字幕列表 影片播放 列印英文字幕 About 15 million years ago, the Central Valley of California was covered by a shallow inland sea. And this sea was filled with exotic marine life - ancient seals, massive sea turtles, and giant megalodon sharks, which, I want to remind you, are super duper extinct. And there were also walruses - three different species of them, which, it turns out, were kind of … odd. Because, at least one of them definitely didn’t have tusks. And the other two probably didn’t, either. Which, just seems strange to me. Like, if they didn’t have tusks, how did scientists know these animals were walruses? Aren’t they a key feature of what we consider to be … walrus-ness? And, if you can be a walrus without tusks, why does the only species of walrus alive today have them? Modern walruses are literally named for these features; their genus name means “tooth-walking,” because they’ve been observed pulling themselves out of the water with their tusks. But there used to be all kinds of walruses in the oceans! The fossil record has revealed a huge diversity of extinct walruses - something like 20 different species - stretching from Florida to Japan and dating back to the early Miocene Epoch. So where did they all go? Why is the modern Arctic walrus the only one still around? The rise and fall of ancient walruses, and how modern ones got their tusks, is a story that spans almost 20 million years. And while there are parts of the story that we’re still trying to figure out, it looks like tusks - which are literally just enlarged upper canine teeth - didn’t have anything to do with how or what these animals ate. Instead, the walrus probably got its tusks because of sex. The earliest known fossils of walruses have been found in Japan, Oregon, and California, and they’re from the early Miocene epoch, around 17 million years ago. They were all smaller than their modern relative, and none of them had tusks. And a couple million years later, when those three species were swimming around the Central Valley, walruses had already started to diversify, developing different body sizes and diets. But again, at least one of them definitely didn’t have tusks. So paleontologists had to use other features to recognize them as walruses. It turns out, tusks are not one of these animals’ defining features. Instead, they can be identified by a wide bone on the upper jaw that’s found only in walruses, as well as a characteristic ear bone, and really big molars. Now, fast-forward to the late Miocene, about 10 million years ago, and we find fossils of another two walrus species in Japan. They were both on the smaller side and at least one of them had sharp teeth for eating fish, but again no tusks. The presence of all these early walruses shows us that these marine mammals diversified really quickly. By 10 million years ago, six species had already become extinct, but three more species had shown up in the fossil record to take their place. And this was just the beginning. Paleontologists think that these animals diversified so quickly because they were geographically isolated. As sea levels rose and fell over millions of years, different populations became isolated from each other and started to evolve independently. Now, looking forward another couple million years, to around 8 million years ago, we find fossils of an enormous walrus in Oregon and California. Pontolis was the size of a modern elephant seal, with a big, long skull. But, while its canines were sharp and fang-like, they didn’t stick out of its mouth. It would take another million years before the first tusked walrus appeared. Behold, Gomphotaria! It was also pretty huge, and it had short, stout tusks on both the upper and lower jaws, as well as these weird, tusk-like third incisors. And it didn’t eat fish, like its ancestors did. Instead, it fed on shellfish - judging from the extreme wear that’s been found on its teeth and tusks, it looks like it just crushed them up, shells and all. So, what drove the evolution of tusks in walruses? This is a question that has puzzled paleontologists for years, because, for a very long time, walruses were obviously doing just fine without them. The first hypothesis was that tusks evolved to stabilize a walrus’s head while it trawled the seafloor for food, kind of like the runners on a sled. But Gomphotaria’s tusks were short and stout, and their shape made them not really useful for stabilizing a big skull. The second hypothesis was that tusks evolved to help walruses to pick their way across ice. Scientists came up with this idea after watching living walruses use their tusks to haul themselves onto ice floes. But Gomphotaria lived in temperate, warm waters where there was no ice. And another species of extinct tusked walrus has also been found in places that were warm and ice-free in the late Miocene - in places like South Carolina, the Netherlands, and Morocco. So, tusks