In 1948, in the Gobi Desert of Mongolia, a Soviet fossil-hunting expedition uncovered
three flat pieces of bone about a meter long.
The paleontologist who described them thought that they must have been the claws of some
big species of turtle or turtle-like reptile that used them to forage for seaweed.
So he gave the fossils a name that translates to “turtle-shaped scythe lizard,” a.k.a.
Therizinosaurus cheloniformis.
But then, over twenty years later, the identity of these claws would change…a lot.
Because they were actually the enormous hand-claws of some kind of dinosaur.
But with so little other material, exactly what type of dinosaur they belonged to was
still a mystery.
Over the next few decades, other bizarre fossils would be discovered in the same area.
Paleontologists found hips, neck bones, and a skull that looked like they all came from
plant-eaters, along with more big claws, and odd four-toed feet.
It seemed like there was no way these fossils
could’ve all come from a single group of dinosaurs.
And if they did, where did they belong in the dino family tree?
Figuring out the identity of these dinosaur mash-ups would take the discovery of more
complete fossils in the 1980s and 1990s.
But the mystery didn’t end there…
Because how the therizinosaurs lived and evolved
ended up being just as weird as their mixed-up anatomy.
For decades, speculation reigned about where all these seemingly-different fossil animals
fit in the family tree of dinosaurs.
Some researchers suggested that the strange remains from the Gobi came from theropods,
the group that includes carnivorous dinosaurs
like Tyrannosaurus rex (maybe you've heard of it), and Allosaurus.
Others went in a completely different direction – thinking that they might be late-surviving
prosauropods, the ancestors of the long-necked giant plant-eaters known as sauropods.
Or maybe they were sauropods!
And it even seemed possible that they were from another, previously unknown branch of
the dinosaur family tree.
The mystery continued until 1993, when a new dinosaur dated to between 125 and 113 million
years ago was described from China.
It was named Alxasaurus and it had all of the strange features of those earlier fragmented
finds combined in one animal, like slender jaws and a long neck.
If the previously discovered fossils from the Gobi Desert were like the mixed-up pieces
of a jigsaw puzzle, then Alxasaurus gave paleontologists
a look at the picture on the box.
This new skeleton showed how the fossils found before, including Therizinosaurus, could actually
be from different species of the same family of just extremely weird-looking dinosaurs.
And this group was unlike anything anyone could have imagined.
These dinosaurs had the hips of a plant-eater, long arms that ended in long claws like a
giant ground sloth, and four-toed feet like prosauropods.
If those features together weren’t strange enough, they also had long necks and small
heads that contained tiny, leaf-shaped teeth like a sauropod, and beaks.
And, along with the discovery of Alxasaurus, came another study that would change the shape
of the dinosaur family tree by figuring out where these weirdos fit into it.
It found that Therizinosaurus and its relatives were actually theropods after all.
And later work would place them in the subgroup
that also includes dromaeosaurs, like Velociraptor,
Deinonychus, and birds.
But these theropods looked nothing like
T.rex or any of their other known carnivorous relatives.
Dromaeosaurs had more narrow hips and were
more lightly built than Therizinosaurus, for example.
They also walked on three-toed feet, had shorter necks, and had larger heads relative to their
body size with serrated, meat-cutting teeth.
And while dromaeosaurs also had long arms, they didn’t have the big claws on their
hands that got Therizinosaurus mistaken for a clawed turtle.
So how and why did the therizinosaurs end up looking so different from their cousins?
Well, the answers to these two questions are actually linked together: these theropods
look the way they do because they evolved into herbivores.
And in 2005, an early relative of Therizinosaurus
that was uncovered in Utah helped us understand how this happened.
It’s called Falcarius and it dates back to at least 125 million years ago, in the
Early Cretaceous Period.
It shows an in-between phase between therizinosaurs
and their more carnivorous ancestors.
Falcarius was much smaller than other therizinosaurs,
weighing around 100 kg.
It had more narrow hips, some pointed teeth that could’ve helped it eat prey, and the
standard curved hand claws of a theropod.
