Around 244 million years ago, what’s now a mountainous area in southwestern China was
part of a vast sea.
It was filled with a thriving marine ecosystem, just 8 million years after the Great Dying
- the extinction that ended the Permian Period and began the Triassic.
And there’s one fossil site there, in Luoping County, that gives us an incredible snapshot
into this window of time when life was recovering from the most devastating mass extinction
ever known.
Most of the fossils from this site are of arthropods, like ancient horseshoe crabs and
crustaceans.
But many different kinds of fish were also present, including coelacanths, as well as
marine reptiles like ichthyosaurs and the ridiculously long-necked tanystropheids that
don’t really look like anything today.
But none of these animals were quite as strange as Atopodentatus.
Figuring out what this creature’s face actually looked like would take paleontologists years,
both because of how its skull fossilized and because of its very odd feeding strategy.
See, Atopodentatus was an herbivore - and herbivorous marine tetrapods, living or extinct,
are pretty rare.
Because, it turns out, switching from eating other animals to eating plants is a harder
transition to make than you might think.
And understanding how and why this weird animal did it can help us shine a light on at least
one way for ecosystems to bounce back from even the worst mass extinction.
In 2014, paleontologists described the first specimen of Atopodentatus from Luoping.
Luoping is considered a Lagerstatte – a site that produces exceptionally preserved
fossils.
There are arthropods from the site with soft tissue preservation and even a coelacanth
with preserved embryos.
And Atopodentatus was yet another example of this beautiful preservation.
But!
The fossil, including its skull, was squashed flat from side-to-side.
What the researchers could tell was that this creature was a marine reptile that would’ve
been about 3 meters long and that it was likely a close relative of the sauropterygians, the
group that includes plesiosaurs and pliosaurs.
It had a slender body, a long neck, and odd hoof-like toe-tips on all four limbs.
And it wouldn’t have been all that strange, at least for the Triassic, if it wasn’t
for its head.
Its upper jaw looked like it was folded in half in the middle, and then split open to
expose its teeth, kind of like the track of a zipper, with the lower jaw forming the bottom
of this super weird triangular snout.
And those jaws were FILLED with tiny teeth - the lower jaw alone contained around 190
needle-like teeth.
That’s where the name “Atopodentatus” comes from - it literally means “unusual
teeth.”
Its peculiar dentition looked like some pterosaurs that are suspected to have been filter feeders
and whose teeth would’ve been too fragile to grasp or crunch prey.
As a relative of the sauropterygians, it seemed safe at the time to say that it was probably
a carnivore, eating tiny invertebrates it filtered from the muddy sea floor.
After all, there weren’t any herbivorous marine reptiles known from this part of the
Triassic - or from any time earlier.
But then!
In 2016, this interpretation had to be completely abandoned when two new specimens were described
that flipped this image on its head.
Literally.
Unlike the first fossil, these specimens had been flattened top-to-bottom rather than side-to-side,
which gave researchers a much clearer view of the skull.
And it was still just as strange.
Instead of having an upper jaw that was vertically split in half, these new skulls showed that
Atopodentatus actually had a hammerhead, with wide jaws that poked out to the sides and
even more exceptionally specialised teeth.
Along the front of its hammerhead, it had spatulate teeth that stuck out, like a scraper.
And those delicate, needle-like teeth that had been important in the original filter
feeding interpretation were actually positioned further back in its jaws.
This whole set-up was so weird that the palaeontologists studying it literally broke out the modelling
clay and toothpicks to try to visualize what Atopodentatus’s head would’ve looked like
in 3D.
And even after that, it still wasn’t entirely clear how this strange mouth was being used
to do what mouths do best: eat.
So the researchers went back to the fossil record to try to find something they could
compare Atopodentatus to - and what they came up with was another odd marine reptile called
Henodus.
Henodus was a peculiar placodont from the Late Triassic of Germany that actually was
a sauropterygian - that group of marine reptiles that Atopodentatus might be related to.
Now, while Henodus didn’t have a hammerhead, it did have a very rectangular head with the
front of its mouth matching the flat scraper-like area that’s found in Atopodentatus.
There were even reports of baleen-like structures being found in its jaws, similar to Atopodentatus’
tiny teeth.
