Ancient Whale Graveyard Mystery Solved

27 February 2014 by Travis Park, posted in Cetacea

In 2010, in the Atacama region of Chile construction workers who were widening the Pan-American Highway struck palaeontological gold. In the quarry they had dug they discovered a treasure trove of marine mammals that is unsurpassed anywhere else in the world. The locality, later named Cerro Ballena (which translates as ‘whale hill’), contained over 40 skeletons of rorqual whales, sperm whales, seals, aquatic sloths, walrus-whales and predatory bony fish. It has been dated to 9.03–6.45 Ma (Late Miocene). However, these fossils couldn’t remain here indefinitely as the road needed to be completed, but just ripping the fossils out post-haste means that any taphonomic information revealing why these animals all died together would be lost. So what did they do? They called in the techno-wizards from the Smithsonian Institution’s 3-D digitisation team.

This is just a couple of the 31 rorqual skeletons that were uncovered at Cerro Ballena. Image from www.cerroballena.si.edu/

This is just a couple of the 31 rorqual skeletons that were uncovered at Cerro Ballena. Image from www.cerroballena.si.edu/

Along with palaeontologists from various institutions in the USA and Chile, the 3-D digitisation team used various laser scanning devices to create high-resolution models of the fossils as they were when originally found, preserving that information for posterity and allowing the palaeontologists to continue to figure out why all these animals died at the same spot. All the data is also on the locality website: www.cerroballena.si.edu/

In this image we can see how the 3-D digitisation team scanne dthe fossils to make their virtual models. A very futuristic looking scanner indeed! Image from www.cerroballena.si.edu/

In this image we can see how the 3-D digitisation team scanne dthe fossils to make their virtual models. A very futuristic looking scanner indeed! Image from www.cerroballena.si.edu/

As it turns out, there were actually four separate layers of these fossils, indicating that this event happened more than once. But what caused it? Possible explanations included herding behaviour, large-scale oceanographic fronts and harmful algal blooms. The theory that best fits the evidence is the harmful algal bloom. These still occur today in the waters off Chile. Nutrient rich minerals (in this case iron) get washed down into the ocean by rivers in the Andes; the algae then feed on these nutrients and undergo a population boom. These algae however produce toxins which are harmful to marine mammals. So if they were to eat the algae or some other organism that has itself eaten the algae they may end up getting poisoned and it is exactly this, except repeated again and again that lead author Dr. Nick Pyenson (Smithsonian) thinks happened. "There are a few compelling modern examples that provide excellent analogues for the patterns we observed at Cerro Ballena–in particular, one case from the late 1980s when more than a dozen humpback whales washed ashore near Cape Cod, with no signs of trauma, but sickened by mackerel loaded with toxins from red tides," said Pyenson, "[h]armful algal blooms in the modern world can strike a variety of marine mammals and large predatory fish. The key for us was its repetitive nature at Cerro Ballena: no other plausible explanation in the modern world would be recurring, except for toxic algae, which can recur if the conditions are right."

Potential proof of the algal bloom suspected of killing these animals: iron-stained traces resembling algal growth structures (L); iron-stained tuft-like forms resembling algae, and possible algal mats covering wave ripples (indicated with arrows), with pen for scale (R). Image from www.cerroballena.si.edu/

Potential proof of the algal bloom suspected of killing these animals: iron-stained traces resembling algal growth structures (L); iron-stained tuft-like forms resembling algae, and possible algal mats covering wave ripples (indicated with arrows), with pen for scale (R). Image from Pyenson et al. 2014.

But is there proof that it was a harmful algal bloom that killed all these animals? Whilst sediment samples under light and scanning electron microscopy lacked distinct algal cell fragments, there were small, spherical apatite grains that may have been the remnants of algae. There were also iron-stained tuft-like forms resembling algae and possible algal mats covering wave ripples in the sediments. The animals, once dead or dying, were washed into Cerro Ballena which in the late Miocene was a tidal flat that had some sort of barrier protecting it. This allowed the specimens to remain articulated and being on a tidal flat also restricted marine scavenging, whilst the desert conditions on land restricted terrestrial scavenging, resulting in the treasure trove we known today.

The fossils have now all been removed from the quarry and are now housed in museums in Caldera and Santiago and the quarry itself is now paved over by the highway. But there is up to an estimated 2 km2 fossiliferous layer at Cerro Ballena which has not even been touched yet. Palaeontologists in Chile are currently working to build a research station at the locality so that the rest of it can be fully explored. I can’t wait to see what they unearth.

Here are some of the fossils from Cerro Ballena now in storage in the museum at Caldera, awaiting further study. What I'd give to be in that room! Image from www.cerroballena.si.edu/

Here are some of the fossils from Cerro Ballena now in storage in the museum at Caldera, awaiting further study. What I'd give to be in that room! Image from www.cerroballena.si.edu/

The paper is open access, so click on the link in the reference section to get your own copy of the PDF!

Also check out the amaing digital data at their website: www.cerroballena.si.edu/

There's also several cool videos on their website, including the one below:

Reference

Pyenson ND, Gutstein CS, Parham JF, Le Roux JP, Carreno Chavarria C, Little H, Metallo A, Rossi V, Valanzuela-Toro AM, Velez-Juarbe J, Santelli CM, Rubilar Rogers, D, Cozzuol MA, Suarez ME (2014) Repeated mass strandings of Miocene marine mammals from Atacama Region of Chile point to sudden death at sea. Proc. R. Soc. B 281: 20133316. http://dx.doi.org/10.1098/rspb.2013.3316


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