by Brian Switek
FIGURE 33 - The “Berlin” Archaeopteryx, as drawn by Gerhard Heilmann.
A second, more exquisitely preserved Archaeopteryx, discovered in 1877, fueled these continuing debates. Found in a quarry in Eichstätt, not far from Solnhofen, it is arguably the most beautiful fossil ever discovered. Whereas the “London specimen” was scrambled, the new specimen was fully articulated, its head thrown back and arms spread wide to display a splash of feathers. The fact that it had a head greatly increased its significance. Although the first specimen appeared to have been
decapitated in 1865, John Evans thought that he had discovered a portion of its toothed mouth on the same slab as the rest of the skeleton. Some said that the jaws belonged to a fish, but Evans did not think it unreasonable that a bird with so many reptilian characteristics would also have teeth. The new specimen confirmed Evans’s hypothesis and refuted that of Owen. Archaeopteryx, like Hesperornis and Ichthyornis, had tooth-studded jaws. This confirmation fed into the ongoing debates of the creature’s affinities, but regardless of what group it was assigned to it was such an enigmatic fossil that it could not be ignored. In time it would be agreed that it was the very first bird, a creature that documented one point in one of life’s major transformations.
There was more to the debate than anatomy and family trees, however. The origin of birds was directly tied to questions about the origins of flight, and an early attempt to tackle this problem was undertaken in 1879 by paleontologist Samuel Williston. Taking a dinosaurian ancestry for birds as a starting point, Williston proposed:It is not difficult to understand how the fore legs of a dinosaur might have been changed to wings. During the great extent of time in the Triassic, in which we have scanty records, there may have been a gradual lengthening of the outer fingers and greater development of the scales, thus aiding the animal in running. The further change to feathers would have been easy. The wings must first have been used in running, next in leaping and descending from heights, and, finally, in soaring.
A similar idea was later developed by the eccentric Hungarian aristocrat, spy, and paleontologist Baron Franz Nopcsa von Felső -Szilvás. He proposed that while pterosaurs evolved from quadrupedal ancestors that lived in the trees and took to the air by leaping, birds had evolved from terrestrial predecessors that jumped and “oared along in the air” with the help of feathered arms.
Yet the “ground-up” origin for flight hypothesized by Williston and Nopcsa failed to gain a firm foothold, and other researchers continued to mull over how flight could have originated. A particularly ingenious solution to the problem was proposed by the American ornithologist William Beebe. Despite the fact that Beebe thought Archaeopteryx more of a “flutterer” than a true flyer, he believed that it might represent an early stage of flight, and he used it as a starting point to predict what its ancestors and descendants might look like.34
Beebe introduced his colleagues to his hypothetical transitional series in 1915. It had all started in the trees. As Beebe had observed in the New World tropics, iguanas sometimes leapt out of trees when frightened, and when they did so they flattened themselves out to slow their descent. In such a scenario longer scales would increase their surface area to further slow their falls, Beebe reasoned, especially if these scales were situated along the arms. But the back end of the animal would have had to have been held up, too, otherwise it would be akin to a reptilian Darius Green, who, like the subject of John Townsend Trowbridge’s poem, would fall “to the ground with a thump! Flutt’rin’ an’ flound’rin’, all’n a lump!”
The key to how these hypothetical creatures stayed aloft was found in living birds. A recently hatched dove Beebe examined had rudimentary feather quills attached to its upper leg, and Archaeopteryx appeared to have long feathers on its legs, too. Thus, Beebe surmised, the ancestors of birds had leg wings that helped balance them out while parachuting and had gone through a “Tetrapteryx stage.” As the scales turned into real feathers and the animals became capable of gliding the forewings became more prominent, and the feathers of the tail became larger to support to back of the body. By combining fossil evidence with studies of living animals, Beebe was able to make a functional prediction of how birds had evolved.
FIGURE 34 - William Beebe’s hypothesis of the evolution of birds. According to Beebe’s scenario, bird ancestors would have started by parachuting in a “Tetrapteryx stage,” and over time the feathers of the front wings would have become enlarged, allowing for powered flight.
