[Fred Nerks]

http://www.gi.alaska.edu/ScienceForum/ASF7/742.html

When the Days were Shorter

[ThePythonicCow]

420 million years old ... the day had only twenty-one hours

I don't think that's short enough, recent enough.

For those big dinosaurs to walk, the earth would have had to had days of (guessing wildly) five to ten hours, and this would have had to have been the case 65 million years ago, when the biggest dinosaurs lived.

Note that a day of 100 minutes would put objects on the surface of the earth at close to escape velocity. That 100 minutes is the approximate time it takes a low earth satellite to orbit.