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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
northern or southern latitudes where pines and fir trees dominate the forests.
Unlike during any of our previous trips, the land is alive with animals and the arthropods, at least, are giants relative to similar species living today. Enormous insects, including dragonflies with nearly yardwide wings, flit about above swampy forests, and even the drier upland areas show a high diversity of both flying and earth-bound insects, intermingled with spiders, scorpions, and millipedes, and many are also giants. But it is the vertebrate life that interests us most, and here too there are giants, at least compared to the very first terrestrial forms of some 40 million years earlier, in the early part of the Carboniferous (but certainly not giant relative to the dinosaurs of the future). In the swamps, giant amphibians, some 10 feet long and massive in girth, lie about like modern-day crocodiles. Land vertebrates, freed from water by the evolution of amniotic eggs, are similarly large. Chief among them is the Sail Back, Dimetrodon, and among its pack scurry smaller reptiles, the eventual rootstocks of turtles, lizards, crocodiles, and dinosaurs.
We turn to the oceans, where a huge diversity of fish is immediately noticeable. The largest are the cartilaginous sharks, some with peculiar heads and teeth, but bony fish are there too. Missing are the placoderms and ostracoderms. Chambered cephalopods compete for dominance with the smaller fish, but gone are the enormous straight nautiloids of earlier times. Almost all nautiloids are gone, in fact, replaced by ammonoids—cephalopods with shells that look like that of the living chambered nautilus but showing soft parts more akin to modern squids than the archaic soft parts of the nautilus. Yet as interesting as the active swimmers are, it is the nature of the attached animals that attracts immediate interest. Moving in shallow water above a wide and warm continental shelf we find seemingly endless fields of flowers. But flowers will not appear on Earth for another 200 million years. A closer look shows only the resemblance of these long-stalked animals with petal-like heads to the flowers we know. They are crinoids, or sea lilies, animals banished in our world to the marginal habitats of the less successful. But there is clearly no lack of success here, and the appearance of square miles of these fantastic stalked echinoderms is