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Dinosaurs are a diverse taxon, with unique combinations of characters to
stimulate one's interest. The skulls and teeth are among the
most important features of fossilized dinosaur bones.
Dinosaurs are clearly members of two distinct orders, Saurischians and
Ornithischians, each of which can be defined on the basis of many
osteological characters. The pattern of extinctions at the end of the Mesozoic era (the Cretaceous - Tertiary boundary) is puzzling because different animal groups were affected differently. Small pterosaurs survived largely unaffected. In the seas, marine reptiles were eliminated completely. Ammonites became extinct, but nautiloids and squids survived. Several groups of microorganisms became extinct, but others including diatoms became more abundant and more diverse than ever before.
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