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  • About 61 million years ago, on the shore of what's now the South Island of New Zealand,

  • there lived a bird.

  • It stood upright, measured about a meter tall, with wings that were pretty short for its

  • large body, and a long, narrow bill.

  • And it couldn't fly, but it used those short wings to propel itself into the coastal waters

  • of the Paleocene Epoch.

  • The scientists who described this creature named it Waimanu manneringi

  • And it's the oldest fossil that we've ever found of a penguin.

  • From the shape of its skeleton, paleontologists can tell that Waimanu was already a flightless

  • waterbird, like modern penguins.

  • But they don't know what came before it -- what the transition from flying bird to

  • early penguin looked like.

  • The fossils that could describe that for us have yet to be discovered.

  • But experimental work on modern birds that can both dive and fly is giving us clues about

  • what it took for penguins to go from the sky to the sea.

  • And the world those first penguins lived in?

  • That helped shape them, too.

  • Because why penguins gave up on flying is just as interesting as how it might've happened.

  • Waimanu lived just a few million years after the extinction that took out the non-avian

  • dinosaurs and all the predatory marine reptiles that had ruled the oceans of the Mesozoic

  • Era.

  • So, in taking to the water, Waimanu managed something its non-avian dinosaur ancestors

  • never had: it became fully aquatic.

  • And Waimanu was only the first member of what would become a very diverse and sometimes

  • strange group of diving birds.

  • Today, we think of penguins as small-ish, waddling, tuxedo-birbs.

  • But they evolved from a flying ancestor, were actual giants for millions of years, and some

  • of them were even dressed a little more casually.

  • The thing to know about modern penguins is that they're really specialized for underwater

  • life.

  • While they still have to molt and breed on land, they've evolved a ton of features

  • that make them dynamic marine predators.

  • For example, they've got unique scale-like feathers that help keep them warm and dry

  • in cold ocean waters.

  • And they've also got structures called rete mirabile systems in their limbs and their

  • head.

  • Incidentally, I saw this pronounced as "ree-tea" and "reet" so for this episode we're going with "ree-tea"

  • These networks of neighboring blood vessels transfer heat between the arteries, where

  • the blood coming from the heart is warmer, and the veins, where blood is colder.

  • This exchange helps keep the penguin's core temperature up by buffering the much cooler

  • blood that is returning to the heart.

  • Their wing joints are also stiffer than those of flying birds, which helps them produce

  • thrust on both the downstroke - like most birds do - and on the upstroke of the wing.

  • And the muscles that raise the wing on the upstroke are much bigger than in other birds

  • - so the place they attach on the penguin's shoulder blade is also much bigger than in other

  • birds

  • This powerful double-stroke allows penguins to move easily through water, which is denser

  • than air.

  • Their bones are also denser than you find in flying birds.

  • After all, penguins aren't trying to stay up in the air; they're fighting against

  • the buoyancy of seawater.

  • AND!

  • Those dense bones are part of what gives penguins such a great fossil record!

  • Now, we haven't yet found enough fossils to completely fill out the penguin family

  • tree.

  • But we know they evolved from a flying ancestor.

  • The closest living relatives of the penguins are the Procellariiformes which includes albatrosses,

  • petrels, and storm petrels.

  • And they all fly.

  • And based on studies that use a combination of genetics, skeletal similarities, and LOTS

  • of statistics, it seems that the split between the two groups probably happened in the Late

  • Cretaceous Period, sometime between about 71 million and 66 million years ago.

  • So why would a bird trade the ability to fly for a lifestyle of full-time diving?

  • After all, flight is energetically expensive, but it helps you do things like travel long

  • distances and avoid predators.

  • Well, one answer is that it's hard to be really good at both, in terms of anatomy and

  • biomechanics.

  • And in a paper published in 2013, researchers compared energy use in two species of living

  • birds to figure out why.

  • They studied thick-billed murres, which are wing-propelled divers, and pelagic cormorants,

  • which are foot-propelled divers.

  • Now, both of these species also fly, so the researchers could calculate the energy costs

  • for that, too.

  • And they found that the more specialized wings become for diving, the worse they were for

  • flying.

  • For wing-propelled diving, you need a large body size

  • and shorter, flatter wings with dense, enlarged bones,

  • because all of these things maximize the length and efficiency

  • of every dive.

  • But these features are also the exact opposite of what you need for efficient flight.

  • So, there's a pretty straightforward trade-off in anatomy here.

  • And it might be that the ecological conditions in which penguins evolved probably made it

  • easier for some birds to give up flight in favor of diving.

  • In the aftermath of the extinction at the end of the Cretaceous, diving predators like

  • small mosasaurs and plesiosaurs were gone, so their ecological niches were suddenly open.

  • And the larger marine reptiles and sharks that had made the seas so treacherous for

  • diving birds were gone, too.

