This sounds about as ridiculous as the idea of aquatic sloths, but I promise both are true and both lived in South America. Thanks to the wonders of plate tectonics, there's a section of the Atacama Desert filled with fossils of marine mammals, including some pretty cool whale specimens; this is where the site's name, Cerro Ballena, comes from (it literally means "whale hill"). The site was uncovered in 2010 when construction workers were trying to build a road in the area, and researchers were able to go in to look at and eventually 3D image the fossils, as the time lapse below shows.
Some of the researchers were able to take this information to come up with a reasonable explanation for the extinction of all these animals in one place. The arrangement of the bones at Cerro Ballena showed that the animals, including whales, seals, and the aforementioned aquatic sloth had all died at sea and washed up on shore. This mimicked another event from the late '80s in which 14 humpback whales died from an algal bloom.
Before you ask, no, it wasn't like the algae formed a giant, amoeba-like mass that strangled the whales. Rather, the mechanism at work here is that of bioaccumulation. A surplus of nutrients can create an algal bloom, and certain types of algae produce neurotoxins, which are obviously bad. Going up the food chain, the problem gets exponentially worse: in the case of a baleen whale, each krill it eats might only have a few units of toxin, but these animals eat a LOT of krill, so the toxin adds up pretty quickly. A slightly different process occurs in top predators: krill might have 1 unit of toxin, a small fish may eat 10 krill, a larger fish may eat 10 small fish, and you get the picture.
So far, this seems to be the most reasonable. A tsunami would've beached a lot more than just big animals, and disease wouldn't have beached so many different types of animals.
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