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It's a case that went cold some 45 million years ago. What killed a couple hundred ancient frogs? NPR's Ari Daniel brings us a mystery that at long last may have been solved.

ARI DANIEL, BYLINE: Central Germany was once a coastal, subtropical swamp.

DANIEL FALK: With loads of creatures, loads of beasts running around.

DANIEL: Including - says Daniel Falk, a paleontology Ph.D. student at University College Cork - ancestral horses, giant crocodiles, huge snakes and plenty of frogs and toads. But in the modern era, things look rather different.

FALK: It's kind of like a fossil crime scene.

DANIEL: The swamp had preserved a couple hundred fossilized frogs and toads.

FALK: And the mysterious question is, like, why did all those animals die? Like, why did those frogs die?

DANIEL: For a long time, scientists thought the swamp had dried out, which could have killed the frogs. But Falk wasn't so sure.

FALK: I basically counted every single bone in every specimen.

DANIEL: The bones were in good shape, so the animals were healthy; there was even fossilized poop in a couple of them, so they didn't starve; and there weren't any predator marks, so the frogs and toads weren't eaten - process of elimination. And what was left, based on similar fossil deposits elsewhere and knowing about modern-day frogs, was that - and here's where it gets a bit gruesome - the ancient animals drowned while mating, especially the females.

FALK: And they sink down in the water. And if the females can't make it up to the surface at some stage, they unfortunately drown.

DANIEL: It's a theory Falk and his colleagues describe in a study published today in the journal "Papers in Palaeontology." If they're right, then it's not the males who revealed what happened; they're long gone. It's the females that have been preserved, whispering their story to us millions of years later. Transcript provided by NPR, Copyright NPR.