Godzilla and the genre of movies it spawned notwithstanding, there seems to be an upper limit to how big an animal can possibly get.

Kaiju-scale sizes seem to be just plain off limits for land animals, and even marine animals are limited by how much food they can find. For a long time, scientists believed the blue whale was the largest animal to have ever lived on Planet Earth.

But a fossil find published in 2023 may have finally unseated the champion. Unsurprisingly, the new possibly-biggest-ever living thing is another whale, one that swam the seas around 39 million years ago. [♪ INTRO] Paleontologists unearthed a partial skeleton of this new creature in Peru. They named their discovery Perucetus colossus, which is a little on the nose but I’ll allow it.

Perucetus was a basilosaurid. The basilosaurid family of whales is now extinct, but they were some of the first fully-aquatic cetaceans. Cetaceans are the order of marine mammals that includes whales, dolphins, and porpoises.

Estimating Perucetus’ weight is difficult based on fossil remains alone, but the scientists that made the discovery conservatively believe it was anywhere between 85 metric tons and 340 metric tons, with their estimates averaging out to 180 metric tons. For comparison, the largest known blue whale was 190 metric tons, though most max out at around 150 metric tons. So this animal was potentially bigger than a blue whale, maybe even way bigger than a blue whale, though we can’t really say for sure.

Estimating the animal’s size is complicated by the fact that the skeleton is really different from modern marine mammals. You see, its bones are thicker and denser than its modern counterparts. It displays a high degree of what the researchers call bone mass increase, where the skeleton piles on more, denser bone both on the outside and within the bone’s internal cavity.

This makes it difficult for researchers to base its body mass on skeletal measurements alone. The whale’s body might have compensated for the heavy skeleton with large amounts of blubber, for example, which isn't as dense as other kinds of soft tissue. So to arrive at the 85 to 340 metric ton estimate, scientists used extreme values measured in living marine mammal species.

Manatees aren’t whales, but their skeletons also have a high degree of bone mass increase. Not as much as Perucetus, but it still invites the comparison. So the researchers used the average ratio of skeletal mass to total body mass of a manatee to arrive at the 85 metric ton estimate.

Unlike manatees, however, most whales tend to have pretty light skeletons. So the scientists used the average ratio of skeletal mass to total body mass of Gervais's beaked whale to arrive at the 340 ton estimate. Basically– was it a sort of big boy with a really heavy skeleton, or was its skeleton more proportional to the rest of its body?

There’s not enough information to answer that question just yet. A whale of this size is a bit of a puzzle compared to what we know about modern whales. Massive size as an adaptation wasn’t thought to have evolved until modern whales emerged around 37 million years ago.

This group contains a lot of fast-swimming animals adapted for the open ocean. The thinking is there’s a positive relationship between being big and mobile, and being able to capture a lot of food to sustain that size. But Perucetus seems to have been more of a slow-swimming, coastal species.

Researchers can make this educated guess based on the weight and shape of the animal’s skeleton. It does seem like Perucetus’ bones were thicker and clunkier for its size compared to modern whales. And they appear to have been normal for the species, though you could be forgiven for wondering if maybe this particular specimen had a bone condition or something.

In fact, in many ways, this whale was more similar to a manatee than to a modern whale. For example, Perucetus’ specially shaped vertebrae would have given it a slow swimming style, like a manatee. The thicker bones would have limited the motion between vertebrae, which would have also slowed it down.

And the shape of the animal's spine would have made it not very flexible. But despite its slower speed, Perucetus’ size could have also given it advantages, like the ability to spend more time near the sea floor. And that larger size could have also helped it remain stable in the presence of strong coastal waves.

This raises questions about what this animal might have eaten. It could have fed on seaweed and seagrass like a manatee, but herbivorous whales aren't really a thing. It's more likely that it was either eating marine animals that live on the seafloor, or that it was scavenging.

Large, dead things are abundant and easy to get, and this type of food could have helped support its large size. Since there are living sharks in similar habitats that also function as scavengers, this last scenario seems totally plausible. Yeah.

Scavenger whale the size of a city block. You’re welcome. This is just one specimen, and we don’t have its skull, so it’s hard to draw any solid conclusions about just how big Perucetus really was or what its life was like.

We do know it has the heaviest known skeleton, but the rest is still uncertain. So although there are limits to how big animals can get, this find opens up the possibility that we haven’t hit that limit yet. Though we’re probably not ever going to find the skeletal remains of Godzilla or something like him, it’s possible that Perucetus isn’t the biggest thing that ever lived.

For now, though, Perucetus may deserve the crown. Thanks for watching this episode of SciShow, and thanks to the patrons who helped make it happen. You’re helping make free educational content accessible to everyone, so pat yourselves on the back.

If you’d like to get involved and join our awesome community, you can get started at patreon.com/scishow. [♪ OUTRO]