The ocean tides on earth are caused by both the moon's gravity and the sun's gravity. In general, ocean tides are not generated by the overall strength of gravity, but instead by the differences in gravity from one spot to the next (the gravitational gradient). Even though the sun is much more massive and therefore has stronger overall gravity than the moon, the moon is closer to the earth so that its gravitational gradient is stronger than that of the sun. Because ocean tides are the effect of ocean water responding to a gravitational gradient, the moon plays a larger role in creating tides than does the sun. But the sun's gravitational gradient across the earth is significant and it does contribute to tides as well.
The primary difficulty is the fact that we do not know all of the organisms that currently exist in the world along with those that existed in the past.
