Or, an alien invasion.

Two days ago I visited the Oregon Museum of Science and Industry which had a T-Rex exhibit. It's amazing to contemplate these animals having lived 65 to +120 million years ago. That they left anything behind that could be reconstructed provides wonder, not only that they could be re-assembled, but the vast space of time. Most of life disappears without trace and we should consider that in choosing how we should live.

Parenthetically, the species depicted going back in time (120 MMY) became progressively smaller. Could some environmental constraint have kept them smaller, and prevented them from becoming the horrific giants that get most of the attention?