Bone Diggers

Bone Diggers

PBS air date: June 19, 2007

NARRATOR: An amateur explorer, cruising the skies of the Australian outback, makes a chance discovery, a prehistoric cave that's never been explored. Inside, something no one's ever seen before: a mysterious fossil.

ULTRALIGHT GUY (Pilot): There was one particular skeleton, and I said, "Well, it doesn't look like an old cow to me. I wonder what it is?"

NARRATOR: The strange bones are the start of an adventure. A team of researchers launch an expedition to hunt for a long lost killer, as fearsome as any top predator today.

JOHN LONG (Western Australian Museum): This is, perhaps, one of the most important Western Australian Museum field trips I'm ever going to do.

NARRATOR: On a continent full of unique animals, they're on the trail of a powerful, prehistoric lion. For years, scientists have had only fragments of the lost predator to study, and its mysteries have remained unsolved.

ROD WELLS (Flinders University): It's an animal that's full of paradoxes. There's nothing quite like it that exists anywhere else in the world. I think it would be quite a fearful animal to encounter.

NARRATOR: Now, struggling in difficult conditions, the scientists have only a short time for a breakthrough.

JOHN LONG: Time to go, guys.

NARRATOR: It's a daring search, deep below the ground...

JOHN LONG: Each time they do it, they've got to treat it like it's their life in their hands.

NARRATOR: ...and a race against the threat of fossil poachers, who could steal the precious bones.

JOHN LONG: Yeah, I wouldn't come any closer, actually. In fact, a whole skeleton could be right here.

NARRATOR: The team will work for three intense weeks to rescue the remnants of a unique ancient beast.

JOHN LONG: You know, this is something that us paleontologists dream of every day.

GAVIN PRIDEAUX (Western Australian Museum): This is, perhaps, one of the most important discoveries ever made in Australia.

NARRATOR: Right now, on NOVA, Bone Diggers.

Major funding for NOVA is provided by David H. Koch and by...

Discover new knowledge: HHMI.

And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

NARRATOR: The Australian outback: a vast, trackless wilderness filled with extraordinary creatures, including more than 200 different kinds of marsupials, which carry their babies in a pouch.

The outback is also home to one of the most ancient cultures on Earth, the aboriginal people who came to Australia about 50,000 years ago.

Their legends conjure up huge bizarre beasts, created in an ancient era called "the dreamtime" —long before the advent of humans—when heroic spirits summoned the landscape into being. The stories speak of wombats the size of buffalo, killer kangaroos, over eight feet tall. Could these aboriginal myths be a distant echo of something real?

A million or more years ago, an array of giant animals flourished here, distantly related to those on the outback today, but all of them died out toward the end of the last ice age.

In Perth, a team of Australian paleontologists are about to embark on a daring hunt for one of the lost prehistoric beasts. John Long is heading the expedition, a journey that started almost completely by chance.

JOHN LONG: I was sitting in my office, looking at the computer, and an email came through. And it was an email from someone I'd never met before. And then I saw there was an attachment. And I clicked on it, and as the attachment slowly opened, my jaw dropped.

NARRATOR: The attachment was a mysterious photo. It was taken in a newly discovered cave. It may be the find of a lifetime: the skeleton of an animal that's puzzled Australian scientists for over a hundred years.

JOHN LONG: I was totally elated and very excited. My first thoughts were that I had to mount an expedition, and we had to go and recover this remarkable fossil.

NARRATOR: Following directions that came with the email, Long and his colleagues set off on three full days of rugged driving, deep into the Australian outback.

JOHN LONG: This is, perhaps, one of the most important Western Australian Museum field trips I've ever been on, and perhaps one of the most dangerous missions I am ever going to do.

NARRATOR: They're heading to the Nullarbor Plain, more than 700 miles of almost uninhabitable land.

JOHN LONG: The Nullarbor is one of the great, vast, flat areas of Australia. It's a huge area covering several thousand square kilometers, very easy to get lost in because there just aren't any landmarks.

