In the aftermath of the giant asteroid that crashed into the Yucatan Peninsula about 66 million years ago, approximately 75% of all species on Earth were wiped out, including the dinosaurs. Among those thought to have perished at this K-Pg (Cretaceous-Paleogene) boundary were the ammonites. These were coiled-shelled mollusks with long tentacles related to modern octopuses and squids, and they are known today for their distinctive spiral-shaped fossils.