Dinosaurs Without Bones (9 page)

To make a long story short, the effects of this impact likely caused what ecologists call a
trophic cascade
, in which the demise of each plant or animal that played a key role in an ecosystem led to further extinctions. Based on the impact scenario, paleoecologists figured that dust caused by both the impact and widespread fires blocked out most sunlight for several years, creating a Mesozoic version of a “nuclear winter.” This effectively shut down photosynthesis in most land plants, thus depriving herbivores of their basic foodstuff. Nearly anything big that required lots of food to sustain its biomass, such as
Triceratops
or
Tyrannosaurus
, quickly went extinct. Yet smaller dinosaurs went with the bigger ones, too. For some reason, and we’re still not sure why, birds were the only dinosaurs to survive this global disaster.

Seemingly every few years, though, paleontologists who intensively search the boundary between Cretaceous and Paleogene rocks end up uncovering a few bits and pieces of dinosaurs from above the boundary, suggesting dinosaurs were still living 63 to 64
mya
. If these paleontologists publish a peer-reviewed article reporting such a find, the peer review does not end then and there. Post-Cretaceous dinosaur bones are always contentious, and in nearly every instance are explained as reworked older material that was deposited again, into a younger formation. In short, there’s just no way to avoid an argument about the veracity of post-Cretaceous dinosaurs if your claim is based on bones.

If only we had some foolproof way of knowing whether any dinosaurs lived into the Paleogene Period. What evidence would convince even the most hardcore, pessimistic paleo-curmudgeon? You guessed
it: dinosaur tracks. Tracks would show not only dinosaurs actively behaving, but also would be in the same place where dinosaurs walked. In contrast, a transported, out-of-place and hence out-of-time track is very easy to identify. Tracks found in situ might even be linked with a general clade of dinosaurs, such as non-avian theropods, hadrosaurs, or ceratopsians, whose bones are in strata just below.

So with this goal in mind, several places have what are among the last dinosaur tracks, coming from just below the Cretaceous–Paleogene boundary. One such site in Utah has hundreds of deeply impressed tracks preserved in strata at multiple levels, one of which is only a few meters below the boundary. Although the authors couldn’t identify the specific dinosaurs that made these tracks, their huge sizes and numbers pointed toward herding herbivores, such as ceratopsians or hadrosaurs. A single—but huge—theropod track from New Mexico, attributed to
Tyrannosaurus rex
, also came from a layer just below the boundary. In Spain, abundant dinosaur tracks, some ascribed to hadrosaurs and sauropods, are preserved only a few meters below strata that preserved fossils of Paleogene fish and mammals. At one site, more than forty horizons contained hadrosaur and sauropod tracks below the boundary, and paleontologists estimated that some tracks were made only 300,000 years before the end of the Cretaceous. Alas, there were no more dinosaur tracks in the Paleogene rocks.

So these few examples show that dinosaurs were indeed alive just before the meteorite impact and mass extinction, which helps us to better clarify whether an overall dinosaur extinction was already happening before the impact or not. If someone does find a dinosaur track above the Cretaceous–Paleogene boundary, it would be a fantastic discovery, worthy of praises and raises. As of now, though, it just doesn’t seem likely that any dinosaur walked for very long after the earth had its very bad day, the last of the Cretaceous. But if you think that is heartbreaking, imagine what it must have been like to be that last dinosaur: walking alone and seeking its own kind but never finding them, its tracks the only ones dotting a desolate and mostly silent landscape.

The Case of the Mistaken Dinosaur

Like all stories in paleontology, that of Lark Quarry—a world-famous dinosaur tracksite in Queensland, Australia—has a vivid geologic history, beginning during the Cretaceous Period at about 98 million years ago. The cast of this tale consisted of more than a hundred dinosaurs, played by:

• One species of a large herbivorous dinosaur (an ornithopod), a mere bit actor, with a cameo appearance before the main act.
  
• Two species of small dinosaurs, one a theropod and the other an ornithopod, who composed most of the cast.
  
• The star of the show, a tyrannosaur-sized theropod who makes a grand entrance toward its climactic finish.
  