NARRATOR: Although it appears empty, the Nullarbor is home to many animals today, distant relatives of those enormous creatures that lived a million years ago.

JOHN LONG: There were giant goannas, the order of six meters in length. There were huge, gargantuan birds. The largest that ever walked on the planet came from Australia at this time, the dromornithids, or thunderbirds. We had a whole range of large animals, a bunch of giant kangaroos, and huge wombats the size of small cars. And at that time, it was kill or be killed. And it was pretty well cutthroat.

NARRATOR: Of all the prehistoric animals, one of the most mysterious is a marsupial lion called Thylacoleo carnifex. Very little is known about Thylacoleo. Fossil fragments have been found scattered across Australia, but never a complete skeleton. And that is what John Long thinks is pictured in the email.

At the end of a long drive, they finally make camp. For three weeks they'll live in harsh and isolated conditions.

JOHN LONG: We were about almost 2,000 kilometers from Perth and almost 900 kilometers to the nearest town. You have to bring in all your food, all your fuel, all your water, all your expertise, and then just do your work.

NARRATOR: In the middle of this desert, they'll be at the mercy of the elements. Temperatures in the daytime soar over a hundred degrees and then drop to freezing at night. And when the wind kicks up, it can literally blow the camp away. But this parched landscape was formed in a totally different environment than it is today.

JOHN LONG: Right here, where I'm standing, 20 million years ago was the bottom of the sea, limestone rock was formed. Today, the Nullarbor Plain is vast and featureless, but below ground is a different story. When ground waters rose up, percolating through that limestone rock, dissolving away calcium carbonate, the result was the formation of caves. Today there are thousands of caves throughout the Nullarbor region.

NARRATOR: Only a few of those caves are known to contain fossils. Concealing their location is critical, because there's a thriving international black market in ancient animal bones.

JOHN LONG: ...very important, because of the security of this cave, that we don't, we do not all walk in the same line.

NARRATOR: They spread out and never take the same route to the cave twice. Leaving tracks in the scrub could give away the location of the site to poachers, and priceless material could be lost to science.

It's going to be tricky just getting inside the cave. The only way down is on a harness and ropes. The scientists carefully build a scaffold at the mouth of the cave so they can lower themselves into the darkness. This will be the only safe way out if anything goes wrong.

JOHN LONG: It's about an 18-meter drop to the floor, and then the cabins go on, you know, in all directions, but for quite a way.

NARRATOR: For many members of the team, this kind of exploring is a completely new experience. They spend most of the first day at the campsite, learning to use their gear. Any mistake could be fatal.

JOHN LONG: Basically, each time they do it, they've got to treat it like it's their life in their hands.

Use your legs and keep straight.

NARRATOR: No matter how experienced the scientists are, the winds at the cave entrance can be treacherous.

GAVIN PRIDEAUX: While you're on the rope or the ladder, you're often getting buffeted around, so you've got to be very careful at all times.

JOHN LONG: Use your legs.

NARRATOR: The next three weeks will be a test of endurance and patience. But it's also the chance for a unique scientific discovery.

JOHN LONG: You know this is something that us paleontologists dream of every day: a wonderful new discovery that we can then go out and collect and work on.

NARRATOR: Tomorrow the real adventure will begin.

JOHN LONG: Time to go, guys.

NARRATOR: The moment has come to descend into the dark depths of the cave.

JOHN LONG: Okay, just give Eve a "hi," and let her know you're coming down.

CAVER: Okay, Eve, I'm coming down.

NARRATOR: It's a 50-foot straight drop from the surface.

JOHN LONG: It's total, pitch black darkness, mostly totally silent. You do get a bit of wind and a bit of air diffusing through the caverns and the rocks at times, but mostly it's pretty isolated, probably about as isolated and lonely as you can get, anywhere in the world.

We dropped down, and we had to go right to the bottom of another large cavern. And then we had to crawl on our hands and knees through another narrow passageway.

This is the first time I've ever had to work in really close spaces in a cave, and it was a bit claustrophobic for me, I had to sort of adapt to it.

NARRATOR: Carefully, the team begins searching the enormous cave.

