66.5 million years ago
April 5 on Logarithmic History marks the most famous mass extinction ever, the one that did in the dinosaurs (okay, okay, the non-avian dinosaurs). Here’s a link to a previous year’s post.
And just let year we had news of one of the most extraordinary fossil discoveries ever, in North Dakota: a graveyard of fish piled on one another by a tsunami-like wave, and mixed with burned trees, and the remains of mammals, mososaurs, ammonites, and insects, and a partial triceratops, formed within hours of the asteroid impact that wiped out most life on Earth. Here is a news release, 66 million-year-old deathbed linked to dinosaur-killing meteor, and here is an article from the New Yorker, The Day the Dinosaurs Died.