The story is filled with dramatic flourishes and tells of a tranquil scene shattered by brutal carnivory in which a quiet Cretaceous lakeshore became a killing ground. The former lakeshore, though,
left no bodies, only tracks. Consequently, we must reconstruct what happened there by using ichnology, a little knowledge of modern animals and their behavior, and our imaginations. And in the spirit of imagination, I will tell the story as a stage play, titled
The Mystery of Lark Quarry
, and attended by an audience of enthusiastic Australians.

The curtain opens, revealing a subtropical lakeshore, and a large ornithopod, similar to the Australian dinosaur
Muttaburrasaurus langdoni
, emerges from a fog and lumbers onto the dimly lit stage. His arms are long and nearly touch the ground, but all of his walking is done with his rear legs. He stops briefly, bellows a low-frequency, didgeridoo-like moan, turns to the left, and exits, enveloped by the mist. Minutes slip by as the stage lights swell in imitation of a Cretaceous dawn. Insect-like buzzes and chirps intrude on the silence, accompanied by bird songs of the Mesozoic.

Soon thereafter, small two-legged theropods and ornithopods, cavorting and gamboling splendidly, enter from stage right in twos and threes. Each pays no heed to the presence of the other species, as neither poses a threat. They wander about and occasionally halt to sip water from the shoreline, but whether stopped or moving, they frequently pop their heads up, down, and around, ever alert to danger. The little theropods behave much like chickens in a … well, a chicken yard. The ornithopods, most about the size of modern emus of various ages, also remind the audience of these modern animals.

At some point the insects, which had reached a cacophonous peak through their blended mating calls, suddenly cease their racket and are quickly joined in silence by the birds. This hush puts the theropods and ornithopods on high alert, the Cretaceous equivalent of a fire alarm. They all look up and to the left simultaneously, where the quiet first descended and then spread like a wave. They begin to sniff the air, but delay expressing full-fledged panic until they actually feel the danger in their feet. There it is, detected by the audience as a Sensurround-like special effect, the
basso profundo
vibrations imparted by each footfall of an approaching multi-ton animal, coming from the left and offstage. Boom. Boom. Boom.

His imposing head appears first, with a mouth full of bladed teeth and nostrils snorting steam into the cool morning air. The rest of his massive body reveals itself with each halting step, his long tail held straight behind him, tiny arms dangling in front. The smell of rotten meat wafts across the stage and into the rest of the theater, a sample of this carnivore’s bacterially fueled bad breath. After stopping once, he crosses his right foot over his left, stops again, crosses his left foot over his right, and then quickens his pace once a target is acquired. He is now stalking his prey.

The small dinosaurs react appropriately to such a sudden and rude interruption of their morning. Acting as one panicked mind, they run to the left as quickly as their short legs can go, straight toward, around, and then behind the marauder. Some of them step into and out of his tracks, leaving their own comparatively tiny footprints inside his. In the slippery mud, though, one ornithopod loses its balance and lands on its side, hapless against the assault, bleating like a sheep about to be sheared.

The theropod roars, pins its struggling prey underneath one 65-cm (26-in) long clawed foot and lowers its head to deliver a fatal chomp. The stage darkens, stirring theme music rises, the curtain closes, and the audience erupts into applause. Peaceful reverie shattered by gruesome death, in which the fleet of foot and the strong survive and the weak perish: what’s not to like?

Then just as the applause begins to subside, the curtain opens again to reveal the star of the show, the villain/hero theropod. The audience claps madly, thinking this is the start of the curtain call. Only, their enthusiasm is abruptly curtailed when they realize the theropod is not bowing, but instead is looking directly at them with a mischievous smirk. This is
not
a tyrannosaur!

Their shock is doubled when the “theropod” pulls off an elastic mask, revealing that it was, all along, a bigger version of the ornithopod seen at the opening of the play. Within seconds, rotten vegetables and fruit, brought in by the otherwise finely attired and
upscale crowd, shower the stage. The curtain closes, and a full-fledged riot engulfs the theater.

A year goes by, and although this displeasing conclusion to the play has not been entirely forgotten, it has been forgiven. Hence, the former audience is glad to see advertisements concerning a remake of the original play. So once again they fill the theater for the grand premiere, an excited buzz filling the air. Maybe it will be better this time, they all say with hopeful expectation.