GAVIN PRIDEAUX: Fossils are not always draped on a nice flat surface. One has to often crawl down between the spaces between the rocks and come in from underneath to get the remainder of the bones. And sometimes the rocks are fairly precarious, so there is an element of risk.

JOHN LONG: It's a good one. Give me your hands, both hands.

NARRATOR: Piece by precious piece, they start to uncover ancient bones. Some are so fragile that just a touch could destroy them.

JOHN LONG: See, this is a major job, to get someone in here, just with a light brush, to brush away the gypsum, consolidate the bone.

GAVIN PRIDEAUX: Yeah, it's a beautifully preserved bone too. Now, of course, if we could find some teeth or a bit of jaw or a bit of skull, that will enable us to put a definite identification on this.

JOHN LONG: Yeah, I wouldn't come any closer, actually. I think there's bone all through this area. In fact, a whole skeleton could be right here.

NARRATOR: The floor of the cave is littered with fossils.

JOHN LONG: Here we have the remains of giant kangaroos, three meters high, huge wombats the size of ponies, and, in addition to that, we've got weird species: kangaroos, wallabies, species that are quite rare and haven't been found in many other places.

NARRATOR: It looks like the cave was a prehistoric deathtrap. They find many of the bones directly under the small opening on the surface.

JOHN LONG: I can imagine that a giant wombat or a kangaroo just running along, didn't see the hole, fell straight down. Who knows? All we know is that, in terms of geological time, you only have to have one animal fall down a hole every hundred years, and over the course of several thousand years, it mounts up to a lot of skeletons.

NARRATOR: They break for the day, but there's a wealth of fossils waiting to be recovered. And they know that somewhere under this ancient soil is the big prize: the skeleton John Long saw in the email, before they left Perth.

They only have three weeks, so time is of the essence. They have to be as efficient as possible.

JOHN LONG: Yeah, I'll get the other guys—Marc and Jeff and maybe Benny—into that new chamber.

GAVIN PRIDEAUX: Move on to the next one.

JOHN LONG: Leave no corner of the cave unsearched...bound to be more treasures hidden there, somewhere.

NARRATOR: As they continue their hunt, they find of hundreds of bones. Many are familiar, belonging to species found at other sites. But then comes a stunning realization. This cave holds hidden treasure, unique new skeletons of extinct species, previously unknown to science.

JOHN LONG: This is the most weirdest kangaroo I've ever seen in my life, great big whacking great horns sticking out over its eyes: bloody devil wallaby.

GAVIN PRIDEAUX: It's certainly a new species. And I think there's an excellent chance that it's probably a new genus and species, so it's a major find...and a complete skeleton, too.

NARRATOR: Over the course of several days, they realize there's an entire community of ancient animals in the cave. They find eight new species of prehistoric kangaroo, and other lost animals that are many times bigger than their relatives today. It's an unprecedented discovery.

GAVIN PRIDEAUX: It's one of the most important finds ever made in the history of Australian paleontology. There's never been a cache of fossils of this age—complete skeletons scattered around the cave floor—so it's given us an excellent window into the fauna of a massive part of Australia, about which we previously knew nothing.

NARRATOR: But just as they had feared, poachers have been here. In one part of the cave, there's evidence that poachers arrived before they did and raided some of the fossils.

JOHN LONG: It hasn't taken long for the cave to start to be trashed. You've got tire marks going in there and then tracks all the way along, then you go to this cave and there's been people in there. Some of the specimens there were disarticulated. But not only that, we don't have the complete picture. When we start logging and mapping each bone by bone, some of the puzzle's missing. So it's just not good.

NARRATOR: Could the poachers have already stolen the skeleton from the email? Hours of searching pass, and then one member of the team makes a promising discovery.

EVE TAYLOR (Archeologist): I was just coming down the rock pile here, and I saw, down the slots, the larger bones of the legs. Then I came around to get a better look, and there I saw some bones.

JOHN LONG: Oh yeah, look at that. There are some big bones there that look like the leg limb bones, and also vertebrae, and a bit of hip and possibly the other lower jaw.

