Why Triassic Animals Were Just the Weirdest

Why Triassic Animals Were Just the Weirdest

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Thanks to Curiosity Stream for supporting
PBS Digital Studios. 220 million years ago, some strange-looking
reptiles lived in the forests of northern Pangaea. They looked kind of like chameleons, with
grasping hands and feet, and long prehensile tails. But they also had really small, bird-like
heads, and sometimes beaks, and these cool spikes sticking out of the ends
of their tails. These were drepanosaurs. And for a long time, paleontologists didn’t
know what to make of them. Sure, they look kinda like chameleons, but
their heads are all wrong, not at all like other reptiles. And chameleons don’t show up in the fossil
record for another 120 million years after these guys. So, for a long time, some experts thought
these animals were close ancestors of birds. I, myself, don’t see it. But it turns out, the Triassic was full of
animals like this — creatures that look a lot like other, more modern species, even
though they’re not closely related to them at all. In addition to the drepanosaurs, the Triassic
was home to the phytosaurs, which could easily be mistaken for crocodiles. And ichthyosaurs, which looked a whole lot
like porpoises, even though they were reptiles. There was a host of creatures whose basic
body plans would show up again and again, much later in time, by completely unrelated
species. So why were the animals of the Triassic like
this? What made them look like animals that lived
much later, and that they weren’t related to? The answer has to do with how evolution works,
and with the timing of the Triassic itself, when life was trapped between two mass extinctions. Drepanosaurs were unknown to scientists until
the late 1970s, when the first species, Drepanosaurus unguicaudatus, was named, based on fossils
found in Italy. And since then, only a handful of others have
been named, all from Triassic rocks in the northern hemisphere. Scientists originally thought that drepanosaurs
were just weird, early lizards. But the more they looked at them, the more
confusing they became. These creatures had grasping hands, arched
backs, and thick muscular tails. Some kinds, like Megalancosaurus, even had
prehensile tails, with a claw-like hook on the end that helped them hold on to branches. And the heads on these animals weren’t like
those on other reptiles. They had pointed snouts, big eyes, and sometimes
beaks instead of teeth, making them look less like lizards and more like … featherless
birds. Their necks were bird-like, too, with vertebrae
that were saddle-shaped, giving them a greater range of motion. Now, because their heads and necks were so
bird-like, some researchers in the ‘90s thought that drepanosaurs must have had something
to do with the early evolution of birds themselves, despite the fact that the rest of their bodies
looked nothing like birds. But in 2017, things started to become more
clear, when the pristine skull of a new drepanosaur species was reported in New Mexico. The new genus was named Avicranium, or “bird
head,” and it brought the relationship between drepanosaurs and other reptiles into sharper
focus. It turned out that any resemblance this drepanosaur
had to birds was really pretty superficial. Its ears for instance, lacked ear drums and
were more like the ears of early reptiles than those of birds. In fact, researchers compared more than 300
of its anatomical features with other early reptiles, and concluded that drepanosaurs
were one of the earliest branching lineages of reptiles, probably diverging late in the
Permian Period. And like all early reptiles, the ancestors
of drepanosaurs probably looked superficially like lizards, even though they weren’t closely
related to them. They had short necks, low-slung bellies, and
long tails, with limbs sticking out from the sides of the body. So, what happened to them? How did drepanosaurs go from being pretty
familiar lizard-like reptiles to animals so strange that it took experts decades to figure
out what they were? And how did these changes happen in such a
short span of time? It’s because the Triassic was basically
the meat in an extinction sandwich. The entire Triassic Period is a window of
just 52 million years, between two major extinction events. At the early end, there was the Great Dying,
which eradicated about 70% of terrestrial species and 90% of marine life. On the other end, there was the aptly-named
End Triassic Extinction, which wiped out more than half of animal species. But in between, there was the whole, wide,
still-mostly-liveable world. And for the survivors of the Great Dying,
this was a completely different world — one with a whole host of ecological niches that
needed to be filled. And at the same time, there was almost no competition. And in any given environment, it’s competition
that usually regulates the rate of evolutionary change among living things. By and large, the less competition there is,
the faster the rate of change will appear to be. That’s because a lack of competition allows
empty niches in an ecosystem to be filled by new species. Then, over time, as those niches fill up,
competition increases, and the rate of evolutionary change appears to slow down. I say it “appears to,” because genetic
