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  • A quarter of a billion years ago, an epic story was starting to unfold all over the

  • world.

  • The heroes of this saga were the survivors of a near apocalypse.

  • They emerged from it as humble creatures, but in time, they came to dominate the Earth.

  • These were the reptiles, which grew to become some of the largest forms of life ever to

  • stomp, swim, and soar across the planet.

  • There were, however, some other noteworthy players in this story -- the animals and plants

  • that diversified in the shadows of the reptiles, many of which would go on to play key roles

  • in our planet’s future.

  • Over millions of years, this whole troupe of characters adapted to a rapidly

  • changing world.

  • But, even though they grew to immense sizes and dominated all environments, many of the

  • reptiles couldn’t adapt to the changes that would bring their reign to an end.

  • This Age of Reptiles was a spectacular prehistoric epic, and it all took place in a single era:

  • the Mesozoic.

  • The Mesozoic Era began 252 million years ago in the aftermath of the most destructive mass

  • extinction of all time, The Great Dying.

  • It brought to a close the previous era, the Paleozoic, and wiped out most marine and land

  • species, leaving a world ripe for the taking, at least foranything that survived.

  • In the Early Mesozoic, Earth’s landmasses had almost finished merging into a single

  • supercontinent called Pangea.

  • And that meant life could traverse the globe, free of ocean barriers.

  • The stage was set for the first act of the Mesozoic, the Triassic Period.

  • At first, the planet was populated only by the survivors of the Paleozoic.

  • On land, there were the amphibian-like temnospondyls and early relatives of mammals called therapsids.

  • Meanwhile, the seas were home to many groups of ancient fish but also lots of reptiles

  • that were adapted to life in the water.

  • These first marine reptiles were mostly amphibious, but they quickly developed fully aquatic traits,

  • including the ichthyosaurs, which came to resemble fish and later marine mammals, even

  • though they were neither.

  • And in the background, a special group of reptiles was beginning to take advantage of

  • this newly open world.

  • These were the archosaurs, a clade that had its origins in the Paleozoic but truly came

  • into its own during the Triassic.

  • Archosaurs include many of the animals that you think of, when you think of ancient life

  • -- like dinosaurs, pterosaurs, and the croc-like phytosaurs.

  • And these creatures became key players in the Mesozoic, because they were very well

  • adapted to its environments.

  • The skulls of archosaurs were lighter than their reptile ancestors.

  • Most archosaurs also had teeth that were set in deep, protective sockets.

  • Plus they pioneered a unique unidirectional respiratory system which was a network of air sacs

  • that let them breathe more efficiently in the low-oxygen atmosphere after the Great

  • Dying.

  • With the help of adaptations like these, archosaurs spread and diversified.

  • And by the middle of the Triassic, around 243 million years ago, the real rising stars

  • of the archosaurs appeared: the first dinosaurs.

  • This is when the earliest known proto-dinosaur appears in the fossil record: Nyasasaurus,

  • in what’s now Tanzania.

  • Soon after, we find evidence of the tiny omnivore Eoraptor and the predator Herrerrasaurus,

  • both in South America.

  • And all of the earliest known dinosaurs were members of the same group, known today as

  • saurischians.

  • They had the same basic things in common, like long necks and tails and a generally

  • reptilian body plan.

  • But the thing I wanna point out here is theirpubis.

  • That’s one of the three bones that makes up the pelvis, including in you.

  • You have a pubis.

  • And in the case of saurischian dinosaurs, the pubis bone always pointed down, and forward.

  • I know itll be hard for ya, but try to remember that word, pubis, because I’m gonna

  • come back to it.

  • Now, as dinosaurs started playing larger roles in their ecosystems, they also became larger

  • and more specialized.

  • And around 230 million years ago, they diverged into two of their most iconic groups: the

  • long-necked sauropods and the two-legged, mostly-meat-eating theropods.

  • But while dinosaurs were becoming more diverse and widespread, another lineage of archosaurs

  • was adapting to another environment: the sky.

  • By the Late Triassic, pterosaurs became the first vertebrates in the history of the world

  • to take flight, with Eudimorphodon and others like it appearing in the fossil record throughout

  • Europe some 210 million years ago.

  • Meanwhile, other, non-archosaurian reptiles were dominating the seas.

  • Ichthyosaurs had been swimming the world’s oceans since near the start of the Triassic.

  • And by 200 million years ago, another group of semi-aquatic reptiles had given rise to

  • plesiosaurs, which adopted a totally different body plan for life in the water.

  • This three-pronged takeover of the land, sea, and sky allowed the reptiles to rule Pangea.