NARRATOR: They immediately recognize that these bones are fragments of a giant extinct lion—a tantalizing clue, but not a complete skeleton.

Then, the moment they've been hoping for. They find the bones from the email, the first complete head-to-toe skeleton of Thylacoleo carnifex ever discovered.

JOHN LONG: I will never forget that moment, as long as I live, when I first laid eyes on the Thylacoleo skeleton. There it was, in reality, not just an image on a computer screen. I was totally gobsmacked, just entirely exhilarated from the inside out. It was a complete skeleton of Australia's largest marsupial killer.

NARRATOR: Every bone is still in place. The skeleton has been resting here in the darkness for thousands of years.

GAVIN PRIDEAUX: It was really quite poignant, because it was possible to tell just from, sort of, looking at the way the skeleton was laid out that that animal had taken its last breath exactly where it was lying. And you can just imagine it wandering along in the dark, dying of thirst or perhaps soft tissue injuries, collapsing, exhausted, and dying. It was quite remarkable.

NARRATOR: It's been a breakthrough day for the team. The find is a landmark in Australian paleontology. There's still a lot of work to do, and not a lot of time. They have an intense two weeks ahead of them as they face the challenge of bringing the precious skeleton to the surface.

Deep in the cave again, they begin to examine the bones more closely and there's more good news. The skeleton is untouched, lying exactly where the animal perished.

JOHN LONG: It's a pristine find. It's never been tampered with, and it gives us scientists the opportunity to go in, and not only to study the bones before we touch them, but to take samples for dating or ancient DNA. And we know there's no contamination down there, and that's very important for the science.

NARRATOR: They take samples for dating, because they want to know exactly how old the skeleton is.

JOHN LONG: It looked so well-preserved, I was thinking it might only be a few thousand years. It might be one of the youngest skeletons of megafauna yet discovered.

NARRATOR: They'll also try to recover ancient DNA from the bones, to look for clues about how Thylacoleo relates to modern species.

JOHN LONG: In modern creatures, it's easy to take a tissue sample and extract the DNA out of it. But when we look at fossils, it's often difficult to get DNA, unless you get very good preservation. Unfortunately, we didn't get any DNA out of those bones. But then the reason why became obvious when the dating results came in. The skeleton was far older than we'd ever imagined. It wasn't a few thousand years; it wasn't a few tens of thousands of years; it was probably half a million to a million years old.

NARRATOR: The skeleton's age and excellent condition make it even more valuable than the researchers had hoped, providing intimate details of the extinct lion's anatomy and behavior.

JOHN LONG: Okay, what I have here is the first-ever assembled tail of Thylacoleo, the marsupial lion. This is the only specimen known that had the tail complete. What you see is they had a long and muscular and powerful tail, the sort of tail befitting an animal that was an active predator.

NARRATOR: Before they can get the bones back to the lab, they need to prepare them carefully. One false move and the precious Thylacoleo skeleton could disintegrate before their eyes.

JOHN LONG: The skeleton was so fragile. Now, had someone actually tried to pick up the skull at that stage, it probably would have just fallen apart in their hands or fallen to dust, because it was very, very delicate.

NARRATOR: They spend days brushing on a special glue to harden the bones. It's a tedious job, but they have to do it before they can even pick up any of the bones.

JOHN LONG: After four days of painstaking work preparing the skull, today, we're finally able to pick it up and look at it in its entirety. And what we see is the powerful dentition: the big stabbing incisor at the front, this big shearing premolar. And when we look at the lower jaw, we see a similar arrangement: a powerful, stabbing incisor at the front and a shearing premolar and molar, which act like a pair of secateurs to cut through the flesh of giant three-meter kangaroos or big diprotodontine marsupials.

NARRATOR: The skull indicates that Thylacoleo's bite was incredibly powerful, equivalent to that of a modern lion, more than twice its size.

As the team continues to explore deeper into the cave, they find more specimens of the fearsome beast.

