ancestor of anapsids during the latter part of the Carboniferous, but anapsids and diapsids have been placed in a single clade (Eureptilia) separate from synapsids. Some lineages of synapsids during the Permian included large herbivorous and carnivorous reptiles called pelycosaurs. Pelycosaurs had elongated, dorsal vertebral spines that formed sail-like structures, which along with their body size (as long as 3 meters) gave them a formidable appearance that understandably resulted in their popularized but mistaken grouping with dinosaurs (Fig. 6.6). However, synapsids also included lineages that later evolved into therapsids, which had some mammal-like characters, and eventually mammals. This means that pelycosaurs are actually more closely related to humans and other mammals than they are to dinosaurs. Mammals are appropriately placed in Mammalia and first show up in the fossil record, at about the same time as the first known dinosaurs, during the Late Triassic.

Diapsida is the clade most pertinent to the discussion of dinosaurs. Diapsids split into two clades, the Lepidosauria and Archosauria, a divergence of lineages that probably happened during the Permian Period. Lepidosaurs are modern lizards, which includes skinks, geckoes, iguanas, Komodo dragons, and their ancestors. A common misconception about large reptiles, such as alligators and crocodiles, is that they are closely related to lizards such as Komodo dragons, but they are

FIGURE 6.7 Thalassomedon, a Late Cretaceous plesiosaur, a marine reptile and an example of a euryapsid. (Euryapsids, and all marine reptiles, were not dinosaurs.) Denver Museum of Science and Nature, Denver, Colorado.

phylogenetically separate, as explained later. Snakes are also lepidosaurs because they share derived characters with lizard ancestors; they even show vestigial pelvic bones. The oldest known snakes in the geologic record are from the Early Cretaceous, thus both lizards and snakes co-existed with dinosaurs during at least part of the Mesozoic (in fact, at least one dinosaur ate a lizard: Chapter 9), and both groups were very successful in later diversification throughout the Cenozoic after the demise of the dinosaurs.

Euryapsids, mentioned previously, are also placed in Lepidosauria because of their inferred common descent from lizard-like ancestors, although they branched into a previously unexplored niche for reptiles, the seas. These diverse, abundant, and often large-bodied marine reptiles of the Mesozoic include the ichthyosaurs, ple-siosaurs (Fig. 6.7), and mosasaurs. Among them were the first vertebrates known to have been viviparous, as shown by a few stunning fossil examples of mother ichthyosaurs with their stillborn young. These fascinating and complex reptiles, like many other vertebrates of the Mesozoic, became extinct by the end of the Cretaceous (Chapter 16). They are sometimes confused with dinosaurs because they were contemporaries and overlapped in size with some of the larger dinosaurs. However, dinosaurs were not only anatomically distinct from euryapsids, they were effectively relegated to completely different environments and niches. Probably the only interactions between these reptilian groups occurred when dinosaur carcasses floated out to sea and were scavenged by euryapsids (Chapter 7).

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