In 2006, a PhD student named Sterling Nesbitt was going through the fossil collections of the American Museum of Natural History, in New York, in search of specimens for his research.

He was studying some of the earliest dinosaurs, ones that lived in the Triassic period, which lasted from about 252 million to 201 million years ago.

Specifically, he was hunting for fossils of Coelophysis, a small meat-eating dinosaur that's well-known from the southwestern United States.

But what he found was much more than he bargained for.

The fossils that Nesbitt was studying had been collected in the 1940s at the Ghost Ranch Quarry in New Mexico, a site that was filled with incredibly well-preserved specimens of Coelophysis.

And after digging up a dozen or so of their skeletons, the field crew just started wrapping up everything that they excavated and sent it off to the American Museum, where it was stored away as bundles of Coelophysis.

Now, imagine Nesbitt's surprise when he looked through one of the field jackets nearly 60 years later and found fossils that were definitely not of Coelophysis.

These bones were from an animal that was around the same size as Coelophysis, and it had long hind legs, like that dinosaur did, but this creature also had a beak.

It would turn out to be just one of many clues that this wasn't the dinosaur he was looking for.

In fact, what he had discovered wasn't a dinosaur at all.

Instead, it was a crocodilian, but it looked a lot like a dinosaur.

This specimen would turn out to be just one of a huge number of croc-like animals that flourished during the Triassic Period.

Back then, dinosaurs had just arrived on the scene, but it was these animals that truly ruled the Earth, becoming both abundant and diverse.

But in the end, their dinosaur-like adaptations wouldn't be enough to save them from extinction.

So how did Nesbitt know that what he was looking at wasn't Coelophysis?

Well, the first giveaway was the animal's hips.

Dinosaurs are unique in having an open hip socket, where the head of the femur is surrounded with bone, and their hip sockets point straight outward.

So the head of each femur locks into that open socket from the side, while the shaft of the femur points downward.

This allowed dinosaurs to stand upright with their legs directly under their bodies, rather than sprawled out to the sides like most reptiles.

But the animal that Nesbitt found in that field jacket had a closed hip socket, and it was angled downward over the head of the femur.

This meant that the hip formed a distinctive shelf of bone that stuck out over the head of the femur, putting the legs underneath, and creating a sturdy pillar-like structure.

So this animal would have walked with an upright posture, like a dinosaur, but with different hip anatomy.

The next clue for Nesbitt was the animal's ankles, and fun fact--dinosaur ankles are a simple hinge joint which keeps the limbs more vertical and restricts the ankles' range of motion.

This creature, however, had what's called a crurotarsal ankle.

It's a more complex joint that has much greater flexibility than what dinosaurs had.

So between the unique structure of the hips and the distinctive ankle joint, Nesbitt was able to recognize this dinosaur for what it really was, an ancient crocodile relative that he named Effigia Okeeffeae-- Effigia, which means ghost, for the Ghost Ranch site where it was found, and Okeeffeae for Georgia O'Keeffe, the famous artist who once lived on that property.

And this creature belonged to a group of ancient croc relatives known as the Rauisuchians, which dominated the land in the Triassic Period.

Dinosaurs were around back then, but they weren't all that common or diverse yet, and the ones that were around were far from the giants that would later evolve in the Jurassic and Cretaceous Periods.

Rauisuchians probably first appeared in the fossil record in the early Triassic, around 250 million years ago.

And Effigia is just one of many Rauisuchians that lived alongside some of the earliest dinosaurs in the late Triassic.

Their fossils have actually been found in almost every continent, except for Antarctica and Australia.

And while Rauisuchians aren't dinosaurs, both are members of a broader group called archosaurs, which includes the common ancestor of birds and crocodilians and all of its descendants.

Now, we don't really understand the evolutionary history of the Rauisuchians very well yet.

So over the years, they've come to be used as an umbrella group for many Triassic archosaurs that are more closely related to crocs than to birds.

But the special hip anatomy that Nesbitt identified in Effigia is found in all Rauisuchians, and it's what gives them their upright dinosaur-like posture.

And Effigia's complex ankle is another feature that all Rauisuchians share, as do all croc relatives.

Even modern-day crocodilians have this unique ankle.

It's what allows them to both sprawl on the ground and to rise up into their signature high walk.

OK, but so if Effigia and other Rauisuchians are croc relatives, why did they look so much like dinosaurs?

The answer, my friends, is convergent evolution-- when similar features evolve in different species, totally independently of one another, as a result of adapting to similar ways of life.

So dinosaurs and Rauisuchians both evolved similar features separately from each other, because they had similar lifestyles.

They lived in the same kinds of habitats and ate similar foods.

So they ended up with many of the same kinds of adaptations.

For example, some species of Rauisuchians, including Effigia, look a lot like members of a group of theropod dinosaurs called ornithomimids.

Both animals walked on their long powerful hind legs and had really eerily large eye sockets.

They also had toothless beaks.

These kinds of shared features tell paleontologists that both groups could likely run really fast, and that they probably had a similar, mostly plant-based diet.

But many other species of Rauisuchians had more in common with carnivorous theropod dinosaurs, like velociraptor and deinonychus.

Poposaurus, for example, is a Rauisuchian that lived between about 201 million to 237 million years ago in western North America.

It was about five meters long and could weigh up to 100 kilograms, making it one of the largest terrestrial predators of the late Triassic, hunting small mammals and even dinosaurs.

Given its bipedal posture and blade-like teeth, it's no surprise that it was originally mistaken for a theropod dinosaur.

So as the Triassic went on, things seemed to be going pretty well for the Rauisuchians.

They were diverse, widespread, and they occupied all sorts of niches, from herbivores to apex predators.

But all good things must come to an end, and for the Rauisuchians, that end came around 200 million years ago.

And that's mostly because, at the end of the Triassic, the supercontinent of Pangaea was starting to break apart.

And this massive drifting of the continents triggered widespread volcanic activity that spewed incredible amounts of carbon dioxide into the atmosphere, leading to extreme global warming.

It was accompanied by big fluctuations in temperature and sea level that, unfortunately, many species couldn't handle.

By the end of the Triassic, roughly 76% of all terrestrial and marine species had gone extinct, including the Rauisuchians, and this left the door wide open for dinosaurs to take over.

Here's a question.

If all the Rauisuchians were so much like dinosaurs, why did they all bite the dust, while the dinosaurs survived?

The short answer is we're not sure.

A lot of things could have made dinosaurs better adapted to survive this changing environment.

It could be that dinosaurs could reproduce faster, which helped them maintain more stable populations, or maybe dinosaurs were better at regulating their body temperature, which would have allowed them to survive those huge temperature fluctuations.

And some paleontologists have proposed that dinosaurs might have had higher metabolisms, which meant that they could have been more active in getting food and would have promoted faster rates of growth.

One study even showed that early dinosaurs had really high levels of variation within individual species compared to crocodilians, and more variation means more opportunities for natural selection.

So maybe this high degree of variation helped dinosaurs develop the ideal combinations of traits that allowed them to survive in a constantly changing environment.

And these are just some of the many possibilities that paleontologists are still studying to get to the bottom of this 200-million-year-old mystery.

Our friend Effigia is just one of the Rauisuchians that lived right at the end of the Triassic, but by the Jurassic, it had vanished.

Understanding what allowed animals to survive through environmental changes in the past, like those at the end of the Triassic, can help us understand how changes going on today will affect modern animals in the future.

And who knows how many other fossils like Effigia are hiding in collections around the world, just waiting for their true identities to be discovered?

So as it turns out, if it looks like a dinosaur and walks like a dinosaur, it might just be a croc.

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