JOHN LONG: This is the eleventh Thylacoleo we've found in these caves, so far. And every specimen gives us new information that other material doesn't show. This particular animal came down in the cave and probably got wedged right here between the rocks. The head of the animal was here, and the front part of the arms fell down here.

NARRATOR: The unprecedented haul of bones poses an unexpected problem for the team. They can't possibly take all the fossils back to Perth.

GAVIN PRIDEAUX: Even if we took these jaws and left the rest....

NARRATOR: But if they leave anything behind, it may not be here when they come back.

JOHN LONG: There is this whole question of cave security and whether other people will get in and pinch bones and walk all over it. I'd still feel better if we at least got some of the better specimens, and we mark exactly where they came from.

GAVIN PRIDEAUX: Yeah, it's a big job. But it's a question of do you want to get the information that's available here out, or do we just want to scoop the goodies off the top and leave it at that?

JOHN LONG: These fossils haven't even been studied or described, and there's whole new species here, so the loss of knowledge, the loss of natural heritage is the main concern to me.

NARRATOR: They decide to take the eight new species of kangaroo, and several Thylacoleo specimens, including the rare intact skeleton. John Long personally tags and wraps each fragile bone for the journey back to the museum.

JOHN LONG: We numbered every single bone in the skeleton. We took loads of photographs documenting the relationship of every bone. And we very carefully hauled it up in drums on a rope to the top of the cave.

NARRATOR: But there's still one final hurdle. The difference in air pressure above and below the ground makes the wind howl through this narrow opening. One wrong move and their precious cargo could be smashed to bits against the limestone rock. Fortunately, their treasure makes the trip safely.

It's been a long, hard three weeks. But the journey of discovery will take a surprising turn as they return home to the museum to start a new exploration of Thylacoleo.

With the priceless skeleton back in Perth, John Long's colleagues from the Western Australian Museum join the investigation. In the lab, they'll be able to conduct a meticulous study of Thylacoleo, from head to toe.

JOHN LONG: Finding a complete skeleton of a marsupial lion is far more important than having hundreds and hundreds of isolated bits and pieces from different localities, because, for a start, we can build up an accurate picture of what the animal was like, and its real proportions. And we can also start to look at its functional morphology—how that animal operated as a creature. You know, how did its feeding mechanism work? How did its hands and feet work? How much movement was there in its shoulder and its backbone? And from these studies we can then start to build up a realistic idea of Thylacoleo as a living animal.

NARRATOR: Nineteenth century naturalists were the first to identify and describe Thylacoleo bones, and even with only fragments to go on, they realized how unusual a creature it was.

ROD WELLS: Sir Richard Owen described this animal back in 1859—the time Charles Darwin had just written his book on the evolution—as "the fellest of predatory beasts," and he called it Thylacoleo carnifex. Thylacis for pouch, leo for lion, carnae for meat and fex for cutting. So this was the "meat-cutting marsupial lion."

NARRATOR: But the name raised more questions than it answered. Marsupials share the characteristic feature of a pouch the females use to nurture their young. The specific bone structure needed to support a pouch in marsupials has been found in Thylacoleo skeletons.

Some marsupials live in other parts of the world, but Australia's geographic isolation contributed to the evolution of many unique species, like wombats, koalas and kangaroos. So the perplexing question is how did Thylacoleo, the extinct "meat-cutting lion," come to share features of both marsupials and big cats?

ROD WELLS: This is one of the most fascinating mammals, I think, that's ever been discovered. It's an animal that's full of paradoxes. There's nothing quite like it that exists anywhere else in the world.

NARRATOR: Rod Wells has been studying Thylacoleo most of his career. The best fossils he's had up 'til now are bits and pieces of skulls and teeth. So he starts his investigation of the new, complete skeleton with the parts he knows best, and right away he finds surprises.

ROD WELLS: This huge, single cutting tooth here, or premolar tooth, and this large premolar tooth, cutting premolar tooth, up here, are unprecedented in the mammalian world.

