Lizard-like brutes, reptiles or birds - during the past 200 years, paleontologists
have developed radically different images of dinosaurs. The three images of
dinosaurs tell a remarkable story of how paleontological discoveries have driven
research that has shaped and reshaped paleontologists' understanding of of the
most famous of all extinct creatures.
Dinosaurs as lizards
The scientific study of dinosaurs began in England during the 1820s, when British geologist and naturalist William Buckland published the first scientific description of a dinosaur. In 1824, Buckland coined the name Megalosaurus (big lizard) for a huge jaw with dagger-like teeth found in Jurassic rocks in Stonesfield (see sidebar). Gideon Mantell, an English country doctor whose avocation was paleontology, had actually discovered dinosaur teeth and bones in Lower Cretaceous rocks in the Weald region of southern England two years earlier, but did not publish his discovery until 1825. He named these fossils Iguanodon (iguana tooth) because they included teeth that resembled those of a living iguana.
This life-sized sculpture of Iguanodon one of the earliest dinosaur fossils discovered completed by Benjamin Waterhouse Hawkins in 1854, still stands in London. Note the ponderous, lizard-like appearance of the dinosaur and the spike (horn) on the nose, characteristics that were typical of early understandings of dinosaurs. Later, scientists learned the dinosaur was more like a kangaroo, and the spike was on the thumb. Image courtesy of Robert M. Sullivan.
The 1830s saw additional dinosaur bones, which were discovered in England and received names such as Hylaeosaurus (Wealden lizard), Cetiosaurus (whale-like lizard), Poekilopleuron (mottled rib) and Thecodontosaurus (socket-toothed lizard). These first dinosaurs, however, were not called dinosaurs; paleontologists of the time saw them as giant, lizard-like reptiles.
For example, Mantell estimated Iguanodons size by simply scaling up a modern iguana to produce a 25-meter-long giant. (Later knowledge of complete skeletons of Iguanodon would reveal that it was only 8 to 9 meters long.) Of course, all the discovered dinosaur fragments from the 1830s resembled the bones and teeth of living lizards (and crocodiles), so the image of dinosaurs as gargantuan lizards was well-founded at the time.
In 1841, British paleontologist Richard Owen boldly united many of these giant lizards into a new group of reptiles, the dinosaurs, literally meaning terrible lizards. Owen brought a much broader and deeper knowledge of comparative anatomy to the study of dinosaurs than had Buckland or Mantell. But while Owen stressed the mammal-like limb posture of dinosaurs, he did not doubt their lizard-like attributes.
In the 1850s, Owen collaborated with artist and sculptor Benjamin Waterhouse Hawkins, producing both paintings and, more notably, a set of life-sized sculptures of dinosaurs for the grounds of the Crystal Palace exhibition in London, where they still stand. These sculptures, completed in 1854, brought the dinosaurs to life as huge, lizard-like brutes establishing their first scientific image.
Dinosaurs as reptiles
In 1855, dinosaur teeth discovered in North America, in the distant territory that is now Montana, captured the attention of Joseph Leidy, a Philadelphia anatomist, inspiring him to study vertebrate paleontology. Soon thereafter, in 1858, a nearly complete skeleton of a dinosaur came out of a rock pit near Haddonfield, N.J. Leidy christened it Hadrosaurus (heavy lizard) and studied its complete forelimbs and hindlimbs to reconstruct the dinosaur as an upright biped, unlike the ponderous quadrupeds of the Owen-Hawkins sculptures. Still, more complete dinosaur skeletons were necessary to remake the dinosaurs image.
After the American Civil War, during the 1870s and 1880s, two ambitious rivals, independently wealthy Edward Drinker Cope of Philadelphia and Yale College professor Othniel Charles Marsh of New Haven, dominated American paleontology. Collectors in the employ of these savants discovered dinosaur fossils across western North America, shipping back many skeletons of bizarre dinosaurs totally new to science, such as Stegosaurus (plated lizard), Triceratops (three-horned face) and Brontosaurus (thunder lizard, now called Apatosaurus, deceitful lizard; see sidebar).
Marshs skeletal reconstructions were particularly significant because they were the first published images of complete dinosaur skeletons. Cope, however, speculated more widely than Marsh did on dinosaur lifestyles first conceiving of active, fighting dinosaurs and formulating the notion (now rejected) that sauropods (plant-eaters) used their long necks as snorkels while ambling across lake bottoms. At the same time as the discoveries rolled in from the American West, complete dinosaur skeletons were found in Europe, notably the 1878 unearthing of exquisite skeletons of Iguanodon in a coal mine at Bernissart, Belgium.