The stage lights dim and the curtain parts. At first, the audience is reduced to squinting into the darkness, confused by strange reflections coming from the stage. The familiar insect and bird sounds from the previous production are present, but something is amiss. As the lights gradually brighten, again in imitation of a dawning day, realization strikes the audience. A large see-through glass container filled with water occupies the stage. The buzz turns to murmuring, followed by shocked silence as the
Muttaburrasaurus
emerges, splashes into the water, and swims from left to right. It is quickly followed by a hundred small ornithopods and theropods which, from the right side, dive one after another in a perfect domino-like effect, then paddle in unison to a blaring waltz-like number, their toes delicately touching the bottom of the pool. The play has been transformed into an Esther Williams-style aquamusical.

“Bloody hell! It’s a pool!” someone shouts. “They’re swimming!” someone else yells. “The music and choreography are dreadful!” screams a third. “Where are the sharks?” demands a fourth. Pandemonium descends, the play comes to an abrupt end, and police—who are waiting in the wings, expecting trouble—politely escort everyone out before the unrest escalates. Needless to say, the reviews are awful.

So with these dramatizations in mind, imagine the first part—large theropod causes a stampede among more than a hundred smaller dinosaurs—being retold thousands of times. Think of this story generating museum dioramas, artwork, a rollicking song, computer-animated recreations, dramatic scenes in mainstream
movies, and a specially designed building with national-heritage status in Australia to help preserve the dinosaur tracksite that inspired it. Except … what happens if the last plot twist is inserted—large theropod was actually a peaceful herbivore—after having been told with a different ending for decades? Even worse, what to do with the remake, with its shocking aquatic take on the tale? Furthermore, what happens when the new stories are scrutinized and found lacking in details, further confusing matters for anyone who wants to know what really happened at Lark Quarry? Not that anything will ever be straightforward about a 98-millionyear-old “crime scene,” and especially one with no bodies left behind.

The History of the Mystery of Lark Quarry

Maybe a visit to Lark Quarry will help us to better understand the conflicting stories gleaned from the dinosaur tracks there. Lark Quarry Conservation Park—the full official name for the tracksite—is in central Queensland, about 110 km (68 mi) southwest of Winton, the nearest town of any size in that part of Queensland. The tracksite is protected from the elements of the Australian Outback by a beautiful, spacious, and environmentally friendly building constructed over and around the tracks in 2002. During my first visit in 2007, I wondered how comfortable I might be spending a night there alone with the dinosaur tracks. This unusual thought came to me upon realizing that I had been accidentally locked in, just after the last tour of the day had departed.

Before talking about that embarrassing episode, though, let’s go back to justifying why these dinosaur tracks deserve such renown and reverence, including their own building in the proverbial “middle of nowhere” and National Heritage status in Australia. I remember first learning about Lark Quarry as a geology graduate student, soon after beginning my studies in ichnology while at the University of Georgia in the mid-1980s. Imagine how thrilling it was for a nascent ichnologist in Georgia to read about a site that held more than three thousand Cretaceous dinosaur tracks in
faraway Queensland, Australia. I likewise dreamed of the ichnologically induced ecstasy that surely would result from a pilgrimage to see it in person. This aspiration became real for the first time in 2007 (that’s when I was locked in) and was fulfilled twice more, once in 2010 with my wife Ruth (who successfully kept us from getting trapped) and in 2011 when I brought a group of university students there as part of a study-abroad program I taught in Queensland that year.

How was such a remarkable dinosaur tracksite discovered? As is still typical for fossil finds in many parts of the world, it was spotted by a sharp-eyed amateur, cattle station manager Glen Seymour, who lived and worked in the area. When Mr. Seymour first saw the tracks in 1962, only a few were visible on a rock surface sticking out of a hillside. I imagine the resemblance of these three-toed tracks to those of emu or brush-turkey tracks caught his eye, his perceptions honed by much time outdoors and looking at the ground. Indeed, he later said he considered them to be fossil bird tracks and was flabbergasted when Peter Knowles, a local photographer and naturalist, identified them as dinosaur tracks. I also can’t help but think, though, that the indigenous people of this area and other parts of Australia, many of whom were expert trackers, probably also noticed these marks of long-gone animal life in the Cretaceous rocks, perhaps incorporating these trace fossils into Dreamtime stories.