If we go to the front of the jaw, here, we have these two front teeth that extend here—a condition that is known as diprotodonte, Di for two, proto: front teeth, "two front teeth." And this is the condition that we find in all of the marsupials that are herbivorous. Things like possums and wombats and kangaroos and koalas all have this diprotodont condition. So here we have an animal that seems to be telling us, with one set of teeth, that it comes from a herbivore ancestry, and with another set of teeth, it's telling us it's a carnivore.

NARRATOR: Most of Thylacoleo's marsupial relatives today are plant eaters, but it seems clear that it was a hunter, like other lions.

To figure out this contradiction, the scientists have to look beyond its teeth.

MARK WALTERS (Research Scientist): I'd like a high resolution scan, one millimeter slices.

NARRATOR: Since this skull is so well preserved, it promises to deliver clues that have eluded previous researchers.

Mark Walters is a doctor who uses forensic techniques to rebuild shattered human faces and skulls. Today he's using a CT scanner for a different kind of detective work. For the first time, he will peer deep inside Thylacoleo's skull, and try to reconstruct the creature's ancient brain.

MARK WALTERS: CT data provides us with a stack of images, and we can bring those into the computer, and then we can create geometrical files. And from those geometrical files we can do a number of different types of manipulations, including taking a cast of the internal surface of the bones.

NARRATOR: The CT images are transformed into a plastic replica of the skull. Then Walters takes a set of precise measurements of the interior cavity. The final result is a remarkable three-dimensional cast of Thylacoleo's brain.

MARK WALTERS: There's a lot of information that can be derived from such a cast. We can see, quite clearly, the lumps and bumps on the bone. And they correspond to different parts of the brain. Well, the very first thing and obvious structure is that we see these very large olfactory lobes. So this animal is going to be able to detect its specific smells over very long distances. Also we can see the parts of the brain associated with sight. And we can also see the big nerves that go to the eyes. And these nerves are quite large, so you can see this animal also required a lot of good vision. So, what we can quickly see by this cast of the brain is that this animal had a very powerful sense of smell. It also had a good sense of hearing, and a very good sense of sight. So it was using all of its senses in its day to day activities.

NARRATOR: Many of today's marsupials are night creatures with keen senses of smell, sight and hearing. These are traits also shared by big cats.

With its powerful bite, Thylacoleo must have been as fearsome a predator as a lion. But the question of how it hunted is complicated by the strange configuration of its teeth.

ROD WELLS: Now, the problem with that is, of course, is how it actually catches its prey, because, in effect, these diprotodont teeth, when they mesh with the other upper incisors here, form something analogous to a parrot's beak. This animal could puncture a hole in the prey quite easily, but it would never be able to hang on to its prey.

NARRATOR: Somewhere in the new bones could be a clue to solve this problem. If Thylacoleo stabbed its victims with its large front teeth, how did it hold on to them while it finished them off?

The researchers find an answer in a part of its body they've never before been able to study in detail.

ROD WELLS: And here you will see the very large slashing claw on the thumb. So here was a mechanism whereby this animal could capture and hold its prey. So once Thylacoleo has grasped its prey, it's then in a position to deal the deathblow. And it can deal the deathblow with these sharply pointed lower incisor teeth. And of course, this would be an ideal way of doing things like severing the spinal cord, or suffocating the prey by grasping around the trachea.

NARRATOR: The evidence suggests that this ancient marsupial lion is fundamentally different from other marsupials, which lack such powerful teeth, and from other lions, which don't have its slashing claw. But without obvious parallels to these familiar predators today, its hunting habits remain a mystery.

Did Thylacoleo chase its prey? Did it hunt in packs? Or was it a stalker, laying in wait for the best moment to strike?

The scientists turn again to the new skeleton for answers. Each bone is carefully scanned so that 3D images can be manipulated on a computer screen.

MARK WALTERS: Oh, this is terribly exciting, because this animal doesn't exist. There's no other animal that exists like it. Through using this type of technology, we can reconstruct the anatomy of these animals to a detail that we've never seen before.

NARRATOR: These images help decipher how the bones were connected to each other and where muscles would have been attached. This gives the scientists a new understanding of Thylacoleo's legs and an important detail about the hunting techniques of the ancient predator.