Beebe’s hypothesis was only one competing among many, though, and no clear consensus could be drawn. Naturalists were unable to confirm whether flight had evolved from the “trees down” or “ground up.” Without knowledge of the ancestral form any hypothesis could be constructed from the scraps of evidence.
Older and more primitive than even the most ancient dinosaurs, the pseudosuchians seemed like good candidates for the ancestors of pterosaurs, dinosaurs, and birds. As proposed by paleontologist Robert Broom, whereas dinosaurs had peculiar specializations that would bar them from being bird ancestors, the psuedosuchians such as Euparkeria were still “generalized” creatures from which both groups could have easily derived. This would make any resemblances between birds and dinosaurs matters of convergence and not real signals of ancestry.
FIGURE 35 - The skull of Euparkeria.
The Danish artist Gerhard Heilmann most forcefully articulated this hypothesis in his 1926 book The Origin of Birds. Some dinosaurs closely resembled birds, particularly the coelurosaurs such as the predatory Gorgosaurus and ostrichlike Struthiomimus, but they lacked one characteristic that barred them from a close relationship to birds: clavicles. According to Dollo’s Law, which was formulated by Belgian paleontologist Louis Dollo, evolution could not be reversed, and therefore dinosaurs could not be bird ancestors as it would require that they regrow clavicles after they had already lost them.35 This left the pseudosuchians the most appropriate stock from which to derive birds.
Heilmann’s work was a classic, and the pseudosuchian origin for birds became the favorite hypothesis during the following four decades. It was so widely accepted that even when clavicles were described among the remains of the small, predatory dinosaur Segisaurus in 1936 no one seemed to notice. (The first specimen of Oviraptor described in 1923 also had clavicles, but they were misidentified at the time.) The problem of bird origins had been solved; all that was needed were fossils to confirm the transition.
With the big question of bird ancestry seemingly resolved, work on the subject slowed during the middle of the twentieth century. Occasional alternate interpretations of Archaeopteryx continued to pop up, some closely linking the bird to dinosaurs, but the pseudosuchian hypothesis remained the favored one. Still, the resemblance between birds and predatory dinosaurs was undeniable. The immense sauropod dinosaurs were often considered to be drab, tail-dragging animals that spent much of their time in swamps, but the small predatory dinosaurs were another matter. Writing in the middle of the twentieth century, paleontologist Edwin Colbert thought the theropod Ornitholestes was an “agile” catcher of lizards and insects, and its compatriot Ornithomimus had “very long, slender hind limbs and very birdlike feet, which indicate that it must have been a rapid runner, much as are the modern ostriches.”
It would take the rediscovery of a dinosaur first found in 1931 for paleontologists to begin to fully realize the significance of the theropod dinosaurs to bird evolution. During the summer of 1964 paleontologists John Ostrom and Grant E. Meyer from Yale’s Peabody Museum were searching for fossils near the town of Bridger, Montana, when they discovered the numerous fragments of an unusual dinosaur. The famous fossil hunter Barnum Brown had found the remains of the same kind of dinosaur, which he informally called “Daptosaurus,” decades earlier, but since he never fully described it few paleontologists knew anything about it. Based upon the more complete remains they had found, however, Ostrom and Meyer knew that Brown had overlooked a dinosaur unlike any other then known.
> They called the new predator Deinonychus (“terrible claw”), so named because of the wicked, sickle-shaped weapon it carried on its second toe. The arrangement of the bones showed that Deinonychus held this claw off the ground, and the tail of the animal was stiffened by ossified rods that would have acted as a dynamic counterbalance. This was not a slow, stupid predator but an agile hunter, and the presence of multiple individuals from the same site associated with bones of the herbivorous dinosaur Tenontosaurus suggested that Deinonychus might have been a pack hunter, something practically unheard of in dinosaurs.36 Of Deinonychus, Ostrom wrote:Deinonychus must have been anything but “reptilian” in its behavior, responses and way of life. It must have been a fleet-footed, highly predaceous, extremely agile and very active animal, sensitive to many stimuli and quick in its responses. These in turn indicate an unusual level of activity for a reptile and suggest an unusually high metabolic rate.
