. Some fossils are as large as 6 feet across, and like the Triassic clams they seem to have been neither carnivores nor herbivores, but instead they were chemosynthetic, using chemical compounds coming up from the reduced carbon–rich mud on which they lay. Ironically, it appears that they were eventually driven into extinction by rising oxygen levels. With the appearance of an ever-more oxygenated sea bottom in the upper Cretaceous, as atmospheric oxygen rapidly rose, the conditions that had succored the flat clams disappeared.
2. The evolution of low-oxygen-tolerant cephalopods. There are many places in the world where marine strata of the latest Triassic age are overlain by Jurassic strata. At such outcrops one can walk through time, and if the strata are continuous, the dramatic events of the late Triassic and early Jurassic are present for all to see. This interval of time and rock preserves evidence of the great Triassic mass extinction, one of the so-called Big Five mass extinctions, a dubious honor of species death. As you walk through upper Triassic beds you are first in strata packed with fossils of the flat clam Halobia; then you move into younger rocks with the even more abundant Monotis, the clam described just above. Then these clams disappear in turn, over only several feet of strata, leaving a long barren interval of rock and time, the last stage of the Triassic, an interval perhaps 3 million years in length known as the Rhaetian stage. Finally, after this thickness virtually without fossils, a new group suddenly appears—the ammonites.
While there are ammonites to be found in the upper Triassic, they are never abundant. Most seem to have gone extinct when the clams