May. 05, 2014

Picture of the Week: Whale Graveyard

by Jessica McDonald

Click to enlarge images
Four years ago, while doing roadwork on the Pan-American Highway, a Chilean construction crew got a surprise when it stumbled upon a massive collection of whale bones. An international group of paleontologists has since unearthed an unprecedented number of whale skeletons from four distinct layers at the site, dating back six to nine million years.
“I never thought I’d have to work with 40 [whale skeletons] at once,” says Nick Pyenson, curator of Fossil Marine Mammals at the Smithsonian’s National Museum of Natural History, who led the excavation. “It was overwhelming.”
The majority of the fossils represented large, filter-feeding baleen whales. But the site—known as Cerro Ballena (Spanish for “whale hill”)—also contained the remains of nine other kinds of marine vertebrates, including seals, sperm whales, dolphins, and one species previously only recovered in Peru: a walrus whale that Pyenson describes as having a “bizarre Admiral Ackbar-looking face.” The findings are described in a recent edition of The Proceedings of the Royal Society B.
Once Pyenson and his team got over the enormity of their task, they began to wonder why the ancient animals had been buried in droves in essentially the same spot repeatedly over time.
Hints came from studying tiny details in fossil scans taken by the “laser cowboys”—two members of the Smithsonian’s 3D digitization lab—whom Pyenson brought in for a week to image the fossils in order to retain information, such as how they were arranged, that would inevitably be lost as the skeletons were removed and shipped off to Chilean museums. 
Pyenson’s team learned that most of the whales were preserved with their bellies facing up, suggesting they had died at sea rather than on land. “If they wash up alive, their blowholes definitely will be pointing up [instead], because they definitely need to breathe air,” says Pyenson.
All skeletons were also remarkably free of bitemarks made by scavengers, indicating that the creatures didn’t stay long in the ocean after dying—rather, they probably washed up on land soon after perishing.
Wondering how the creatures at Cerro Ballena could have died, Pyenson recalls a similar mass death that occurred in the late 1980s, when 14 humpback whales mysteriously washed up on the shores of Cape Cod. “The humpbacks showed no signs of trauma,” he says. “In some cases they were seen on a whale watch 90 minutes before they washed up dead.” Subsequent testing found that the whales died from asphyxiation after feasting on Atlantic mackerel loaded with saxitoxin, a neurotoxin found in dinoflagellates. The microorganisms had exploded in population off the Cape Cod coast in what’s called a harmful algal bloom.
Could such a phenomenon explain the ancient Chilean deaths, too? Algal blooms are known to recur off the western coast of South America, fueled by the iron-rich runoff from the Andes Mountains in combination with ocean upwellings that bring nutrients to the surface. Pyenson speculates that such a bloom occurred millions of years ago, creating a toxin that poisoned all of the marine animals found at the Chilean site. Strong currents soon pushed the carcasses onto the Cerro Ballena tidal flat, where they were stranded once the water receded. Thousands of years later, a similar scenario occurred, taking down more animals. The process repeated two more times, laying the groundwork for the site as we know it today.
Given the staggering number of whale fossils recovered, Pyenson says that Cerro Ballena should be recognized as a UNESCO World Heritage site, calling it Chile’s version of the United States’ Dinosaur National Monument. But as unique as the area seems, the algal bloom hypothesis suggests that there are other sites like it. Indeed, areas off the western coasts of California and Namibia, where upwellings are common, have probably experienced similar conditions that have created whale graveyards, as yet undiscovered.
About Jessica McDonald

Jessica McDonald is a health reporter for WHYY, Philadelphia's public radio station. She is also a former SciFri intern.

The views expressed are those of the author and are not necessarily those of Science Friday.
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