FIGURE 36 - A modern restoration of Deinonychus.
Deinonychus stood in sharp contrast to the traditional image of dinosaurs. Even though nineteenth-century naturalists like Owen, Cope, Huxley, and Seeley thought that dinosaurs were warm-blooded animals, the consensus since that time had shifted to envision dinosaurs as larger versions of living lizards and crocodiles. Like their living counterparts they would have required a warm environment in order to be active, but the details of their physiology were unknown. What was supposed about their biology had been inferred from living reptiles in studies like that carried out on alligators by Edwin Colbert, Charles Bogert, and Raymond Cowles in 1946.
In order to approximate dinosaurian physiology, the trio of scientists carried out the unenviable task of sticking thermometers in the cloacae of American alligators. Several specimens, ranging from one to seven feet in length, were placed in the sun or shade and had their temperature taken every ten minutes. (Larger animals would have been better, but as the researchers explained, “the difficulties of making temperature experiments [on fully grown alligators] would be great and can be best left to the imagination.”) What the scientists found was that the larger alligators warmed up and cooled down slower than the smaller ones. It took about a minute and a half for the small animals to warm up one degree Celsius, while it took the largest animals five times as long to do the same. This was regulated by their internal volume. As the size of a body or object increases, its internal volume increases exponentially. An ostrich egg, for instance, is only about two and a half times as large as a chicken’s egg, yet it contains about twenty times as much fluid and tissue inside. (If you wanted to make a hard-boiled ostrich egg it would take much, much longer for the heat to cook it than it would for a chicken’s egg.) Likewise, the larger alligators had more internal volume and so took a greater amount of time to heat up or cool down. Extrapolating these differences up to the size estimates for dinosaurs, the authors wrote that it would take a ten-ton dinosaur around three and a half days of basking out in the sun to raise its body temperature one degree Celsius!
But as the researchers found out the hard way with two of their test animals, prolonged exposure to the hot sun could be deadly. It was absurd to think that dinosaurs had to sunbathe for so long to become active. (They revised their figures in later publications, writing that a large dinosaur would have to spend most of one day heating up, but this was still an unreasonable amount of time to spend sunbathing.) It was more likely that the large size of many dinosaurs shielded them from fast heat fluctuations, and that they benefited from a high, stable body temperature that would have allowed them to be active much of the time.
This only made sense for the largest dinosaurs. At only one meter tall Deinonychus was too small to maintain a near-constant high body temperature, yet it was adapted for a very active life. Was it possible that some dinosaurs maintained a high body temperature internally? Ostrom and his student Bob Bakker thought so, and French paleontologist Armand de Ricqlès came to a similar conclusion almost simultaneously through his work on the microstructure of dinosaur bone. This launched a lively, and sometimes acrimonious, debate about the lives of dinosaurs.
After simmering for several years, the debate over “hot-blooded dinosaurs” came to a boil during a 1978 symposium hosted by the American Association for the Advancement of Science. While no clear consensus could be reached, it was apparent that the phrases “warm-blooded” and “cold-blooded” were easily misused. A better understanding of the physiology of many different organisms revealed a wide diversity of metabolic strategies that were not easily categorized. An animal that controls its body temperature internally, maintains that high temperature regardless of external temperature, and has a high metabolic rate while at rest is called “endothermic.” Animals traditionally called “cold-blooded,” on the other hand, do not have constant, internally regulated body temperatures. Their metabolic rates can be high or low depending on external factors, giving them the label “ecotherms,” and they can be just as active as endothermic animals under the right conditions.37
The question that remained was whether dinosaurs were endotherms or ectotherms, but without living subjects to observe it was difficult to know for sure. As paleontologist Peter Dodson opined, it was perhaps best to consider “dinosaurs as dinosaurs.” But what if dinosaurs did have living descendants, after all? The discovery of Deinonychus and the debate over dinosaur physiology reinvigorated interest in the idea that birds had evolved from dinosaurs, and if this was correct then the physiology of birds would be a model for understanding the lives of dinosaurs.
