8. STRATIGRAPHIC OCCURRENCES
When adding the time scale to the phylogeny, we see a certain pattern in the evolution of ichthyosaurs. Ichthyosaurs were quite diverse by the end of the Early Triassic. By early Late Triassic, there already were forms approaching the typical body profile seen in truly fish-shaped ichthyosaurs. So, the fish-shaped body plan, which, persisted for about 140 million years once established (until the extinction of the group>, evolved somewhat quickly (within 20 million years or less).
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Last updated on November 15, 2000