Some paleontologists have recently put forth a controversial idea—that they’ve found a place where the time of year the dinosaurs died is captured precisely in the fossil record.

It’s called Tanis, in North Dakota. But it was once the northern end of an inland sea.

66 million years ago, when the Chicxulub asteroid struck the Yucatan Peninsula, it instantly wiped out all life within 1,000 miles. But Tanis is nearly 2,000 miles away—what happened here?

Glass spherules, made of quartz, rained down from the heated atmosphere.

Shock waves from the huge earthquake caused by the asteroid sloshed the inland sea, in waves up to 30 feet high.

Fish died with spherules caught in their gills. Schools of them were found preserved in rocks, with open mouths suggesting death by suffocation.

Their bony plates show they had just begun the rapid growth of spring but had not reached the maximum growth of summer.

Other animals and plants were buried upright, frozen in place, not flattened like typical fossils.

If the scientists are right, this pegs the asteroid impact to a spring day in the Northern Hemisphere, which would have made its animals and plants more vulnerable than those in the Southern Hemisphere, where they could have already begun winter hibernation.

More fascinating clues in our quest to understand the event that made way for mammals, like us, to inhabit the Earth.