Pachycephalosaurus Timeline

Pachycephalosaurus Timeline

Time	Event
Approximately 220 million years ago	First dinosaurs evolve during the Triassic period.
Approximately 76 million years ago	First Pachycephalosaurus appears during the Cretaceous period.
Approximately 65 million years ago	Cretaceous-Tertiary extinction event: All non-avian dinosaurs (including Pachycephalosaurus) go extinct. The K/T Extinction Event may have been caused by an asteroid impact:
1850s	Unknown fossil hunter finds Pachycephalosaurus in the Lance Formation in Southeastern Montana. The earliest name used to refer to the animal is "Tylosteus". Map of Hell Creek and Lance Formations:
1872	Joseph Leidy identifies the fossil as belonging to the armor of a reptile or an armadillo-like animal (more than a century later it turned out to be a bone from the knobs on Pachycephalosaurus's skull). Joseph Leidy:
1931	What is today known as Pachycephalosaurus wyomingensis is named by Charles W. Gilmore as Troodon wyomingensis. Charles W. Gilmore:
1945	Barnum Brown and Erich Maren Schlaikjer, using newer and more complete specimens, establish the Genus Pachycephalosaurus. They identify two species: Pachycephalosaurus reinheimeri and Pachycephalosaurus grangeri (although neither are generally recognized as distinct species today). Barnum Brown:
1985	Donald Baird successfully campaigns to get the officially recognized name for the Genus to be "Pachycephalosaurus", even though the normal rules of zoological nomenclature would dictate "Tylosteus", since this was the first name used. As a result of this the only species generally recognized today is Pachycephalosaurus wyomingensis. Pachycephalosaurus wyomingensis at the Academy of Natural Sciences, in Philadelphia, Pennsylvania:

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