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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.
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