In the dry, outback landscape of Queensland,
Australia, there’s a huge swath of sedimentary
rocks that today is known as the Winton Formation.
But back in the Late Cretaceous Period, this
arid scene was a lush floodplain, streaked
with rivers and swamps that drained into the
retreating Eromanga Sea - an interior seaway
that at its peak covered much of the eastern
part of the continent.
And this marshy habitat was full of life!
In the rivers, there were lungfish, turtles,
crocodilians and even aquatic lizards.
Meanwhile, on land, dinosaurs dominated - with
the megaraptor known as Australovenator perched
at the top of the food chain.
And around 95 million years ago, on one of
those Australian riverbanks, a group of small
dinosaurs were going about their business,
when they were suddenly interrupted by the
arrival of a big theropod heading straight
toward the flock, looking for an easy meal.
The small dinosaurs panicked and stampeded
away, running behind the predator to avoid
becoming lunch.
But what if … they hadn’t?
What if, instead, that theropod wasn’t a
theropod at all, but a big plant-eater, and
the small running dinosaurs weren’t running
away from it, they were just … running?
Or maybe none of those dinosaurs were even
there at the same time, and they left their
marks at this site on different days!
This site, now called Lark Quarry, was - and
still kind of is - a puzzle for paleontologists.
Because, there are no fossils of dinosaurs’
bodies to be found here.
All we have to work with are the footprints
they left behind.
So, to try to solve the puzzle of Lark Quarry,
experts have turned to a special subfield
of paleontology -- paleoichnology, or the
study of trace fossils -- to reconstruct exactly
what happened on that spot, on that day, nearly
100,000 millennia ago.
Trace fossils are literally the fossilized
traces of animal activities.
They’re also called ichnofossils, and they
can give us important clues about things like
ancient environments and animal behaviors.
They come in all kinds of forms, from burrows
- like the Devil’s corkscrew - which can
show us how and where animals lived, to coprolites,
which are just fossilised poop.
Now, it can be pretty easy to figure out what
made a modern trace - because we can actually
observe it happening - like, watching a crocodile
walk through mud, leaving its footprints behind.
But with ichnofossils, it’s a lot trickier.
In those cases, we can’t ever see the trace
being made, so without pretty spectacular
evidence, we can never be certain what species
made it.
So instead of saying an ichnofossil is a T.
rex footprint, for example, the trace is given
its own two-part name, with an ichnogenus
and an ichnospecies.
These work exactly like the binomial names
given to all organisms, and they help us group
trace fossils into useful categories.
Now, the trace fossils from Lark Quarry are
footprints, and there are over three thousand
of them.
The site was discovered in the 1960s, and
a team mostly made up of volunteers had to
remove over 60 tons of rock to reveal the
full site.
And in 1979 and 1984, two papers describing
the site were published that caused quite
a stir.
In these papers, two researchers proposed
a dramatic reading of the newly uncovered
tracks - where a peaceful day at the river
became a stampede to avoid being eaten by
a hungry carnivore.
The smaller footprints at Lark Quarry, they
said, belonged to two species of small dinosaur.
The most common tracks were given the ichnogenus
name Wintonopus and are thought to have been
made by small ornithopod dinosaurs.
They are a diverse group of plant-eating
ornithischian dinosaurs that includes animals
from the little Dryosaurus to the much larger
Edmontosaurus and Iguanodon.
The dinosaurs that made these tracks would
probably have been swift and bipedal - something
like Dryosaurus.
The second set of smaller tracks were given
the ichnogenus Skartopus
And these tracks are thought to have been
made by tiny theropods called coelurosaurs
But the third, much larger set of tracks,
looks like it was also made by a theropod.
This trackway was made by a single individual,
entering from the northeast.
The researchers interpreted this trackway
as having been made by a carnosaur, a theropod
much, MUCH larger than the little track makers.
So, their explanation for what happened is
that the sudden entrance of this carnosaur
triggered the small dinosaurs to stampede
away and behind it, some laying down footprints
within the footprints of the carnosaur.
Bit of a risky escape strategy, if you ask
me.
And this layering of tracks on top of one
another is what gives us the sequence of events;
when we have one footprint with a second footprint
on top of it, we know that the lower track
must be older.
But this isn’t where the story of Lark Quarry
ends.
More than 30 years later, other scientists
were starting to question the original interpretation
of the trackways - especially the part about
a big, hungry theropod crashing the party.
Some dinosaur tracks are pretty easy to identify,
but the differences between the tracks of
theropods and ornithopods can be much harder
to spot.
They both have three-toed prints, and while
theropods often have longer toes that end
in sharp claws instead of the rounded toes
of ornithopods, this detail is not always
preserved.
This is part of what makes the interpretation
of the Lark Quarry trackways so hard, and
interesting!
So this group of researchers used a method
called photogrammetry to construct a detailed
3D picture of the tracks so they could be
re-analyzed.
With this new analysis, it seemed that the
tracks of the supposed “carnosaur” were
not made by a large theropod at all.
