The evolutionary story is one of constant change. It proposes that simpler life forms evolved into complicated organisms whose offspring branched out in ever more diverse directions. But the modern forms of some creatures are so similar to their ancestors’ fossils that it is clear they haven’t changed much at all. If some species diversified, why didn’t others?

Fossils from South China are being touted as the first animals to have evolved on the planet. A recently-published study on these tiny organisms, however, presents a fresh evolutionary puzzle. Although the common assumption is that the “first animals” emerged from the ocean, the Chinese Doushantuo Formation looks like it came from lake deposits.

The remains of a dinosaur found in the Hell Creek Formation of North Dakota are so well preserved that some scientists are just “gobsmacked.”

For decades, museums and textbooks confidently asserted that mud rocks—such as limestone, siltstone, mudstone, and shale—were formed over vast eons as super-fine sediments slowly settled to the bottom of shallow lakes or seas.