How to Build a Dinosaur (14 page)

But our view of dinosaurs changed as our knowledge of birds increased. One of the scientists who helped change our view of both dinosaurs and birds was the late John Ostrom, one of the great paleontologists of the twentieth century. Two discoveries were of key importance.

In 1964, Ostrom, of the Peabody Museum of Yale University, found some unusual bones at a site in the Cloverly Formation near Bridger, Montana. The site, which came to be called the Shrine, was south of Billings, about halfway to the Wyoming border. The site, dating to about one hundred twenty million years ago, in the early Cretaceous, had been excavated once before by Barnum Brown of the Museum of Natural History around 1930.

For the next few years, through the field season of 1967, he and his crew collected more than a thousand fossil bones representing at least three individuals of a new dinosaur. They stopped work because the fossil finds were decreasing and the rock that had to be removed was getting harder and deeper. The dinosaur, which Ostrom named
Deinonychus antirrhopus
(literally “counterbalancing terrible claw”), was named for its two most striking features, a long, stiff tail, and recurved, slashing claws on each of its hind feet.

In 1969 he published a paper naming
Deinonychus
and describing the kind of dinosaur it was—fast, smart, with slashing claws. Ostrom portrayed it as a quick, fierce animal that was smart enough to hunt in packs and had a metabolism that could support sustained effort. It was likely to have been warm-blooded, Ostrom argued, meaning that, like birds and mammals, it could regulate its body temperature separately from the temperature of its environment. Reptiles like turtles, lizards, and alligators depend on the outside temperature to warm them up and cannot function when the temperature drops. At least this is the simple version of what was the common view of science at the time, which was that reptiles did not have independent regulation of bodily heat to any significant degree. Dinosaurs were undoubtedly reptiles, but they did not fit this picture.

With
Deinonychus
Ostrom helped start a revolution in our understanding of dinosaurs, a revolution that I became swept up in, and was able to contribute to, with finds like colonial nesting grounds that also suggested that dinosaurs were unlike the animals we had imagined up to that point.

Ostrom came at the dinosaur/bird connection from both ends. Shortly after
Deinonychus
he made another remarkable discovery, this time in a museum. He found a misclassified specimen of
Archaeopteryx lithographica,
the most famous ancient bird, and the one that produced the most famous fossils, remains in fine-grained limestone that have the quality of masterful etchings. The fossil has the name
lithographica
precisely because of the German limestone deposits, a source of superb material for lithography.

The first fossil skeleton of
Archaeopteryx
was discovered in 1861. It shows us, as Chiappe describes it, “a toothed, crow-sized bird with powerful hand claws and a long bony tail.” It was the oldest, most primitive bird known when the fossils were first found, and it still is. That first specimen was sold to the British Museum of Natural History and it is still there. A nearly complete skeletal impression of a comparable fossil of
Archaeopteryx
was found in 1877 in another quarry not far from the town where the London fossil was found.

Archaeopteryx
is dinosaurlike in many ways. But of course it had abundant feathers, which marked it as a bird immediately. Had it been known at the time that other fossils that were clearly dinosaurs had feathers, the classification might not have been so obvious, since it has many characteristics that make it far different from modern birds, not the least of which are its long tail and teeth. Its skull is reptilian. It is a mixture: long tail, but not as long as its ancestors’, primitive spine but not as primitive as those of earlier dinosaurs, and claws at the end of its wings. But it was clearly a bird or a transitional animal between birds and reptiles. Today it is considered a bird, and the earliest bird fossil we have, but not the first bird ever. The study and interpretation of bird fossils show that there must have been earlier birds.

Ostrom made his find because he was working on the origin of flight, and it was for that reason he wanted to examine a specimen of a pterodactyl in the Teyler Museum in Holland. As Pat Shipman describes in her book
Taking Wing,
once the slab in which the fossil was embedded was brought out to him “he carried the slab over to the window where the light was better. In the next instant the oblique sunlight illuminated the slab and brought up the impression of feathers.” He knew right away what it was.

