The enigma of Chile’s whale fossil graveyard finally explained

Thousands of 5 million year old whale fossils. One location by the Pan-American Highway. No explanation…

Until now.

Such discoveries get palaeontologists up in the mornings to dig painstakingly away at relics of a lost time. The discovery was not unexpected since Chile’s Atacama Desert is renowned for having some of the finest preserved whale fossils in the world, with these particular ones jutting out of rock faces. Granting this particular location its name: Cerro Ballena, or “whale hill” where the Pan-American Highway is starting to be widened onto. Luckily for American and Chilean researchers, this gave them the opportunity to get a much more in-depth look at the fossil beds to try to understand why this was such a fossil hotspot. However, time was of the essence. They had only two weeks to complete their field work and get out of there before the construction on the new road was going to begin.

The team first brought in a huge amount of digital techniques to record the fossils in as much detail as they could, utilising 3-dimensional models of the skeletons in situ, whilst also extracting some bones for further laboratory analysis. In just 240 m of sediment, they found 40 rorquals (large whales such as blue, fin or minke whales), along with many marine predators and grazers such as extinct ‘walrus whale’ – dolphin that bizarrely evolved a walrus-like face. It gets stranger; even peculiar aquatic sloths were discovered too.

Interestingly, the murder-mystery’s plot thickens since all the skeletons are nearly complete, showing little sign of scavenging after death, and most are facing in the same direction or upside-down. This implied all the creatures had succumbed to the same, sudden catastrophe. However, the fossils were found at different levels, also suggesting that there was not just one single event, but actually four, separate but identical events spread over only few thousand years – just a glimpse of a second on the geological timescale.

Their findings concluded in a report in the Royal Society journal that the whales all died suddenly at sea from poisoning.[1] The most likely cause of this was toxins secreted by algal blooms which indeed cause many mass whale strandings, where often hundreds of whales would all become stranded on beaches, as we see today. Such toxins were concentrated from one organism to the next along the food chain (bioaccumulation) or were simply inhaled by the whales, causing rapid death.

Indeed, the morphological layout of the coastline at Cerro Ballena during the late Miocene Epoch (23.8-5.3 million years ago) was such that these carcasses would have been channelled into the area in which they are found today, possibly thrown onto the sand flats by strong storm waves, thus accounting for the high density of fossils. Marine scavengers would not have been able to access the carcasses, and the lack of land scavengers, due to its desert location, left whale fossils intact. The research was so detailed that the fossils at Cerro Ballena were found to only show slight chips inflicted by foraging crabs.

However, there is no ‘smoking-gun’ evidence for the algal bloom hypothesis as there are no algal cell fragments found in the sediments. There is some evidence of many tiny grains (20?m in diameter) encrusted in iron oxides, which could suggest algal activity in the past, and possibly be dinoflagellate cysts (a rock containing a type of algae) according to Dr Pyenson, a palaeontologist at the Smithsonian’s National Museum of Natural History. These are found in algal-like mats across the site and so support the idea of a toxic algal bloom.

Many more fossils are still waiting to be found though and to that end the University of Chile in Santiago is trying to establish a research base to further investigate the site. If you would like to investigate the site for yourself then go to cerroballena.si.edu to see 3D scans and maps of the fossils! Who knows what more there is to discover.