At a site in North Dakota, researchers have discovered a multitude of fossils of animals that died together about 66 million years ago. They were likely buried by an enormous wave of mud, generated by the asteroid impact that marked the end of the reign of the dinosaurs.
Shaped by the impact
The site, called "Tanis" by the scientists, was seeded with tiny beads called spherules, formed from droplets of molten rock that were hurled into the atmosphere after the asteroid struck. They rained down into Tanis shortly before the site was buried under a wave of mud.
Glass at the center
At Tanis, spherules, also called tektites, were covered with clay but had a glass core.
Caught in the gills
Study co-authors Robert DePalma and Jan Smit photograph freshly exposed impact spherules in fish gills at Tanis.
A cross-section of the Tanis deposit shows some animal fossils and layered stratigraphy from two surge pulses.
Animals that were preserved at Tanis died at the same time "on the last day of the Cretaceous," lead study author Robert DePalma told Live Science.
Multiple fish carcasses (R) and tree logs were tossed together by a massive inundation surge, forming part of the mass-death layer at Tanis.
Peering at the past
Study co-author David Burnham examines a specimen collected at Tanis.
Fish that fossilized after they were buried 66 million years ago maintained a 3D shape, after their bodies filled with mud.
Asphyxiated then buried
Spherules were caught in the gills of about 50 percent of the freshwater fish that the researchers examined at Tanis. The fish likely aspirated the spherules that rained down on the river valley after the faraway asteroid impact, prior to the giant wave of mud that buried them.
Marine mollusk fossil
A small fragment of fossilized shell at Tanis belonged to an ammonite — an extinct group of marine mollusks related to modern octopus, cuttlefish and squid. This and other marine fossils were swept into the river valley by an enormous wave from the sea nearby.