ROD WELLS: There is nothing in the back part of the skeleton to suggest this animal was, in any way, a fast-moving animal. The hind legs are low-geared. They're powerful. And, indeed, if we look at the position where the muscle—the muscle scar—where the muscle comes off the thigh bone for rotating the thigh—about its fulcrum point here, it's attached way down here. So it's a very, very low-geared and powerful hind limb.

NARRATOR: Legs that are powerful but low to the ground mean Thylacoleo couldn't run fast for any significant distance. Chasing its prey would be out of the question; it would never have enough speed.

ROD WELLS: So I think this animal really is one that carefully stalks its prey. The speed comes with the speed with which it can slash with that forearm.

NARRATOR: As all of the new information about Thylacoleo's anatomy is falling into place, a scientific illustrator begins reconstructing its size and shape. The only things left to artistic license are the color and markings of its coat.

With the finished drawings as a guide, a scaled down model of Thylacoleo slowly takes form. Lasers scan the completed 3D model to produce digital images. And finally, the lost prehistoric predator comes back to life.

It was a fierce and stunning beast with a crushing bite and powerful legs, but it was low to the ground and heavy, not built for speed. Its feet had claws like a lion, but it walked flat like a bear. Since it was slow-moving, and probably able use its claws to climb trees, it may have ambushed its prey like some big cats today, laying in wait from above and then springing down on an unlucky victim.

And there's one final dramatic clue that seals its image as a deadly predator. The clue comes from the tail they discovered, intact, in the cave.

ROD WELLS: One of the interesting features of the tail is that...these little tiny bones here—these little v-shaped bones that are called chevrons, like on, like a roof or a chevron—and what they actually do is they protect the blood vessels that run underneath the tail at the point where the tail flexes down to the ground.

This is the sort of thing that you see in kangaroos. And kangaroos are capable of propping themselves up on the tail, tripoding, if you like, with the tail and the two hind legs. If this animal is actually catching its prey with its front legs and its slashing claw, being able to tripod back on the tail and the hind feet frees up the arm for slashing or grasping.

JOHN LONG: I have absolutely no doubt that an animal the size and bulk of Thylacoleo could have pulled down much larger animals, bigger than itself, like a giant wombat—phascolonus, the size of a small car—or giant herbivorous kangaroos. That is the sort of creature that would have been fodder for Thylacoleo.

ROD WELLS: My perceptions at this moment are of an opportunist carnivore scavenger that hunted at night, probably operated in woody shrub lands. I wouldn't want to have been a leaf-eating kangaroo with one of these guys around.

I would imagine that he operated on his own...probably sneak within very close range of the prey, and then leap at it...now either leap directly at it and grasp it with those big claws, maybe slash at it with the big claws, maybe leap onto its back and sever the spinal cord with the incisors, and then, just sit down and sheer off great lumps of flesh. I would imagine, if there were any dead animals around—this guy has got a very good sniffer—it would probably pick up the smell of carrion very easily. I think it would be quite a fearful animal to encounter.

NARRATOR: Thylacoleo ruled the Australian outback for more than a million years. Of the many large and formidable prehistoric animals, it was the deadliest, the king of beasts.

But toward the end of the Ice Age, something toppled Thylacoleo from its throne and drove it into extinction. How did this powerful killer, that could attack animals many times its size, become a victim?

CHRISTINE JANIS (Brown University): People always think that big predators are like the top of the heap, the king of beasts, the king of the jungle. And in fact, those big predators, particularly ones that sort of highly specialize in just eating meat, they're very vulnerable.

NARRATOR: Scientists are sharply divided about what caused Thylacoleo and all the other giant animals in Australia to die out. One view is that the final climax of cold conditions toward the end of the Ice Age, 30,000 years ago, pushed the great creatures into oblivion.

JOHN LONG: If you go into a maximum ice age and out of it again, in the period of 10,000 years, then you could certainly have major changes that put pressure on the environment and the animals that live in it.