The
Archaeopteryx
fossil, misclassified until then as a pterodactyl, was a powerful reminder of how close dinosaurs and birds were. Shipman writes, “As a consequence of this two-part discovery, Ostrom began to revive Huxley’s dinosaur hypothesis of bird origins. Birds, he argued with the passion of a sudden convert, are so like small theropod dinosaurs that an unfeathered early bird specimen could easily be mistaken for such a dinosaur.”

I first met John Ostrom in 1978 when he came to Princeton, where I was working as a preparator, to talk to my boss, Don Baird, about footprints in the Connecticut Valley. Bob Makela and I had already found the first fossils of baby duck-billed dinosaurs, a complete surprise to the world of dinosaur science because baby dinosaurs were almost never found, and their rarity was a disturbing puzzle. Ostrom looked into the lab where I worked, and commented on the tiny sizes of the baby duckbills. We talked about duckbills and their skulls, in particular about whether bones in their skulls moved when the animals were feeding. Both bird and lizard skulls have this feature, cranial kinesis. John had written that duckbill skulls were akinetic , like those of crocodiles and alligators, and I had presented some evidence at a conference that the duckbill skulls were movable, like those of birds and lizards.

Over time we became friends and in 1995, I invited John to join the Museum of the Rockies crew in the field in Montana to see what we had been doing with his
Deinonychus
site. In 1993 I was interested in the life histories of dinosaurs, particularly whether they had lived in social groups. All of the other sites we had explored were of herbivorous dinosaurs, the prey. With John’s permission I sent a field crew to reopen the Shrine. The operation required removal of hundreds of tons of hard rock, using jackhammers, picks, and crowbars, and in the end, very few bones were found, confirming John’s good judgment in not continuing to attack the site. The crew did find important fossils that have led to a much better understanding of what the skull of
Deinonychus
actually looked like, but at quite a price. And the quarry gave me precious little information about the social behavior of
Deinonychus
.

John, like many of the earlier generation of paleontologists, was a gentleman with a wonderful social presence. Although we had become friends—and he had been extremely complimentary about my work and an earlier book about finding the skeletons of baby duckbills
, Digging Dinosaurs
—I still considered him a great scientist, an inspiration, paleontological royalty. So it felt like a privilege to guide him around the site of the discovery for which he was probably best known. It was bittersweet, because he was aging, and one of the deepest satisfactions for any dinosaur scientist was slipping away from him, the prospecting, the excavation, the time travel by shovel and pickax and jackhammer. It was on his mind as well. After we toured the site, and John had seen our excavation, he told me he didn’t think he would be venturing out into the field anymore. Then he gave me his hat.

I can’t say hats are as precious to paleontologists as they are to Texans, but they can be something of a signature, or talisman. Think Indiana Jones, without the bullets and Nazis and special effects. Excavations are never, ever done in the shade. Where there is erosion and exposure, there is inevitably sun, and a hat, which is absolutely necessary, can gather memories and significance. John Ostrom’s hat is on the wall of my office, where it will stay. He died a few years later and we heard the news when we were in the field. The whole crew was shaken.

John did not just discover an unusual dinosaur, he made a comprehensive argument supporting the descent of birds from dinosaurs. In a 1975 article he summed up other views and presented his own argument with evidence to back it up.

First, he noted that the idea that birds were descended from reptiles had long held sway. “Over the years, several different reptilian groups have been suggested,” he wrote, “but for the past fifty years or more the general consensus has placed the source of birds among a group of primitive archosaurian reptiles of Triassic age—the Thecodontia.”

The thecodonts were the precursors of crocodilians, pterosaurs, and dinosaurs. They were land animals that had succeeded some of the huge amphibians that evolved as animal life exploded in its colonization of the land. But, John argued, presenting a thorough and detailed analysis of the fossils of
Archaeopteryx,
that were available, this bird fossil was so similar to theropod dinosaurs, specifically the gracile, swift, and predatory coelurosaurs, like
Deinonychus,
that the line of descent to birds was obvious.

