Growth rings in fossilized clams known as rudists, such as this one, record the changing length of Earth's day due to the gravitational interactions between Earth and the Moon. A recent study found that, near the end of the time of the dinosaurs, each year consisted of 372 days, which means a day was about 30 minutes shorter than it is now. Earlier studies had shown that the length of a day was even shorter in the more distant past. [Wilson 44691/Wikipedia]