mutations keep showing up like they always do. But if those mutations don’t offer big advantages
right away, they get weeded out, and body plans keep looking basically the same. So, if you’re an animal that’s found your
niche — say, preying on fish in rivers — then natural selection tends to let you keep doing
what you’re doing. Your rate of evolutionary change, at least
outwardly, appears to be slow, because you’ve found a body plan that works. Likewise, it’s hard for other lineages to
move into that role. So, natural selection usually leads them to
keep doing what they’re doing, too, rather than try to compete directly for a niche that’s
already been filled. But! When extinctions occur, the opposite happens! Lots of niches open up, and there’s much
less competition. There’s plenty of room for everyone. And that’s when the rate of evolutionary
change takes off. Organisms quickly move into unfilled niches
and diversify into new species, as they arrive at the adaptations that help them succeed
in certain roles These explosive bursts of evolutionary change
are known as adaptive radiations, and they typically happen after mass extinctions. And this is exactly what happened to drepanosaurs,
and other reptiles at the start of the Triassic. Before the Great Dying, in the Permian, the
ancestors of all reptiles looked pretty lizard-like, and they filled similar niches that many lizards
do today. But the earliest members of the drepanosaur
line started to adapt to a specific niche — that of tree-climbing insectivores. And within just 20 million years, the first
drepanosaurs appear in the fossil record as their own distinct lineage of reptile. Those chameleon-like hands and feet helped
them climb trees. The arched shoulders and flexible necks may
have helped them catch bugs or peel away tree bark. And their bird-like heads and beaky mouths
allowed them to probe tight spaces for food. As the Triassic wore on, more empty niches
were filled, and the rate of evolutionary change, and the appearance of new species,
slowed down. Soon, a new normal was established. For example, some lineages of reptiles took
on the role of apex predators on land, and by 240 million years ago, a group of archosaurs,
the rauisuchids, appear in the fossil record. Meanwhile, with these predators running around,
another line of plant-eating archosaurs, the aetosaurs, came to be covered in bony armor. And another lineage became adapted for catching
fish. These reptiles, called phytosaurs, developed
long, narrow snouts lined with teeth in just a few million years, thanks in part to the
lack of competition for the role of river-dwelling fish-eaters. But, you know what happens next! Starting around 201 million years ago, a wave
of volcanic activity, along with Pangae’s break up, released tons of CO2 into the
atmosphere, causing rapid global warming and ocean acidification. It brought an end to the Triassic Period — and
also the end of about three quarters of the world’s species, including the aetosaurs,
phytosaurs, and the drepanosaurs. This time, it was the dinosaurs who were the
big winners, because they were small and unspecialized, which may have helped them survive. And then they went on to fill the empty ecological
niches that were once filled by Triassic animals, setting off a whole new round of adaptive
radiation. The aetosaurs, for example, were gone, but
stegosaurs and ankylosaurs eventually took their place as armor-covered herbivores. Likewise, with the disappearance of the phytosaurs,
spinosaurids and crocodiles were able to move in to dominate as freshwater predators. In fact, most dinosaur groups replaced earlier
species that had filled a similar niche. And they often wound up repeating some of
their body plans, too — a classic example of convergent evolution, where similar selective
pressures can lead to similar physical results. And this whole cycle has repeated itself throughout
history — extinctions strike, only to be followed by bursts of evolutionary innovation
through adaptive radiation. In fact, after the non-avian dinosaurs were
wiped out, the surviving mammals and birds went through the same rapid burst of evolutionary
change, filling the niches left empty by the dinosaurs. Even though the history of life is marked
by devastating mass extinctions, the survivors rally each time, with adaptive radiations
produce a dizzying number of new species, and body plans keep showing up, molded as
they are by the same selective pressures. It’s a pattern that will go on probably
forever — including after the next mass extinction. As some guy once said, life finds a way. Thanks to Curiosity Stream for continuing
to support PBS Digital Studios. With CuriosityStream you can stream documentaries and programs about science, nature, and history, including Curiosity Stream originals! In fact, Season 2 of Ancient Earth is now
available, featuring brand-new episodes about giant insects, feathered dinosaurs and, aptly
enough, the dawn of mammals! You can learn more at curiositystream.com
slash eons, and when you sign up, use the code EONS. As always, thanks for joining me and be sure to let me know in the comments what you want to learn about ancient life. You guys seriously have the best ideas! Also if you haven’t already go to youtube.com/eons and subscribe. Now if all this talk about climate change
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