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! Become an Eonite by supporting us at patreon.com/eons. Remember Eonites get perks like submitting
a joke for us to read! Like this one from Charles Copley: What do
you call an ugly dinosaur? An eyesaur. Also check us out on social media and celebrate
a bunch of random holidays - such as Geological Map Day and National Velocirator Awareness
Day! Yes, those two days both exist. Find us on Instagram, Twitter, and Facebook. And you can follow me on Instagram at fossil
underscore librarian. And as always thank you for joining me in
the Konstantin Haase studio. Subscribe at youtube.com/eons for more creature
features.
Her pronunciation was great with no hesitation .
sounds like the weird tooth dino was home schooled.
I wish there were more channels like this...
I love Eons so much. <3
Eons is great! But worried about the long term monthly supporter Steve being gone though, they've been mentioned in almost every video.