The newly discovered complete skeletons of dinosaurs forced a makeover of the Owen-Hawkins image of dinosaurs. One particularly interesting revelation concerned the bony spike that was among the first fragmentary bones of Iguanodon discovered: Mantell and Owen had both identified the spike as a horn on the tip of the dinosaurs nose, but the complete Iguanodon skeletons from Bernissart revealed it to be a thumb spike.
Dinosaurs emerged as unique reptiles, walking on upright limbs and totally unlike any living lizard in overall shape and appearance. Still, dinosaurs generally gigantic size and obvious reptilian affinities (particularly their close relationship to living crocodiles) heavily colored conclusions about dinosaurian biology. Paleontologists of the late 19th and early 20th centuries perceived dinosaurs as ponderous, slow, coldblooded and dimwitted. They also realized that dinosaurs were extinct, having disappeared at the end of the Cretaceous period 65 million years ago, leading to the second dictionary definition of a dinosaur as something unwieldy, outmoded or unable to adapt to change.
The best images of this perception of dinosaurs as reptiles came from another paleontologist-artist collaboration between Henry Fairfield Osborn, the American Museum of Natural Historys premier paleontologist and a protégé of Cope, and Charles Knight, a remarkably talented painter and sculptor (see story, this issue). Knights images portrayed unique but reptilian dinosaurs, such as can be seen in his classic painting of a confrontation between Triceratops and Tyrannosaurus. These images were so influential that they pervaded dinosaur art for more than half a century and shaped the work of many artists. To this day, Knights paintings of dinosaurs remain some of the most familiar and copied images of dinosaurs. Indeed, the first Hollywood dinosaurs, including those felled by King Kong in his 1933 movie debut, came straight out of Knights paintings.
Dinosaurs as birds
It is fair to say that the paleontological image of dinosaurs as reptiles, well-established by the 1920s, changed little from the 1920s until the 1970s; dinosaur science was no longer a forefront of paleontological research. Those who did study dinosaurs continued to work within the framework of dinosaurs as strange and unique reptiles, but still as biologically reptilian. Researchers in the 1970s changed this trend, beginning a makeover of the dinosaurs image that continues today. The new image of dinosaurs strongly allies them both evolutionarily and biologically more to birds than to reptiles.
This skeleton with wing and tail feather impressions is the 10th specimen of the first known bird, Archaeopteryx. The new specimen provides important details on the feet and skull of these birds and strengthens the widely accepted argument that modern birds arose from theropod (meat-eating) dinosaurs. Image courtesy of G. Mayr/Senckenberg; Science.
Most paleontologists now point to the work of Yale paleontologist John Ostrom, who passed away last year, as the key to this makeover. In the 1960s, Ostrom had begun to question in print the notion of dinosaurs being slow and ponderous. Particularly significant was his monographic analysis in 1969 of a striking, new kind of meat-eating dinosaur, Deinonychus (terrible claw) from the Lower Cretaceous of Montana. This 3-meter-long (10-foot) biped has a fierce, 10-centimeter-long (4-inch) claw on each hind foot, slender and hollow limb bones, and bony tendons that held its tail out as a rigid counterbalance to its body. Ostrom described it as an active and very agile predator with a metabolism more akin to that of a modern bird or mammal than to a lizard.
In the 1970s, Ostrom also revived the idea (first suggested by Charles Darwins great supporter Thomas Henry Huxley in 1868) that birds descended from dinosaurs. He did so by making a compelling case that the oldest bird, the 150-million-year-old Archaeopteryx from the Jurassic in Germany, was little more than a meat-eating dinosaur with feathers. Ostrom thus reasoned that if birds are highly active and warmblooded, so too were their immediate ancestors among the dinosaurs. In 1975, one of Ostroms students, Robert Bakker, published a provocative article in Scientific American, asserting that all dinosaurs were warmblooded (endothermic), and the dinosaur renaissance (as Bakker aptly titled his article) had begun.
The dinosaur renaissance continues today, with more scientific attention lavished on dinosaurs than on any other group of fossil vertebrates, with the possible exception of the fossil hominids central to understanding human evolution. Bakkers 1975 article and others that followed sparked a lively debate about dinosaur metabolism that forced a reevaluation of long-cherished notions about the terrible lizards.