  • But it was not until the very end of this period that the last remnants of Paleozoic

  • life would truly be swept away.

  • 201 million years ago, Pangea began to break apart, as North America drifted away from

  • the rest of the continent.

  • This caused a spike in volcanic activity that sent greenhouse gases into the atmosphere.

  • The resulting rise in global temperatures triggered an event known as the End-Triassic

  • Mass Extinction.

  • The casualties included most of the therapsids and other holdovers from the Paleozoic.

  • And this left many niches open for the dinosaurs, pterosaurs, and other reptiles that would

  • come to define the Mesozoic.

  • Which brings us to the second act of this story: the Jurassic Period, when the Age of

  • Reptiles reached its peak.

  • Hence the name of the park

  • By this time, dinosaurs had already acquired a variety of body plans and adaptations.

  • And one of the most important was the spread of the Ornithischians

  • Now, remember that pelvic bone I was talking about?

  • The pubis?

  • You remembered! Very good

  • Well that’s where a major change occurred in ornithischian dinosaurs.

  • Instead of pointing down and forward, in ornithischians, the pubis was reversed.

  • It pointed backward.

  • This allowed ornithischians to have a larger gut cavity that could hold expanded digestive

  • organs, and this helped them eat some of the toughest plants of the Mesozoic.

  • Because of this adaptation, and with the help of new, chisel-like teeth, ornithischians

  • became the eating machines of the Mesozoic!

  • Ornithischians probably first appeared back in the Triassic, but this was when they started

  • to spread and diversify.

  • The group became so successful that eventually it would grow to encompass many of the most

  • prolific kinds of dinosaurs -- all the hadrosaurs, ceratopsians, armored dinosaurs and pachycephalosaurs,

  • to name just a few.

  • So, in the Early Jurassic, reptiles took center stage, moving into most of the large animal

  • niches.

  • But one group of fuzzy therapsids managed to survive into the Jurassic, and for a long

  • time its members played only minor roles.

  • These were the very first true mammals, like Megazostrodon -- a tiny, nocturnal insectivore

  • that scurried around the feet of the dinosaurs.

  • While there’s some debate over whether mammals actually appeared at the end of the Triassic,

  • they certainly diversified in the Jurassic.

  • By the middle of the period, 164 million years ago, mammals had diversified beyond little

  • shrew-like things to include species that could swim like beavers and glide like flying

  • squirrels.

  • And alongside these new types of mammals was a group of theropods that would also take

  • to the air, acquiring the first complex wing feathers.

  • By 150 million years ago, the first paravian dinosaurs -- or stem birds -- were taking

  • to the wing, although these animals, like Anchiornis, probably weren’t very good fliers.

  • As for the non-avian dinosaurs, the Late Jurassic was when their really famous forms appeared.

  • Legendary characters like the spiky ornithischian Stegosaurus, and the carnivorous Allosaurus

  • patrolled the plains of North America.

  • Meanwhile, giant sauropods like Giraffatitan roamed Africa.

  • For these reptiles, the Late Jurassic was a golden age, where they were the most obvious

  • forms of life all over the planet.

  • But as the period came to a close, Jurassic Earth was changing.

  • The breakup of Pangea was still underway.

  • Sea levels began to rise, creating shallow seas in North America and Europe

  • And as these landmasses moved, more events unfolded that led to a complex series of extinctions

  • about 145 million years ago,

  • Rather than being a single clear incident, these losses were the result of a constantly cycling

  • climate of cooling and warming, and a jolt of volcanic activity again in the Pacific Ocean.

  • These events ushered in the third act of the Mesozoic: The Cretaceous Period.

  • I know that traditionally the third act is supposed to be the shortest

  • part of any drama but in this case the Cretaceous is

  • actually the longest period of the Mesozoic. And the Cretaceous saw some of the most extreme changes

  • ever recorded in both flora and fauna.

  • One of the first breakthroughs of the Cretaceous was the appearance of flowers, which appear

  • in the fossil record about 130 million years ago.

  • Before these early bloomers, conifers, ferns and cycads were the dominant plants.

  • Now they had competitors, although it would be a while before flowering plants became

  • major players in the landscape.

  • Meanwhile, dinosaurs were going through their own revolution.

  • Feathered theropods, called coelurosaurs, rarely got larger than dogs during the Jurassic,

  • but they reached new heights in the Cretaceous.

  • By 125 million years ago, big, predatory coleurosaurs like Utahraptor were roaming North America,

  • while Yutyrannus was hunting in China.

  • In the middle Cretaceous, a new group of sauropods, the titanosaurs, were outgrowing all of their

  • Jurassic relatives.