Instead, they looked like the tracks of a
large ornithopod, like Muttaburrasaurus, which
is known from other sites in Australia.
If their new interpretation was accurate,
it would mean that there never was a large
theropod at Lark Quarry that day.
The panicked dinosaur stampede never happened.
But the story was about to get even more complicated.
This same group of researchers also suggested
that the way we’d been thinking about the
timing of the event was all wrong.
Because, it turned out the tracks weren’t
made at the same time!
Instead, each part of the story may have taken
place over the course of hours or days.
The evidence for this comes from drag marks
that were probably left by branches or other
debris that were pulled along the river bed
by the current.
Some of these drag marks cross over the larger
tracks and then have smaller tracks on top
of them.
This suggests that the largest trackmaker,
whatever it was, was not on the riverbank
at the same time as the smaller trackmakers.
It came and went before they even got there!
So is the original stampede hypothesis completely
dead in the water?
Not necessarily.
In 2017, yet another group of researchers
took a different approach by actually making
a physical model of an Australovenator foot,
and using it to make footprints in sediment.
They wanted to see whether a theropod could
have made the large tracks, or whether it
was an ornithopod.
They even studied the biomechanics of emus
walking through similar sediments so the movements
of the model foot could be perfected.
And the prints made by the model foot closely
matched those larger tracks found at the site!
In the footprints made by the model, the claw
imprints weren’t always visible, which means
that the tracks from a large theropod could
easily be mistaken for those of an ornithopod.
So maybe the best explanation for what happened
at Lark Quarry is a mixture of the two main
hypotheses:
It’s possible that a large theropod lived
in the Winton Formation, but it may not have
been there at the same time as the smaller
trackmakers.
And while it looks like the smaller dinosaurs
might have been running across the muddy ground,
the larger dinosaur probably wasn’t the
reason why they were running.
In fact, there's even some debate about whether
these small dinosaurs were actually running!
Other researchers think that some of the "stampeding"
traces were actually scratch marks left as
the small dinosaurs swam or waded across shallow
water!
Either way, we can learn a lot about how the
dinosaurs of the Winton Formation lived, based
on their tracks.
For example, trackways are one of the best
sources of evidence for establishing how fast
extinct animals could move.
By following a pretty simple formula that
takes into account the size of the animal
and the distance between footprints, we can
figure out how fast an animal was moving.
And from this, we can estimate that, while
the larger dinosaur was plodding along at
a leisurely 7 kilometers per hour, Wintonopus
and Skartopus were moving at 16km/h and 12km/hr
respectively.
These smaller dinosaurs were clearly heading
somewhere in a hurry, and the sheer number
of fast moving individuals makes this site
unique.
And ...the similarity between these sprinting
dinosaurs and a certain movie scene has even
led some to call this discovery the real scientific
underpinning to the famous “they’re flocking
this way” scene in Jurassic Park.
Now, we can also use tracks to figure out
how many dinosaurs were there.
The 3300 tracks preserved represent at least
130 different dinosaurs, all but two of whom
- our large mystery dinosaur and another ornithopod
who came and went before the action - were
Wintonopus or Skartopus trackmakers.
So they were moving in big groups.
And by looking at the dimensions of each track,
we can also figure out a rough size estimate
for the track maker.
Based on this, we can say that the Wintonopus
trackmakers ranged in size from 158 centimetres
all the way down to a tiny 14 centimetres
at the hip.
The Skartopus track makers were even smaller
with hip heights between 22 and 13 centimetres.
This variation in track size, especially between
Wintonopus tracks, could suggest that dinosaurs
of different ages lived in the same herds!
So, overall, it seems from the research that,
while a large dinosaur - maybe a theropod,
maybe not - did stalk the riverbank of Lark
Quarry, it may not have been there at the
time of the apparent stampede.
And although we may never know exactly what
happened to create these incredible trackways,
we can still learn a lot about the lives of
the animals that made them and the ecosystem
they lived in.
Ichnofossils can be a gateway into individual
moments in the distant past, a snapshot of
the Cretaceous - even if that picture
is a little blurred by the passage of time.
The site at Lark Quarry is such an exceptional
and intriguing mystery that it has been preserved
as Dinosaur Stampede National Monument.
And it attracts thousands of visitors every
year seeking to step back in time and walk
in the footsteps of dinosaurs.
Aren’t trace fossils the best!?!
If you want to learn more about who made that
crazy spiral shaped burrow we mentioned earlier,
check out our episode “Untangling the Devil's
Corkscrew”.
Also thanks to this month’s Eontologists
for making muddy footprints with us: Lucan
Curtis-Mahoney, Sean Dennis, Jake Hart, Jon
Davison Ng, Patrick Seifert, and Steve!
All patrons have access to the Eonites Only
Podcast where I interview people in STEM and
ask them questions provided by our patrons.
Join the fun at patreon.com/eons.
And as always thank you for joining me in
the Konstantin Haase studio.
Subscribe at youtube.com/eons to explore more
stories of the ancient past!