He noted similarities in the vertebrae, the forelimb, pelvis, hind limb, and a bone called the pectoral arch. He also dismissed the idea that lack of clavicles or collarbones in theropod dinosaurs meant they could not have given rise to birds, in which right and left clavicles have fused to become what we call the wishbone. Ostrom pointed out that clavicles had indeed been found in several dinosaurs, and that even if they had not been found, negative evidence is never conclusive. Given the rarity of fossils, absence of a characteristic only proves that we haven’t found a fossil with it, or we haven’t noticed it.

In fact, he concluded, the only characteristics that made
Archaeopteryx
a bird were its feathers and its wishbone. He did not believe
Archaeopteryx
could fly, and suggested that feathers had evolved for insulation, anticipating that other, nonavian dinosaurs would have evolved feathers. Without those two characteristics the skeleton would have been classified as a theropod.

Now is a good time to tackle how such classifications are made. When Ostrom was publishing his work he was tracking descent, a fairly straightforward idea, which led to evolutionary trees much like family trees. Instead of parents and great-grandparents , you would have parent species or genera and great-grandparent species or genera. But genealogy and phylogeny were both alike in that they traced actual descent, trying to establish who fathered cousin Fred and what particular genus of dinosaur gave rise to the first birds. They were, in effect, using the same charts.

Gradually this has been supplanted by cladistics, which is significantly different—even revolutionary—in how it changes the way we think about the past. Cladistics is used not to track ancestors, as in genealogy, but as a way to look at the changing characteristics of organisms over vast stretches of time. It abandons the search for a specific ancestor to any species or genus. Instead it tracks evolutionary change by looking for new characteristics, like feathers or hair or walking on two feet.

A cladistics diagram, or cladogram, starts out with very large groups that share very basic characteristics. Branches appear when new characteristics evolve. These are called derived characteristics because they are derived from a more basic or primitive state. Vertebrates are a very large clade including all animals with backbones. Within that clade are mammals, which have backbones, but also have derived characteristics that they share only with other mammals, hair and mammary glands. Evolution can be tracked from the largest to the smallest clades, as life explodes in diversity and new characteristics keep popping up.

The differences between this approach and older approaches are subtle and profound. Instead of looking for the specific ancestor of birds, for instance, what we try to do is to look at the characteristics birds share with other groups, like the dinosaurs, and what new characteristics they have. There is quite a bit of judgment involved in making sensible groups, or clades, based on specific characteristics. But the close study of old and new traits makes the classification of birds as dinosaurs unavoidable. For example, some of the characteristics that we might think of as being exclusive to birds, like the wishbone, feathers, hollow bones, and oblong eggs, are found in dinosaurs, where they evolved first. There are many more shared features, but most are obscure, like the shape of the wristbone that allows a bird to fold its wings to its sides. If we were to try to do a similar motion with one of our arms, we would have to be able to bend our wrists to the side, rather than front-to-back.

The way we track evolution, shared characteristics like feathers or particular shapes of wristbones mean a common ancestor. Of course, this commonality may be so broad that it is not very helpful. All organisms that have cells with nuclei share a common ancestor, but a characteristic that is shared by ants, falcons, and corals doesn’t give us much information about evolution. When groups share a great number of characteristics, then that means they have a common ancestor not very far back. Birds share almost every characteristic that we have noticed with a group called the dromaeosaurid dinosaurs. In fact, it’s hard to tell them apart. And birds share so many more characteristics with dinosaurs than they do with other groups that are candidates for avian ancestors, like archosaurs, that we put them in the dinosaur clade.

Cladistics is merely a tool, however, a way of thinking about and categorizing fossils. It is the fossils themselves that are the source of information and, sometimes, exhilaration. In the past fifteen years a series of finds by native and foreign paleontologists in China have produced shock waves of excitement about the ancestry of birds and the nature of dinosaurs.