K-Pg Boundary Microtektites
Hell Creek Formation
Garfield Co., MT, USA
These aren't fossils, but are relevant to the extinction of the non-avian dinosaurs, pterosaurs, large marine reptiles, and many other species of flora/fauna at the end of the Cretaceous.
When a large meteor/asteroid struck the earth ~ 66 mya, it sent molten ejecta across the world. Some of this molten material, sourced from the impact site, was shaped by its trajectory through the atmosphere and cooled into small, glassy droplets. The black blobs you see are those droplets, called tektites (each typically ~ 1 mm in diameter).
The layer which these came from is more famous for its unusually high concentration of iridium (which is more common in meteorites than on Earth). However, in some locations, tektites have been preserved. In this matrix sample, I've also found carbonized plant material (charcoal), which suggests fires that could be associated with the impact event.