A whale of a discovery: Perucetus colossus, aka “the monster”

The discovery

For me the adventure began during my PhD, a bit more than ten years ago. Back then I was in Paris, at the Muséum national d’Histoire naturelle. A friend and colleague of mine, Manuel Martínez-Cáceres, came back from his last trip from Peru with a surprise: a few thin-sections. He told me that the field palaeontologists have found something very weird in the Ica desert (Southern Peru). Mario Urbina, who made the initially discovery, is usually able to recognise right away what he finds, because he has been excavating these dunes for several decades. But this time was different. What was sticking out of the rock was big and weirdly shaped, and lacked the porosity usually seen in fossil bone. Some of his colleagues told him “this does not look like a fossil, it might just be a rock”– but he is a stubborn character, and organised for some thin-sections to be made, to clarify the mystery. Those were the thin-sections I had in front of me back then, and the conclusion was clear: they were entirely made out of compact bone.

El monstro

From then on, I continued my PhD (on another Peruvian wonder, the aquatic sloth Thalassocnus), and I did not hear from this mysterious new fossil for years. It’s only about five years ago that I received a phone call from my PhD advisor, Christian de Muizon. He told me that some excavations have taken place years after years to collect what turned out to be gigantic vertebrae of an undescribed animal, that they colloquially called “el monstro”. Back then, just a couple could have been transported to the Museo de Historia Natural in Lima to be prepared. C. de Muizon then sent me some pictures, which utterly blew my mind. To those who work on the evolution of land animals (tetrapods) who returned to the aquatic environment (such as whales and sea cows), it is very clear from the first glimpse what’s going on with these gigantic bones: they are pachyostotic, an adaptation that entails additional deposition of bone tissues on the external surface of bones. This is well-known in many other groups of aquatic tetrapods, such as manatees for instance. But in the case of “the monster”, this adaptation reached an unprecedented – and ridiculous – degree. And its discoverers wanted me to help studying it!

The plan

I could not believe how lucky I was. However, I had other projects to attend to, and the excavations were still under way. I was not involved in the field trips, but I am told they had to go back roughly 20 times to free what turned out to be 13 vertebrae, four ribs, and a hip bone. In the meantime, we started discussing the study design with the other co-authors. Quickly someone asked whether we were looking at the animal with the heaviest skeleton. I am not sure I can imagine a more exciting challenge: we are dealing with an extraordinary animal, but we know only a few bones from its skeleton. How can one make a robust estimation of the animal’s total skeletal mass? So we devised a plan: We would assess the internal structure of the bones with bone drills and reconstruct the volume of the missing parts using data from close relatives with complete skeletons. After a few more years of patience (including the COVID-19 Pandemic), we planned the trip for the end of Spring 2022 to study the fossils, and off we went to Lima.

Hard as concrete

Seeing the vertebrae first hand was simply amazing. But we had much work to do, and the duration of our trip was limited. First we surface-scanned the vertebrae, so we could precisely measure their volume and look at them from all possible angles. This is not trivial for such large objects. Because each vertebra probably weighs almost 200 kg, one does not just flip them over that easily.

But my particular task was too look inside the bones. I had prepared myself well (or so I thought). I had ordered a custom core drill bit, diamond-coated and all, following the advice of a colleague who uses those to study the bones of large dinosaurs. But starting to drill into the fossil, we quickly realised it would not be an easy task. And despite our efforts to cool down the drill bit, it literally melted at the first sample site. We had to make the emergency purchase of a sturdier tool (usually made to pierce metal), a quite stressful endeavour. In the end we managed to collect just enough samples to perform the analyses.

A record breaker

Back in Stuttgart, my colleague Christoph Wimmer-Pfeil prepared all the thin-sections, and after digitising them, I was ready to run all the numbers. The story basically wrote itself: with a lower estimate of 5.3 tonnes, the skeleton was indeed the heaviest of all mammals or aquatic vertebrates. Most appealing to many people is of course to know how heavy the whole animal was during its lifetime. To calculate that, we have used the ratio between the skeleton mass and total body mass in recent marine mammals. Even the lowest estimate (85 tonnes) entails that Perucetus colossus (as we have called the new species) can compete with some adult blue whales, making it a true giant. This is a big deal, because so far giant cetaceans (the group comprising these early whales as well as living whales, dolphins, and porpoises) were thought to have only evolved recently (up to 10 million years ago), and only in an open-sea environment. P. colossus did not care about the open sea; all clues agree with the fact that it was a coastal dweller.