  • Some, like Argentinosaurus, are thought to have grown over 30 metres long and weighed

  • nearly 70 metric tons!

  • In the skies, pterosaurs also got much bigger, and by the Late Cretaceous, they became the

  • largest animals ever to fly.

  • These were the giant azhdarchids, which were as tall as giraffes, had wingspans the size

  • of small airplanes, and were more than capable of feeding on small dinosaurs.

  • Beneath the wings of these animals, the Cretaceous continents continued to drift apart, and dinosaur

  • groups became more and more isolated, and also more distinct.

  • For instance, titanosaurs became much more common on the southern landmasses, while in

  • the north, a group of feathered coelurosaurs was reaching the rank of apex predator.

  • These were the tyrannosaurids, the largest of the tyrannosaurs.

  • They first appear in the fossil record in the middle Jurassic, but by the late Cretaceous,

  • they had developed powerful crunching jaws and swift legs to deal with a whole new cast

  • of ornithischians on the northern continents.

  • Some of their prey, like the ceratopsians, grew wild head-gear, while ankylosaurids acquired

  • armour to attract mates and fend off predators.

  • But the duckbilled hadrosaurs were the most prolific herbivores in the north, thanks to

  • their powerful, beaky mouths and complex teeth that allowed them to eat just about any kind

  • of plant.

  • By the Late Cretaceous, many of these dinosaurs were the largest and most bizarre that these

  • groups had ever produced, showing just how successful the reptiles had become

  • Yet, they were about to see their Age come to an end.

  • For reasons that experts aren’t quite sure about, some dinosaur groups, like hadrosaurs

  • and ceratopsians, were becoming less diverse toward the end of the Cretaceous.

  • The low species density means that some dinosaur groups were already vulnerable to extinction.

  • And as it happened, doom was on the horizon.

  • Rocks dated to 66 million years ago from a region of India called the Deccan Traps show

  • signs of massive volcanic eruptions.

  • These were among the largest eruptions in Earth’s history, lasting for tens of thousands of

  • years, and the volcanic gases likely had powerful effects on the air and oceans.

  • And in the midst of these eruptions, another disaster came: A giant asteroid struck the

  • Gulf of Mexico, spewing ash into the atmosphere, creating an impact winter that starved plants

  • and phytoplankton.

  • These twin disasters threatened all life on Earth, but the largest animals -- the ones

  • that needed the most food -- were most affected.

  • The giant titanosaurs, and ceratopsians, and other herbivores wouldn’t have been able

  • to find enough plants to sustain their bulk.

  • And their decline meant that the large carnivores, like the tyrannosaurs, were doomed as well.

  • The seas saw similar losses in reptile groups, including the extinction of the plesiosaurs

  • and mosasaurs.

  • Other marine reptiles, like the ichthyosaurs and pliosaurs were already long gone by this

  • time.

  • Nearly 200 million years after they pulled themselves from the ashes of the Great Dying,

  • all of the giant reptiles were wiped out.

  • It’s now known as the K-Pg Extinction, after the German abbreviation for Cretaceous and

  • Paleogene, the two periods whose boundary is marked by this event.

  • And it brought about the disappearance of 75% of the world’s species.

  • But, thanks in part to their small size and their more varied diets, many Mesozoic animals

  • survived into modern times.

  • The three modern mammal groups -- the placentals, marsupials and monotremes -- all made it.

  • The flowers that first bloomed in the Cretaceous are now more numerous than ever.

  • And even some of the mighty archosaurs have persisted into our day, as crocodilians and

  • .. birds, the last surviving saurischian theropod dinosaurs.

  • But for the giant reptiles of the Mesozoic, their dominance turned out to be their downfall.

  • When disaster struck, the niches that demanded large size and specialization were the first

  • to go.

  • It turned out playing smaller parts in the story of our planet was a key to survival.

  • And so as the next era dawned, the Cenozoic, it would be those once-minor characters that

  • would inherit the Earth.

  • Thanks for sticking around for this long story about the Mesozoic, I appreciate it

  • But please, tell me what you want to learn about.

  • Because you have a lot of good ideas!

  • So leave me a comment, and don’t forget to go to youtube.com/eons and subscribe.

  • By the way, have you checked out Physics Girl?

  • She’s been nominated for a Webby, and if you check out her channel, youll understand

  • why.

  • Your brain will thank you!

A quarter of a billion years ago, an epic story was starting to unfold all over the

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三幕爬行动物的时代(The Age of Reptiles in Three Acts)

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    joey joey 發佈於 2021 年 04 月 30 日
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