  • So, with a new potential source of food and fewer predators, those penguin ancestors that

  • were better divers might have had an advantage, being able get more food while using less

  • energy.

  • And once they started down the path towardunderwater flight,” there was no stopping

  • them.

  • They spread quickly, by geological standards, to most of the places that we find them today.

  • We don't know what those first penguins looked like, because we haven't found any

  • fossils that show this transition directly.

  • But what we can say is that they were probably around 1 kilogram.

  • Because, that's the biggest that living birds get that can do both wing-propelled

  • diving and flying.

  • And it's also the size of the smallest living penguin.

  • From there, true penguins probably evolved in what's now New Zealand in the early Paleocene

  • Epoch, around 61 million years ago, with Waimanu and its relatives.

  • From there, they made it to Antarctica by the late Paleocene, between 59 million and

  • 56 million years ago.

  • And they arrived in South America by the middle Eocene Epoch, around 42 million years ago.

  • By the late Eocene, between 37 million and 34 million years ago, there were at least

  • two genera of penguins in Australia.

  • Oh, and did we forget to mention that many of these penguins were huge?

  • Well, huge for a penguin.

  • The largest living penguin is the emperor penguin, which stands a little over a meter

  • tall and can weigh up to 40 kilograms or so.

  • It's not a smol birb.

  • But it would've looked smol next to some of its Paleocene relatives from New Zealand.

  • Crossvallia was just over 1.5 meters tall, and weighed nearly twice as much as an emperor

  • penguin, between 70 and 80 kg.

  • And Kumimanu was even bigger - it stood about 1.7 meters tall and tipped the scales at just

  • over 100 kg.

  • Both of these species show up not long after penguins first evolved.

  • This tells paleontologists that not having to fly anymore meant they could go all-in

  • on becoming more efficient divers, and several groups independently developed large body

  • sizes as a result.

  • And those guys weren't even the biggest penguins that we've found.

  • Antarctica in the Late Eocene, was home to at least two more species of giant penguin.

  • Anthropornis was about the size of Kumimanu - close to 2 meters tall and about 100 kg.

  • Very respectable for a giant penguin.

  • But one species in the genus Palaeeudyptes was the penguin heavyweight champ.

  • It stood just over 2 meters tall and weighed about 115 kg.

  • And finally, another of these Late Eocene giant penguins wasn't the dapper

  • bird we picture today when we think of penguins.

  • We call it Inkayacu, and it lived in what's now Peru

  • It was found with fossilized feathers that contained preserved melanosomes, the parts

  • of cells that make and store pigment.

  • And its melanosomes didn't look like the ones found in living penguins.

  • Instead of making its feathers black and white, they looked like those of other modern birds

  • that are gray and reddish-brown!

  • The giant penguins were successful for millions of years, but they'd mostly disappeared

  • by about 23 million years ago and were totally gone by 18 million years ago.

  • And the cause might have been the rise of marine mammals, especially new groups of toothed

  • whales that happened in the Oligocene Epoch.

  • Toothed whales and pinnipeds, like seals and sea lions, might have competed with the giant

  • penguins for food.

  • Or they might've seen the giant penguins as food.

  • And social pinnipeds might've competed with the penguins for safe places to breed, too.

  • But the jury is still out, because testing hypotheses about competition in the fossil

  • record is hard.

  • All we can say for sure is that these groups overlapped in time and space, and the giant

  • penguins aren't around anymore.

  • Today, penguins still live on all of the southern continents, from Antarctica all the way up

  • to the Galapagos Islands, which they reached about 4 million years ago, and where cold

  • ocean currents keep the waters cooler.

  • And we know a lot about their extinct relatives, thanks to their dense bones and preference

  • for living on the coast, because these things made them more likely to be preserved than

  • birds living farther inland.

  • We're still missing the fossils that capture their transition from flying to diving, and

  • there's definitely more to figure out about how and why that happened.

  • But it looks like the story of penguins may have been tightly linked to the rise and fall

  • of the other organisms around them.

  • The extinction of the marine reptiles at the end of the Cretaceous left a lot of empty

  • niches that they were able to dive into.

  • ...see what I did there?

  • And then they got huge!

  • Which I, for one, am pretty sad to have missed out on.

  • They spread throughout the southern continents, until some of them lost out to new competitors,

  • the marine mammals.

  • And while we think of them as smallish, formally dressed, flightless birds, for almost forty

  • million years, penguins were some of the big predators in the Cenozoic oceans - all because

  • they could take advantage of one of our planet's major extinctions.

  • And the penguins that are still around today are the descendants of that ancient lineage

  • that went from the sky to the sea.

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  • Oversized flipper high fives to this month's Eontologists: Patrick Seifert, Jake Hart,

  • Jon Davison Ng, Sean Dennis, and Steve!

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About 61 million years ago, on the shore of what's now the South Island of New Zealand,

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When Penguins Went From The Sky To The Sea

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    joey joey に公開 2021 年 05 月 01 日
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