The blue whale is not only the largest animal alive today, it is the largest one that has ever lived. Now, analysis of a fossil found on the shore of an Italian lake hints at when, and perhaps how, the blue whale became such a behemoth.

The beast’s very large skull, described today in the journal Biology Letters, confirms that this ancient blue whale is the largest known in the fossil record, reaching a whopping 85 feet long. That’s just shy of the largest modern blue whales on record, which reach up to a hundred feet. Perhaps even more surprising to scientists, though, is the fact that a whale of this size swam the seas around 1.5 million years ago, during the early Pleistocene—far earlier than previously thought.

View Images An illustration shows how a modern human diver would have measured up to the ancient whale. Illustration by Alberto Gennari

“The fact that such a large whale existed that long ago suggests that large whales had been around for quite a while,” says study coauthor Felix Marx, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels. “I don’t think species can evolve to such a size overnight.”

A whale of a find

Figuring out how blue whales came to be so big has been a challenge, as large whale fossils from the past 2.5 million years are rare. This is likely because the planet went through a number of ice ages during this period, when plenty of water froze into ice and sea levels dropped dramatically. The remains of whales that died in those days, even if they stranded on land, may now be many dozens of feet below sea level. (A 27.5-million-year-old fossil recently found in New Zealand belongs to one of the oldest known ancestors of baleen whales.)

In 2006, a farmer near the southern Italian town of Matera saw some very large vertebrae sticking out of the clay on the shore of a lake he uses to irrigate his fields. Over the course of three fall seasons, when it was possible to lower the water level without ruining the harvest, Italian paleontologist Giovanni Bianucci of the University of Pisa and his team dug out the remains.

The team at the time thought the fossils might belong to a blue whale, and the new anatomical studies have now confirmed it.

View Images The fossil skull of the Matera whale (left) helped scientists create this reconstruction of the full skull for an anatomical analysis. Photograph by Akhet s.r.l.; drawing and composition by G. Bianucci and F. Marx

The new fossil might also help reveal that the rise of giant whales has been more gradual than previously believed, argues Marx. In 2017, a study analyzing the body size of all known baleen whale species, many of them only known from fossils, suggested an increase in body size may have happened rather suddenly, likely some 300,000 years ago but possibly as far as back as 4.5 million years.

When Marx included the new fossil in this analysis, however, “the most probable date was pushed back to 3.6 million years, and likely even further, possibly as far back as six million years.”

Surplus of small fossils

Graham Slater of the University of Chicago, who did the original analysis, points out that 3.6 million years still fits in the rather large time window he had found. And even if the most probable date for the size jump is pushed back that far, he says, the revised date of 3.6 million years ago makes sense.

Around that time, a global decrease in ocean temperature likely changed the availability of food to whales, creating patches of very high prey density where there was upwelling of cold water from the deep, which he believes was “important for supporting really large whales.” Slater does not agree with Marx that the new analysis favors an even older origin for blue whale bigness. (See a prehistoric “sea monster” that lived about 200 million years ago and was about the same size as this fossil whale.)

It is true that the analysis as such does not directly confirm that scenario, Marx admits. But his point of view is informed by what he believes is yet to come. Because large whale fossils are difficult to collect, study, and describe, our view of body-size evolution in whales may be distorted. Marx is involved in a project in Peru that has found multiple whale fossils that have not been recovered yet. Although the data is preliminary, including them in the analysis further weakens the impression of a sudden shift, he says.

“I’m aware of multiple large whales of at least the same age that haven’t been described yet” in the scientific literature, he says. Every additional large whale fossil we find and document, he thinks, will make the idea of a gradual change more likely.

View Images A chart shows the changing body length of baleen whales plotted against time. Red circles show the posistions of the Matera whale along with three unexcavated fossils found in Peru. Diagram modified from Graham J. Slater et al.; blue whale drawing by Carl Buell

Paleontologist Cheng-Hsiu Tsai of the National Taiwan University described the sparse remains of what was likely the second-largest fossil whale found so far, a fin whale from California. He has been arguing for a while that baleen whales became big much earlier than was generally believed, and he largely agrees with Marx’s conclusions.

“To be honest, this fossil does not surprise me at all,” Tsai says. “I expect that we should find something bigger and geologically even older soon.”