Based on these features, Henodus is thought to have used the front of its mouth to scrape
vegetation up.
Then it opened its mouth and throat wide to gulp in the now-floating plants, filtering
out the water with those baleen-like structures and leaving it with a mouthful of food.
If Atopodentatus was doing something similar, that means that it was probably also a herbivore
- not the tiny crustacean-eating carnivore it was originally interpreted to be, making
it the oldest herbivorous marine reptile in the fossil record.
Which would mean that it not only looked weird, but that its lifestyle was pretty strange,
too.
Because!
Herbivorous marine tetrapods are very rare - there were just a handful in the Mesozoic
and there are only a few alive today, with manatees and dugongs, marine iguanas, and
green sea turtles being pretty much it.
And, to make things more complicated, Atopodentatus probably came from a carnivorous ancestor.
See, there are some difficulties involved in switching from a carnivorous to an herbivorous
diet that Atopodentatus or its ancestors would’ve had to overcome during its evolution.
Plants are a much trickier food source to digest than meat, largely because of the cellulose
in their cell walls, so herbivores have adaptations to deal with this that carnivores don’t
have.
For example, some rely on doing tons of chewing to physically break down the tougher plant
material.
But Atopodentatus definitely wasn’t doing this with its weird little needle teeth.
What it was doing, though, is still kind of a mystery.
It’s possible that it did something like what we see in marine iguanas today, and had
endosymbiotic bacteria in its gut that helped it digest its dinner.
Or maybe it had other physiological adaptations, like longer intestines for fermentation that
helped it get the most out of its food, which is how manatees make herbivory work.
Unfortunately, these kinds of things are rarely preserved in the fossil record, so we can
only make educated guesses based on what we know about living marine herbivores for now.
So why even become an herbivore if it’s so difficult?
Well, there are some real advantages to herbivory.
One is that you don’t expend a lot of energy chasing down your lunch every day, like you
would as a predator.
Another is that there’s generally more food easily available at lower trophic levels.
Higher up the food web, there’s just less biomass - the mass of all the organisms that
occupy that level.
As plants are primary producers that make their food from sunlight, they form the lowest
level and have the greatest biomass.
And, remember, Atopodentatus wasn’t alone in its ecosystem.
The other fauna at Luoping may have also played a part in pushing Atopodentatus toward herbivory.
The diversity of fish and marine reptiles present at Luoping - from tiny ones all the
way to top predators - is not exceeded at any time in later Triassic fossil sites.
This means that roughly 8 million years after the Permo-Triassic extinction, the biggest
mass extinction ever, Atopodentatus was living in a fully recovered ecosystem
With such a range of carnivorous marine reptiles present in the formation, it may have been
advantageous for it to do something different, to avoid competition for resources.
This is called niche partitioning and I’ve talked about it before, like in When Sharks
Swam the Great Plains.
And on top of this, prehistoric Luoping would’ve been an ideal environment for marine plants
to grow.
We know that it was a near-shore, shallow water environment, because terrestrial plants
and arthropods are found alongside the marine species that make up most of the fossils at
this site.
The shallow water would’ve allowed sunlight to penetrate the waves, which is necessary
for plants to be able to photosynthesise.
And these conditions coming together may have helped drive the evolution of a large herbivore
like Atopodentatus!
Despite the remarkable discoveries of new Atopodentatus fossils that helped paleontologists
figure out what it looked like and how it might’ve lived, there are still mysteries
to be solved about its evolutionary history.
For example, we don’t really know where it came from or what its ancestors were.
And we don’t know what happened to it, other than that, given how rare herbivory is as
a strategy for marine tetrapods, it seems to have been an “evolutionary dead end.”
But while it was around, its odd feeding strategy must’ve helped it thrive in a post-extinction
world.
The recent discovery of Atopodentatus reminds us that the strange time in Earth’s history
known as the Triassic hasn’t given up all its secrets just yet.
And I, for one, can’t wait to see what we find next.
Let’s face it...this month’s Eontologists are awesome: Sean Dennis, Jake Hart, Annie
& Eric Higgins, John Davison Ng, and Patrick Seifert!
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