Sheets are a great step forward for postal authorities: instead of selling stamps one at a time, you sell
them wholesale. They also enable designers to let their imagination run wild over a larger area. For collectors,
they are a mixed blessing at best, and they certainly hit you hard financially.|
Anyway, the first sheet with a prehistoric theme is this one, issued in 1990 by the South American country of Guyana, with twenty South American prehistoric animals in one go.
When I created this page, I had in mind only sheets containing stamps with different designs but the same face value. However, as Robert Margulski pointed out, Bulgaria issued a six-value prehistoric animals set a few months earlier in 1990 - and also issued it in a collective sheet.