The Way of the Dinosaur

I hate the phrase “going the way of the dinosaur.” I cringe almost every time I see it. Political and business journalists are the worst offenders. When a politician begins to lose favor or a company is outmoded, such writers often draw a parallel between their subjects and the classic image of dinosaurs as stupid, swamp-dwelling brutes who ultimately lose life’s race to the quicker, smarter mammals. This metaphor has been around for a century, at least, and has its roots in a time when dinosaurs were thought to be creatures that became so big and fierce that they could no longer survive.

As music composer Deems Taylor explained before the prehistoric segment of 1940′s Fantasia, dinosaurs were once seen as “little crawling horrors,” “100 ton nightmares”, “bullies” and “gangsters.” Dinosaurs had come to rule the world through strength alone and evolution ultimately left them behind as imperfect monsters. The mystery wasn’t why dinosaurs died off, paleontologists believed, but how they had managed to dominate the planet for so long.

Some paleontologists believed that dinosaurs simply walked off the evolutionary stage when their time had run out. This was an extension of a weird idea known as “racial senescence”–a discarded idea that flourished during a time when paleontologists disagreed about the causes of evolution and extinction.

Even though Charles Darwin had beautifully articulated the idea of evolution by means of natural selection in 1859, and many naturalists subsequently agreed that evolution was a real phenomenon, natural selection was  frequently criticized. Some scientists were disgusted by the violence that seemed inherent in natural selection–the emphasis on competition for survival–and, alternatively, others argued that a gradual, stepwise process was not powerful enough to affect major change. As historian Peter Bowler has documented in books such as The Eclipse of Darwinism and The Non-Darwinian Revolution, late 19th and early 20th century naturalists often turned to alternative evolutionary mechanisms to explain fluctuations in form through time –bizarre, difficult-to-define forces that somehow dwelt inside organisms and drove the creation of new forms.

Racial senescence was one of these ideas. Paleontologist Richard Swann Lull explained the concept in his 1917 textbook Organic Evolution. Just as an individual creature was born, grew up, declined in health and expired, species also went through a similar pattern of birth, growth and decline. In fact, naturalists believed that there were tell-tale signs that a lineage was at death’s door. Following an outline by colleague Arthur Smith Woodward, Lull identified signs of “racial senescence” as a relative increase in size, a tendency for organisms to grow spectacular spikes and spines (old lineages no longer had the ability to control the wild growth of their skeletons, in his view) and a general pattern of “degeneracy”, such as the loss of teeth and other prominent characteristics.

Lull cited dinosaurs as examples of some of these trends. The immense Jurassic sauropods “Brontosaurus” and Brachiosaurus seemed perfect examples of increased size preceding extinction, as both were then believed to be among the last of their kind. (The wealth of fantastic Cretaceous sauropods we now know had not yet been uncovered.) Likewise, Tyrannosaurus–among the largest terrestrial carnivores of all time–lived at the terminal point of dinosaur history.

Stegosaurus was an even better example of senescence. Not only was the dinosaur large and apparently the last of its kind–at least as far as paleontologists knew circa 1917–but the dinosaur also displayed a “marvelous overgrowth of armor plates and tail spines which heightens the bizarrerie of this most grotesque of beasts.” Naturalist Charles Emerson Beecher tried to explain the mechanism by which this would lead to extinction in his book The Origin and Significance of Spines. Beecher considered spines and other ornaments to be outgrowth of “dead tissue,” and as a species accumulated such adornments there was less available space and energy for “living tissue.” Dinosaurs such as Stegosaurus and Triceratops, therefore, might have painted themselves into an evolutionary corner by developing wonderful armaments.

Regarding teeth, Lull explained that dinosaurs such as the sauropod Diplodocus and the ostrich-like Struthiomimus were suffering a reduction in the number of teeth–what the paleontologist thought was an almost sure sign the animals had reached evolutionary old age. Other naturalists concurred. Horatio Hackett Newman borrowed some of the same examples for his 1920 textbook Vertebrate Zoölogy, and considered dinosaurs such as the sauropods to be unfortunate geriatrics. Contrasted with species in their prime, Newman wrote than an old lineage “is characterized by sluggish behavior, by herbivorous habits or feeding habits involving little exertion, by structures on the whole specialized or degenerate, often by giant size or bulky build, and by accumulations of inert materials such as armor, spines, heavy bones or flesh.”

Yet the distribution of supposedly degenerate dinosaurs perplexed Lull. Some forms he identified as “senescent”–such as the stegosaurs and sauropods–supposedly slipped into extinction long before the final disappearance of dinosaurs as a group. Turtles and birds also underscored this problematic wrinkle–Lull considered that both turtles and birds were degenerate because they lacked teeth, yet turtles had been around longer than the dinosaurs and birds showed no sign of dying out. Nevertheless, Lull was confident that the dinosaurs had “died a natural death.” Their time had simply run out, although the puzzle was why such apparently unhealthy and degenerate creatures were able to survive for so long. Only mammals–creatures thought to be more evolutionary “advanced” than the dinosaurs–were thought to suffer rapid, catastrophic extinctions due to forces such as changing climate.

Dinosaurs seemingly couldn’t help themselves. They just got bigger and stranger until they simply could not change anymore. The “way of the dinosaur” was one of weird extravagance ultimately culminating in an extinction pre-ordained by evolutionary paths. Yet, even when such ideas were in fashion, contradictory evidence had to be overlooked.

According to the outline of racial senescence, dinosaurs should not have survived past the Late Jurassic, yet they thrived for millions and millions of years after the time of Apatosaurus and Stegosaurus. Additional fossil finds have also documented that many of the so-called degenerate lineages did not actually go extinct when Lull, Newman and their peers thought, and the recognition that natural selection is the primary driving force behind evolutionary change disintegrated muddled ideas about internal life forces and evolutionary life clocks. Dinosaurs did not die out because evolution programmed them to self-destruct. The Dinosauria was one of the most successful vertebrate lineages in all of history, and the ultimate extinction of the non-avian forms around 65.5 million years ago was simply an unlucky turn. At this point in time, paleontologists have turned Lull’s question on its head–we are getting a better idea of why dinosaurs dominated the planet for so long, and their ultimate disappearance has become ever-more perplexing.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.