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 Iguanodon’s 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).
Marsh’s 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 dinosaur’s
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 History’s premier paleontologist and a protégé
of Cope, and Charles Knight, a remarkably talented painter and sculptor (see
story, this issue). Knight’s 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, Knight’s 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 Knight’s 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 Darwin’s
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 Ostrom’s 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. Bakker’s
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.
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |