An ancient whale-like animal may be the biggest to have ever lived
One reason dinosaurs are so popular with schoolchildren is that many of them are so extraordinarily big. Tyrannosaurus rex, a meat-eater, was up to 12 metres longabout half the length of a tennis courtand is thought to have weighed up to 7 tonnes. Some of the herbivorous dinosaurs were bigger still: Patagotitan mayorum, discovered in 2014, was 37m long and may have weighed 69 tonnes. But the true giants do not live on land, where the pull of gravity imposes limits on how heavy an animal can grow. They live in the sea, where their enormous bodies can be supported by the water. The most massive animal to have ever lived is still swimming around today. The blue whale, Balaenoptera musculus, regularly tips the scales at over 100 tonnes. Some estimates have pegged individuals at 190 tonnes. But now, it seems, the blue whale has a challenger. On August 2nd, in a paper published in Nature, Giovanni Bianucci, a palaeontologist at the University of Pisa, and his colleagues describe Perucetus colossus, an extinct marine mammal that swam the ocean shallows around 40m years ago. It may have weighed up to 340 tonnes, making it by far the most massive animal ever to have existed. Or at least, that is the upper estimate. Dr Bianucci and his team, working in the Ica Valley, an arid desert in southern Peru, were able to recover only an incomplete skeleton, consisting of 13 vertebrae and four ribs. Working out exactly how much flesh was once attached to those bones is not easy. Using skeleton-to-body-mass ratios from existing marine mammals, the researchers arrived at a lower estimate of 85 tonnes, and a central figure of 180 tonnes. Exactly why the animal was so big is unclear. To be sure, size has its advantages. Temperature control is one. Mammals must keep their body temperatures within a tight range. As they grow bigger, their surface area rises more slowly than their volume. That means bigger creatures have an easier time staying warm in the comparatively cold ocean. But there are downsides, too. The most obvious is that a bigger body has a bigger appetite which is harder to satisfy. Biologists believe that whales attained their modern proportions only about 5m years ago, when climatic changes produced the rich feeding conditions necessary to support their enormous bodies. Since P. colossus lived millions of years before those changes, something else must explain its heft. Working out how an extinct animal lived is even harder than deciding just how big it was. But there are clues. One striking feature of P. colossuss bones is their density. All the bones the researchers found were unusually thick and heavy. Some lacked a medullary cavity, a hollow, marrow-filled section found in the middle of most mammals bones. These days, deep-diving marine mammals like the sperm whale, which can descend to well over 1,000 metres, tend to have comparatively light skeletons. It is those that live in shallower waters, such as manatees, which sport dense bones. For that reason, the researchers think that P. colossus was likewise a creature of the comparatively fertile coastal shallows. Without a jaw or any teeth to study, exactly what it ate is unclear. One theory is that it was a herbivore, munching its way through prodigious quantities of seagrass and seaweed. It may have been a filter-feeder like modern baleen whales, straining small creatures from the water. The researchers speculate that it may even have been a scavenger, subsisting on the corpses of other animals. Definitive answersand an update to the record booksmay have to wait until a more complete skeleton is discovered. Curious about the world? To enjoy our mind-expanding science coverage, sign up to Simply Science, our weekly subscriber-only newsletter.