Hot-blooded dinosaurs and textbook cardboard

Our understanding of dinosaurs today is a far cry from the massive, crocodile-like beasts envisioned by Richard Owen and William Buckland, but the way in which ideas about dinosaurs held by earlier paleontologists are presented has been troubling me lately. In many documentaries it is fashionable to say that dinosaurs were traditionally viewed as big lizards, making them slow, dumb, and cold-blooded animals, but the more I have read about the early days of paleontology the more I've come to doubt that such generalizations can really be maintained.

I should probably preface my remarks by saying that I have by no means conduced an exhaustive search of early paleontological literature, but the actual history of dinosaur restorations certainly differs from the standard version. Perhaps it is most fitting to start things off with Iguanodon (discovered in 1822 and first described in 1825), a dinosaur originally conceived as being an enormous iguana. This was primarily due to the fragmentary nature of the earliest fossil discoveries of this animal and the similarity of the teeth to that of living lizards, and so it became relatively easy to create a super-sized lizard.

By 1842, however, Richard Owen gave Iguanodon something of a makeover, keeping the overall form (although somewhat shorter than some of the fanciful estimates that envisioned Iguanodon as being over 100 feet long) but making it much more mammal-like. Indeed, Owen's vision of dinosaurs had the creatures carrying their legs underneath their bodies, making Iguanodon look more like a scaly rhinoceros than a glorified lizard. (As Adrian Desmond notes in his book Archetypes and Ancestors, we know exactly how Owen viewed dinosaurs as he was a major force in planning the form of the dinosaurs that would appear at the Crystal Palace in 1854.)

There were anatomical reasons for the changes, but the new design of dinosaurs also was a strategic attack on the "transmutationist" ideas of Jean-Baptiste Lamarck; Owen stated that dinosaurs probably had near "perfect" circulatory systems with four-chambered hearts as Lamarck used cardiovascular criteria to help arrange his concept of the arrangement of life. To Owen, dinosaurs were the "highest" of reptiles that approached mammals in a number of characters, and the living reptiles of his time were mere degenerates in comparison to Iguanodon and Megalosaurus. In 1878 the famous Bernissart Iguanodon specimens were found in side a coal mine in Belgium and completely revolutionized the way this dinosaur was reconstructed, but even earlier discoveries from North America anticipated the Bernissart discoveries.

His curiosity sparked by some bones that had been dug from a New Jersey marl pit in 1838, William Parker Foulke uncovered the first recognized remains of Hardosaurus in 1858, the bones eventually being described by the naturalist Joseph Leidy in 1860 (although his description was not published until 1865). Three years later, Benjamin Waterhouse Hawkins (the artist behind the Crystal Palace dinosaurs) helped to create a mount of what Hadrosaurus may have looked like for the Academy of Natural Sciences, but this dinosaur was quite different from the Iguanodon he had created in the previous decade. Hadrosaurus was a bipedal dinosaur, not a reptilian pachyderm, and this discovery (along with E.D. Cope's diescription of "Laelaps" in 1866) arrived right on time to support some of T.H. Huxley's views of the relationship between reptiles and birds.

Since 1863 Huxley had been teaching his students that birds and reptiles were closely related (grouped together within what he called the Sauropsida), and to further the connection he proposed that dinosaurs had "hot blood" as well as hearts and lungs that were more avian than reptilian. (Around this time it was also proposed that pterosaurs were hot-blooded, particularly since some considered them the best candidates for the ancestors of birds.) Bipedal, hot-blooded dinosaurs with bird-like circulatory systems fitted Huxley's program much better than the ponderous creatures Owen envisioned, and the discovery of the small dinosaurs Compsognathus (described in 1858) and Hypsilophodon (described by Huxley in 1869) did away with the problem of birds being so small while dinosaurs were so large. (It should be borne in mind, though, that Huxley did not think that birds evolved from dinosaurs, only that dinosaurs approached the ancestral type from which both groups evolved and were therefore "persistent" types.)

On the other side of the Atlantic other paleontologists were recognizing the similarities between birds and dinosaurs, too. In 1868 E.D. Cope published a paper on "The Fossil Reptiles of New Jersey" in which he stated that Hadrosaurus and "Laelaps" approached mammals and birds in a number of characters (even though they were overwhelmingly reptiles). Most telling is Cop's description of "Laelaps," a dinosaur he saw as the ultimate terror of ancient New Jersey shorelines;

We can, then, with some basis of probability imagine our monster carrying his eighteen feet of length on a leap, at least thirty feet through the air, with hind feet ready to strike his prey with fatal grasp, and his enormous weight to press it to the earth. Crocodiles and Gavials must have found their bony plates and ivory no safe defence, while the Hadrosaurus himself, if not too thick skinned, as in the Rhinoceros and its allies, furnished him with food, till some Dinosaurian jackalls dragged the refuse off to their swampy dens.

This carnivore, then, is an interesting link between those of the mammalian series, and the carnivorous birds.

This image was most strikingly brought to life by the artist Charles R. Knight who (under Cope's supervision) created this striking image of "Laelaps" shortly before the paleontologist died in 1897;

All of this does not mean that dinosaurs were unquestionably seen as being warm-blooded, though. Popular restorations often portrayed them as active animals, often fighting each other, but the scientific debate over whether they were warm-blooded or not was certainly contentious (and it seems like it slipped into the background until the description of Deinonychus in 1969). In the 19th century the issue was heavily involved with the concept of evolution, the warm-bloodedness of dinosaurs being more important in linking groups than figuring out the physiology of the animals themselves.

19th and early 20th century reconstructions face us with something of a paradox, then. There was plenty of debate over the physiology and affinities of dinosaurs within scientific circles, but the animals were often restored in active poses. Many sauropods were shown lurking in swamps and theropods dragged their tails, that is true, but dinosaurs appear to have been viewed as unique and dynamic animals (and not merely big lizards) from 1842 onwards. If this is true, can we really continue to say that our intellectual forebears saw them as giant iguanas and nothing more? I don't think so. The history of this topic is far more complex than is often acknowledged, so much so that a short post that I was hoping would only be a few paragraphs has ballooned up into this monster. Such is the price of tossing out textbook cardboard.