Compelling evidence from new kinds of analyses, such as histology the study of microscopic structure of dinosaur bone challenged old ideas about dinosaurian metabolism and activity levels. Ostrom, for example, estimated dinosaurian blood pressures (using the vertical distance between the heart and the brain as a proxy, which suggested some very high dinosaurian blood pressures) and reviewed where dinosaurs lived (they even extended into the poleward regions of the globe) to argue for some degree of dinosaur warmbloodedness.
The current consensus is that many dinosaurs (especially the small meat-eaters) had a metabolism akin to warmblooded mammals and birds, whereas others (the giant sauropods) were coldblooded behemoths whose gargantuan size guaranteed a nearly constant body temperature. To understand this range, we would do well to remember the variety of dinosaurian body sizes, from agile, chicken-sized predators to school bus-sized (and larger) herbivores. Much as living mammals from tiny shrews to blue whales display a range of metabolic activity determined in part by body size, dinosaurs employed a range of metabolic strategies.
Dinosaur posture and locomotion have also proved lively areas of discussion. Dinosaur footprints, long regarded as curiosities, now hold center stage as the best prima facie evidence of how dinosaurs walked (see story, this issue). They also provide the soundest basis for estimating dinosaur walking and running speeds. Analyses of dinosaur footprints supports the upright walking postures long inferred from skeletal evidence, but indicates rather slow to moderate walking speeds (5 to 8 kilometers per hour) for the average dinosaur. Prancing sauropods and galloping stegosaurs seem to be the excesses, not the well-established inferences, of the dinosaur renaissance.
The relationship of dinosaurs to birds looms large in current paleontological thinking about dinosaurs. Crocodiles are the closest reptilian relatives of dinosaurs, a conclusion well-accepted since the 1800s. But this relationship is a relatively distant one, like that of cousins. By contrast, the dinosaur-bird relationship is that of ancestor and descendent, a much closer relationship analogous to that between parent and child.
Paleontologists are almost unanimous in their agreement that the dinosaur-bird relationship mandates a more bird-like biology of dinosaurs than previously thought, and recent discoveries support that inference. For example, we now know that the furcula, or wishbone, which is the fused collarbones long thought to be unique to the avian skeleton, was present in several kinds of meat-eating dinosaurs, including the legendary Tyrannosaurus rex. And from 125-million-year-old Lower Cretaceous lake beds of northeastern China have come Caudipteryx (tail wing) and other small meat-eating dinosaurs sheathed in feathers. Indeed, new discoveries have truly blurred the distinction between dinosaurs and birds.
Refining the view
The story of the different images of the dinosaurs as lizards, reptiles and birds is a tale rooted in the discovery of new fossils. Although new kinds of analyses, such as bone histology, have also provided modern paleontology with novel insights not previously available, the discovery of new fossils drives paleontology. It was the discovery of complete dinosaur skeletons during the 1870s and 1880s that revised the Owen-Hawkins images of dinosaurs as lizards. Beginning in the 1960s, the discovery of Deinonychus and other bird-like dinosaurs drove acceptance of the dinosaur-bird link that proved to be pivotal to the current image of bird-like dinosaurs.
The current rapid pace of dinosaur discovery and research continues to sharpen this image of bird-like dinosaurs. Recent discoveries in Africa revealed bizarre new dinosaurs that are shaking the evolutionary tree so long accepted by paleontologists. And in 2003, Chinese paleontologists described Microraptor, a small (77-centimeter-long) meat-eating dinosaur from the Lower Cretaceous lake beds of northeastern China. Covered with feathers, Microraptor was an arboreal glider that well represents the evolutionary intermediate (missing link) between a ground-running, meat-eating dinosaur and its descendants, the earliest birds.
Through all this, the reptilian nature of some key aspects of dinosaur biology has been upheld. For example, dinosaurs reproduced by laying eggs and most had naked, scaly skins like those of living reptiles. Yet the dinosaur-bird link has forced a serious rethinking of much of dinosaurian biology and appearance that now pervades our current image of the terrible lizards.
And ongoing research on baby dinosaurs, and particularly on how and at what rate dinosaurs grew, promises a more profound understanding of dinosaur biology. Dinosaurs thus continue to be one of the hottest areas of paleontological research.
The dinosaur formerly known
Paleontologists who discover new dinosaurs also name them. A set of rules,
the International Code of Zoological Nomenclature, governs the scientific names
given to dinosaurs (and to all animals). These rules promote the consistency
and stability of scientific names. They thus mandate that all names be in Latin
and that the first name correctly proposed for an animal takes priority over
subsequently proposed names. The history of Brontosaurus one of
the most familiar of dinosaur names provides a classic example of the
code in action.