But it also had some leaf-shaped teeth that point to it having a more omnivorous diet
than a typical theropod, because this tooth shape is what we see in some plant-eating dinos.
And Falcarius had a much longer neck than most theropods of its size, too.
This let it reach about 1.5 meters off of the ground, which may have allowed it to
browse on leaves.
By the Late Cretaceous, therizinosaurs took these adaptations one step further.
Their hips evolved to become wider and more robust, which made room for a bigger gut to
digest plant material, a trait we see in many herbivores.
Their hands began to have a greater range of motion, and their claws gradually became
better-adapted as grasping hooks for foraging,
similar to the claws of extinct giant ground sloths.
And their necks trended longer to browse on even higher foliage, while they also increased
in overall body size.
But how could a theropod evolve to eat plants in the first place?
We’ve seen how their bodies changed over time, but what drove that change?
Well, in 2009, a study proposed two possible scenarios for how this could’ve happened.
One scenario pointed out that we already know of several groups of theropods that shifted
from a more carnivorous diet to a more omnivorous one.
These include the ornithomimosaurs and the oviraptorosaurs.
So it’s possible that this change in diet happened multiple times independently in different
groups of theropods.
The second scenario pointed out that these two groups and the therizinosaurs belong to
a specific subgroup of theropods called Coelurosauria,
along with dedicated carnivores like T. rex.
This subgroup is thought to share a common ancestor, which might’ve had a more omnivorous
diet to start with.
This would mean that the therizinosaurs went from
omnivory to herbivory, and were never carnivorous at all!
And the thing that drove this group to herbivory?
Well, Therizinosaurus and its ancestors probably adapted to eat plants to avoid competition
with their carnivorous relatives.
Falcarius shared an environment with a host
of dromaeosaurs, troodontids, and early tyrannosaur
relatives, so competitive pressure among carnivores
was likely pretty high.
And the specific adaptations of the therizinosaurs – like long necks and claws – probably
also helped them avoid competition with
other herbivorous dinosaurs, like hadrosaurs and ceratopsians.
Therizinosaurs that lived in the same place at the same time even became more specialized
to avoid competition with each other!
So, amid all of this competition, the therizinosaurs were pushed into their
weird combination of features to carve out their own ecological niche.
And in solving this fossil mystery, the big picture that the adaptations of Therizinosaurus
and its relatives paint for us is ultimately a much more diverse picture of theropods – they’re
not all just variations of T. rex!
Not only that, but they’re also pretty weird for herbivorous dinosaurs, combining many
of the traits we see in other groups of plant-eaters
into one unique family.
From their initial mistaken identity as giant-clawed
turtles, to the debates about how they evolved, the therizinosaurs
have been surprising paleontologists for decades.
They’ve both shaken up our view of the dinosaur
family tree and shown us how much of the fossil record we still have to explore.
Thanks to this month’s claw-some Eontologists!
I'm just getting used to it now.
Like there's just gonna be a pun every week, fine.
Gale Brown, Juan M., Jacksy Weiss, Melanie Lam Carnevale,
Annie & Eric Higgins, John Davison Ng, Colton, Chase Archambault, and Jake Hart.
Become an Eonite at patreon.com/eons and you
can get fun perks like submitting a joke for me to read.
Like this one from The Burgins.
"How many dinosaurs does it take to screw in a lightbulb?"
I'm not even gonna look.
I'm gonna say...11.
"Ha, trick question! No dinosaur has paid its electric bill in over 65 million years."
Then, there's the question of like, is the electric bill - did they use fossil fuels???
Riddle me this.
Now the joker has become the jokee.
Or is it...?
Now the jokee has become the joker.
And as always thanks for joining me in the Adam Lowe studio.
Subscribe at youtube.com/eons for more evolutionary escapades.
These dinosaurs had the hips of a...
I don't know why that cracks me up...
has the hips of a plant eater.
[sighs] I'm unhinged now and that's the kind of stuff I'm getting in mind...
[Paige, the script editor of this episode:] "Hips Don't Lie..."
Exactly! The hips don't lie!
Now I have to get back on track.
I don't get paid by the hour.