(host) 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 mega raptor known as Australovenator perched at the top of the food chain. And around 95 million years ago, on one of those Australian river banks, a group of small dinosaurs were going about their business when they were suddenly interrupted by the arrival of a big theropod heading straight towards 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, the 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. The site now called Lark Quarry was and still is kind of a puzzle for paleontologists because there are no fossil dinosaur 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.

(Describer) Title: Eons.

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 fossilized 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 3,000 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 they are thought to have been made by small ornithopod dinosaurs. They're 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 the 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 well 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 trackway so hard and so interesting. So this group of researchers used a method called photogrammetry to construct a detailed 3-D picture of the tracks so they could be reanalyzed. With this new analysis, it seems that the tracks of the supposed "carnosaur" weren't 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've 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 riverbed by the current. Some of these drag marks crossed over the large tracks and then have smaller tracks on top of them. This suggests that the largest track maker, whatever it was, was not on the riverbank at the same time as the smaller track makers. 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 the large theropod could easily have been 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 track makers. 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 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 the 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, Wintopus and Skartopus were moving at 16 kilometers per hour and 12 kilometers per hour 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 in a certain movie scene has even led some to call this discovery the real scientific underpinning to the famous, "They're, uh, they're flocking this way," scene in "Jurassic Park." Now we can also use tracks to figure out how many dinosaurs were there. The 3,300 tracks preserved represent at least 130 different dinosaurs, all but two of whom are large mystery dinosaur and another ornithopod who came and went before the action were Wintonopus or Skartopus track makers. 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 track makers ranged in size from 158 centimeters all the way down to a tiny 14 centimeters at the hip. The Skartopus track makers were even smaller, with hip heights between 22 and 13 centimeters. 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 same 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 to 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. Accessibility provided by the U.S. Department of Education.

(Describer) Titles: Kallie Moore: Content Consultant, Host PBS Accessibility provided by the US Department of Education.

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