A key piece of new evidence in this reinvestigation came from a mislabeled specimen in a museum. In 1855, five years before the first Archaeopteryx feather was found, Hermann von Meyer acquired what appeared to be a pterodactyl skeleton from the German limestone quarries. When Ostrom saw it over a century later, however, he knew it was no pterodactyl. It was a specimen of Archaeopteryx that had been misidentified, and it was strikingly similar to Deinonychus. After carefully studying the “new” specimen, Ostrom came to the same conclusion English zoologist Percy Lowe had arrived at in 1936 (albeit by a different route). “The osteology of Archaeopteryx, in virtually every detail, is indistinguishable from that of contemporaneous and succeeding coelurosaurian dinosaurs,” Ostrom wrote, confirming that the first bird was a theropod dinosaur.38
The revival of the avian dinosaur hypothesis was not immediately well received. The pseudosuchian hypothesis still held strong, even as the pseudosuchia (now sometimes called thecodontia) was recognized as a taxonomic wastebasket that did not constitute a natural evolutionary group. Slowly, however, many paleontologists came around to the view that birds might be the direct descendants of dinosaurs, even as the fossils that would confirm the transition remained elusive.
If Ostrom was right that coelurosaurs gave rise to birds, then it was likely that there were other feathered theropods waiting to be discovered. 39 The likelihood of finding feathered dinosaurs, however, was slim. Even under the best of circumstances fossil preservation is a capricious thing. Fully articulated skeletons are rare, and rarer still are fossils that preserve any indication of body covering or soft tissues.
It was for just this reason that a snapshot circulated at the 1996 Society of Vertebrate Paleontology meeting held at the American Museum of Natural History caught paleontologists off guard (John Ostrom among them). It showed a little theropod dinosaur not unlike Compsognathus with its head thrown back and tail pointed straight up, and along its back was a strip of fuzzy feathers. Although no scientific study had yet been undertaken (the fossil had only come to the attention of Canadian paleontologist Phil Currie and paleo-artist Michael Skrepnick two weeks earlier), the specimen confirmed the connection between dinosaurs and birds that had been proposed on bones alone. The new dinosaur was dubbed Sinosauropteryx, and it had come from Cretaceous deposits in China that exhibited a quality of preservation that exceeded that of the Solnhofen limestone.
Sinosauropteryx was only the first feathered dinosaur to be announc
ed. A panoply of feathered fossils started to turn up in the Jurassic and Cretaceous strata of China, each just as magnificent as the one before. There were early birds that still retained clawed hands (Confuciusornis ) and teeth (Sapeornis, Jibeinia), while non-flying coelurosaurs such as Caudipteryx, Sinornithosaurus, Jinfengopteryx, Dilong, and Beipiaosaurus wore an array of body coverings from wispy fuzz to full flight feathers. The fossil feathers of the strange, stubby-armed dinosaur Shuvuuia even preserved the biochemical signature of beta-keratin, a protein present in the feathers of living birds, and quill knobs on the forearm of Velociraptor reported in 2007 confirmed that the famous predator was covered in feathers, too.
As new discoveries continued to accumulate it became apparent that almost every group of coelurosaurs had feathered representatives, from the weird secondarily herbivorous forms such as Beipiaosaurus to Dilong, an early relative of Tyrannosaurus. It is even possible that, during its early life, the most famous of the flesh-tearing dinosaurs may have been covered in a coat of dino-fuzz.
FIGURE 37 - A Velociraptor attempts to catch the early bird Confuciusornis. Both were feathered dinosaurs.
The coelurosaurs were among the most diverse groups of dinosaurs. The famous dinosaurs Velociraptor and Tyrannosaurus belonged to this group, as did the long-necked, pot-bellied giant herbivore Therizinosaurus and birds. What is remarkable is that, with the exception of the ornithomimosaurs, every branch on the coelurosaur family tree contains at least one feathered dinosaur, and it is expected that fossils of even more feathered coelurosaurs will be discovered as investigations continue. This suggests that, instead of evolving independently in each group, feathers were a shared trait for coelurosaurs that was inherited from their common ancestor.40 Most, if not all, coelurosaurs probably had some kind of feathery covering for at least part of their lives.41
