Sharks weren't always the huge, vicious alpha predators that they are today. Hundreds of millions of years ago, they were just another tiny little harmless fish species hanging out in lakes and rivers. So yeah...things have changed slightly since then.
The most striking evidence of sharks' humble beginnings has been found, in all places, in the central Asian nation of Kyrgyzstan. It's hard to find a spot on Earth much further from the oceans, and it's been that way for a long time - this region was similarly isolated during the Late Triassic 230 million years ago. And yet it's here that paleontologists have found fossilized teeth and egg capsules belonging to the tiny ancient shark Lonchidion ferganensis. You can see some examples of these egg capsule fossils down below, along with a reconstruction.
The fossils represent two species of ancient shark, one of which belongs to the hybodontid family, while the other is known as a xenacanthid. These egg capsules represent the first known instance of sharks reproducing in freshwater - all sharks alive today reproduce exclusively in the oceans. It's unknown whether the parent sharks also remained in these ancient lakes and rivers, or whether they mostly lived in the open seas, and only swam hundreds of miles upstream for breeding purposes.
Either way, the basics of shark reproduction remain remarkably unaltered after 230 million years. While the location has shifted from lakes to oceans, the tiny sharks of the past and giant sharks of the present both seek out shallow waters for spawning. Because sharks so rarely fossilize - a byproduct of their cartilage skeletons - this discovery of even just a few teeth and egg capsules represents one of the biggest hauls in history, as researcher Michael Budwitz explains:
"The fossil record of sharks is no laughing matter; a spine here, a tooth there, or three miniscule denticles [small spines of the skin] picked from a 10 kilogram sample. Therefore, dozens of egg capsules alongside juvenile teeth in one deposit is a dream come true!"
As for the ancient sharks who left behind these fossils, we do at least know their ultimate fate. The xenacanthid didn't make it out of the Triassic, going extinct about 200 million years ago. The hybodontids, on the other hand, proved much more successful, surviving until about 65 million years ago, when they likely succumbed to the same mass extinction event that wiped out the dinosaurs.