Messier 104, The Sombrero Galaxy: 65.2 Million Light Years
The K-T boundary, a sedimentary layer marking the transition from the Cretaceous to the Tertiary periods, was laid down about 65 million years ago, when the light from M104 was starting on its journey here. The layer of sediment is unusually high in the element iridium, which occurs in high concentrations in meteors. Dramatically different kinds of fossils appear on either side of this boundary. Below this layer we have dinosaurs; above it we don't.
Apparently, about 65 million years ago a 10-20 kilometer wide asteroid slammed into what is now the tip of the Yucatan peninsula, leaving a buried impact crater near the village of Chixculub. Whether or not this event caused or simply accompanied the mass extinction is open to some debate, but it sure didn't help. That's a shuttle image on the right, below, marked up by JPL to show the crater outline.