NARRATOR: But a rival theory places the extinction much earlier, around 50,000 years ago. At that time, climate changes were probably less severe. And the first humans to arrive in Australia, the ancestors of today's aboriginal people, were entering the continent.

GIFFORD MILLER (University of Colorado): If we look at what was happening in Australia, 50,000 years ago or so, we can see a relatively stable climate, not that much different than today. And we also see that that's when the first humans colonized Australia. So it raises the question: could it be a human impact that's the main driver for these big extinctions?

NARRATOR: There's no way to prove that humans hunted prehistoric Australian animals aggressively enough to kill them all off, but other human activities could have helped eliminate the beasts.

GIFFORD MILLER: When humans come to Australia, we know they have fire on-demand. And so that's one of the tools people use in modifying landscapes to their benefit. And humans burn for a whole range of reasons. I mean, partly, they just burn to clear the land so that they can move across it. They hunt along the fire front, they signal distant bands, they promote the growth of plants that respond after burning. So there's many reasons why humans might be burning the landscape in a way that they thought was a positive impact.

NARRATOR: Whatever the reason, burning could have disrupted natural cycles of wildfire and re-growth, changing the vegetation and altering the habitat the giant animals depended on. Fires set by humans would narrow the range of plants to fire-resistant species. The landscape would turn to desert scrub land, and many of the browsing animals, like giant kangaroos, wouldn't have enough food to get by.

A predator like Thylacoleo, that couldn't suddenly switch its diet or hunting habits, would be highly vulnerable as its supply of prey dwindled.

GIFFORD MILLER: If the humans reduce the prey, that is, these other large browsers, on which the big carnivores are dependent—the carnivores are very sensitive to prey abundance—and Thylacoleo, which is not a running animal, it's an animal that depends on its prey walking by underneath it, so it can leap down on it where it's extremely efficient—but if it reduces the number of potential prey below a threshold, then Thylacoleos will starve to death.

CHRISTINE JANIS (Brown University): It's sort of like a snowball rolling uphill, if you like. The big predators are the ones who are going to feel any effects first, because they're the ones on top of the pyramid, and they're dependent on everything below them.

NARRATOR: But exactly how big a role humans played in the extinctions, alongside climatic factors, is still hotly debated, and not only in Australia, but on other continents, where a similar pattern of extinction unfolded as the Ice Age ended and humans arrived.

GIFFORD MILLER: This question is much larger than Australia, because one of the characteristics of the last 100,000 years is the disappearance of almost all the large animals almost everywhere on the planet. And the one piece of evidence that's consistently present in all extinction cases is that they occur about the same time that humans first appear on the landscape.

CHRISTINE JANIS: I think the lesson to be learned for the current day is that these big animals are very vulnerable. We think of them, because they're big and powerful and could kill us—either on purpose or by accident—as being these strong, kind of, survivors. But they're very fragile. And once they're gone, they're gone.

NARRATOR: Long after the magnificent skeleton takes its final resting place on proud display at the Western Australian Museum, research will continue on the unique role Thylacoleo played in the natural history of the outback. And the scientists who found it believe this is just the beginning of many more discoveries.

ROD WELLS: If you pause for a moment and think about Australia, what is Australia's great heritage? Australia's great heritage is its flora and its fauna, and the marsupial fauna is absolutely unique, and this is the biggest marsupial carnivore that ever lived on this continent.

JOHN LONG: I think the discovery, all out—not just the Thylacoleo but including everything we found in those caves—was a quantum leap in our knowledge of Australian megafauna of that age. Looking back on those halcyon days of the Thylacoleo expedition, it was one of the greatest times of my career as a paleontologist, certainly one of the most exciting times to be able to go out, launch an expedition go down those caves, and find these wonderful skeletons. It was just the most amazing thing. Every paleontologist dreams of something like that happening. There's still...a very big part of my heart is still with Thylacoleo and always will be.

On NOVA's Bone Diggers Web site, see a gallery of Australia's extinct giant animals, from the marsupial lion to a carnivorous kangaroo. Find it on PBS.org.

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