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  • 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 ofodd.

  • 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 bewalrus-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 meanstooth-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 polygynousliving 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.

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  • 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!

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  • Also thank you for joining me today in the Konstantin Haase studio.

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About 15 million years ago, the Central Valley of California was covered by a shallow inland

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B2 US walrus selection sexual competition teeth fossil

How the Walrus Got Its Tusks

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    joey joey posted on 2021/05/03
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