probably didn’t evolve as ice picks. That leaves the third hypothesis: That tusks were used in competition between males for mates - but not always in the way you might think. Walruses today spar with their tusks, competing to monopolize groups of females. The males posture and jab at each other, sometimes even drawing blood. So that made scientists wonder whether male competition was at work in early walruses, too. Because, in animals where males compete for groups of mates, you tend to also see differences in body size between the sexes, with males being bigger than females. And looking back in the fossil record, we can find evidence that one of the species from California’s Central Valley had bigger males than females. In fact, if you look even further back in their history, you’d find this pattern of having larger males in the ancestors of walruses that lived more than 20 million years ago. So it looks like walruses were polygynous - living in groups of one male and many females - from the very start of their lineage. And this suggests that competition between males was common, and may have eventually driven the evolution of tusks. But here’s the thing: big tusks can be used as weapons, but they’re also useful for display. And traits like these can be acted on by sexual selection. Sexual selection is a type of natural selection that’s driven by competition for mates. It includes males fighting other males over females, and also includes one sex preferring certain traits in the other sex. As I’m sure you’ve noticed in other animals, sexual selection tends to produce big, showy structures in males - like deer antlers or peacock’s tails. In fact, the most well-studied example of sexual selection is probably in deer. The fossil record shows that, as deer diversified into many species, their antlers diversified, too. And in deer, paleontologists think that antlers evolved as weapons for fighting at first, but eventually took on a new, separate role as a sexual display. And in walruses, evolutionary biologists see the incredible diversity of tusks over the last 8 million years as similar evidence of sexual selection. When tusks became indicators of fitness, the thinking goes, male competition for mates didn’t have to be lethal. Instead the dudes could just instead intimidate with displays. So, the fact is, the most elaborate structures in males rarely inflict damage in fighting - instead, they serve as indicators of a male’s status, size, and health. But having these huge tusks came at a cost. Because, like I mentioned, the earliest walruses were fish-eaters. But walruses with tusks don’t eat fish because their giant canines get in the way of catching them. So, these walruses shifted to another mode of feeding as their tusks got bigger and bigger, due to sexual selection. We can see this, for example, in the Pliocene walrus known as Valenictus. It had no teeth except for its tusks. But the arched palate in its mouth allowed it to create a vacuum that it could use to suck up mollusks from the sediment. And today’s walrus does have teeth, but it doesn’t use them for feeding. It just noses through sediments for mollusks, like clams, feeling around with its sensitive whiskers and just sucking the animal right out of the shell. Mmm. So, in their long history, walruses evolved from having sharp teeth for eating fish to having giant tusks for competition and display. And the evolution of tusks coincided with the incredible diversification of walruses. Over time, there have been at least 21 species, in at least 18 different genera, with today’s walrus being the only surviving representative of the group. Now, there are still gaps to fill in the fossil record, but it’s clear that the diversity of these animals was a roller coaster - rising and falling several times as climate and sea levels changed over millions of years. Walruses suffered their final dropoff in diversity after the close of the Pliocene Epoch, 2.6 million years ago. Changes in climate and sea level, and the presence of humans converged to reduce them to the one species we see on Earth today. So while we’ve learned that tusks don’t make a walrus, the tusks of the last surviving species are an impressive reminder of the enduring power of how sex can drive natural selection. Do you want more Eons content? Of course you do Then be sure to follow Eons on social media! You can follow us on Instagram, Twitter, and Facebook. And you can join me on Instagram at westerndigs. And tusk you very much--which doesn't make any sense but Kallie is making me say this to this month’s Eontologists: Sean Dennis, Jake Hart, Lucas Curtis-Mahoney, Jon Davison Ng, Patrick Seifert, and Steve! Be sure to check out the upgrades to our tiers and become an Eonite at patreon.com/eons. Also thank you for joining me today in the Konstantin Haase studio. Be sure to subscribe at youtube.com/eons for more adventures in deep time.