The Most Famous Extinction

Jingo the Stegosaurus was all armor plate and no brains. That’s the slogan that the pacifist Anti-Preparedness Committee attached to the ponderous reptile, in any case. A symbol of foolish investment in offensive and defensive weaponry rather than intellect, the spiky-tailed mascot was a portent of what anti-WWI protesters believed would become of the United States if warmongers had their way – total and irreversible extinction. And from the time of Jingo on, we’ve often looked at dinosaurs as the ultimate symbols of extinction. In so doing, we’ve thoroughly misunderstood dinosaurs.

Political caricature though he was, there was some scientific backing to Jingo. At the time, decades after Charles Darwin’s and Alfred Russel Wallace’s promoted the idea of evolution by natural selection but still years before that natural mechanism was generally accepted as being the main driver of transmutation, paleontologists tried to understand the rhythm of evolution and extinction according to internal drives and seemingly pre-ordained evolutionary trajectories. Both the perplexing manifestation of creatures as odd as the dinosaurs, and their ultimate oblivion, were seemingly the products of such unseen urges.

Early dinosaurs were small and relatively mundane, paleontologists observed, while later forms were enormous and often bristled with spikes, plates, horns, and other intimidating accoutrements. To experts such as Richard Swann Lull, this trend indicated that some kind of internal driver that forced dinosaurs to become bigger and more bizarre, investing more of their vital energy in developing monstrous bodies while their mental faculties remained unimpressive. (The brain of the bulky, 30-foot-long Stegosaurus, for one, was said to be about the size of a walnut.) By trading size and ferocity for cerebral capability, dinosaurs effectively evolved themselves into a corner. If they didn’t go extinct by simple virtue of their excess, then the smarter, more enterprising mammals in the undergrowth quickly out-competed the tottering, sluggish reptiles.

A nice and neat morality fable, worthy of Aesop, but even Lull was perplexed by inconsistencies in the story. Dinosaurs persisted for a great span of Earth history, and some of the largest and most extravagant forms – such as Stegosaurus and the 80-foot, long-necked, small-skulled Brachiosaurus – lived right in the middle of the so-called Age of Reptiles, only to be replaced by dinosaurs such as Tyrannosaurus and Triceratops that were just as enormous and weird. What researchers thought of as the dinosaurian interlude – a long intermission where reptiles unjustly kept our mammalian ancestors and kin down – stretched on far long than would seem probable for animals believed to be victims of their excess. “[T]he marvel is, not that [the dinosaurs] died,” Lull observed, “but that they survived so long.”

In time, the self-destructive tendencies Lull and other researchers toyed with were recognized as fantasy and the undirected, unpredictable influence of natural selection became established as the prime force behind evolutionary change. Yet the mystery of dinosaur extinction remained, and the puzzle became a fertile playground for anyone with a notion for why they disappeared. By the middle of the 20th century, the question of the great dinosaur disappearance was open to all comers. Paleontologist Michael Benton has rightly called this the “Dilettante Phase” of dinosaur extinction studies, when any armchair expert of whatever discipline could forward their pet notion for the Mesozoic calamity.

As they had been since the early 20th century, dinosaurs were still viewed as aberrations that ruled the planet for an unjustly long amount of time. And instead of being broken down from within, they were thought to have been brought to their knees from without – too stupid, slow, and large to adapt and survive. Mammals clearly had the right qualities to persist and thrive in a changing world. Else we wouldn’t be here to mull over all this history. Then again, depending on who you asked, dinosaurs might have been wiped out by lack of sex drive, cataracts, the climate getting too hot, the climate growing too cold, radiation from a nearby supernova, and, perhaps most imaginative of all, slow starvation after caterpillars ate all the Mesozoic plant life.

Researchers inside and outside of paleontology continued to spin wild ideas as dinosaurs at long last received a major makeover in the 1970s and 80s – the “Dinosaur Renaissance.” This major pulse in argument and research was spurred by Deinonychus – a roughly human-sized, bird-like, switchblade-clawed carnivore of the same archetype as Jurassic Park’s villainous Velociraptor. Yale paleontologist John Ostrom, who described the Cretaceous predator in 1969, could not believe that such a lithe, menacing predator was an idiotic tail-dragger (least of all because Deinonychus had a tail stiffened by specialized tail bones, creating a sturdy counterabalance for the rest of the carnivore’s body). And as this dinosaur changed perceptions about what dinosaurs were, the conundrum of dinosaur extinction became stranger still.

The time had come to follow through with an idea that researchers had been toying with for decades – maybe dinosaurs weren’t actually sluggish reptiles writ large. Perhaps their biology and behavior were more unusual than anyone had supposed, and, based on the similarities between Deinonychus and Archaeopteryx (the earliest known bird), dinosaurs emerged as the best candidates for avian ancestors. And if birds could be counted as living dinosaurs, maybe avian traits – such as feathers, a hot-running metabolism, parental care – originated among creatures like Apatosaurus. In time, fossil discoveries and analysis of old bones would make dinosaurs far more bird-like than previous generations of researchers could have anticipated.

The Dinosaur Renaissance didn’t solve many of the questions the debate itself raised. Some aspects of dinosaur lives – such as their physiology – remain mysterious and hidden by the gulf of time between us and their kind. But the renewed interest in dinosaur lives caused many researchers to question what had always been assumed about the charismatic prehistoric celebrities, and dinosaurs metamorphosed into highly-active, behaviorally-nuanced creatures that dominated the planet for over 160 million years before suddenly disappearing at the height of their diversity. Dinosaurs were not a cautionary tale of investing too much energy in size or armaments, nor were they evolutionary mistakes that were given a perplexingly and unfairly long tenure on the planet. If dinosaurs really were so wonderful, and their demise could not be attributed to any kind of inherent inferiority, then why did the entirety of the non-avian dinosaur family tree disappear?

Elsewhere in the annals of paleontology, other researchers were carrying on in a similar spirit. While Ostrom and many others started focusing on dinosaurs as living animals, not just osteological aberrations, researchers in other parts of the field were fomenting what historian David Sepkoski has called the “Paleobiological Revolution.” Paleontologists were no longer content with just cataloging the past, and, tired of being derided as stamp-collectors by geneticists and members of other disciplines, a cadre of paleontologists vied to turn their discipline into a theory-producing branch of science that could explain essential aspects of evolution and extinction. Stephen Jay Gould, Niles Eldredge, Elisabeth Vrba and many others pushed this new form of paleobiology forward, as well as researchers such as Jack Sepkoski and David Raup who focused on trying to comprehend the major pulses of extinction through time and their influence on life on Earth. The end-Cretaceous catastrophe was one of these pivotal events. Mass extinction stopped being just a footnote in the history of life on Earth – an occasional and expected clearing of the evolutionary slate – but something puzzling and worth exploring in detail as our own fears of overpopulation, global warming, and mutually-assured nuclear destruction came to the fore.

As a reinvigorated paleontology bubbled and frothed with new ideas and arguments, the geologist Walter Alvarez turned his attention to the band of clay-rich rock between the very end of the Cretaceous and the beginning of the Paleocene preserved at a site in Italy – a thin layer that’s currently called the K/Pg boundary. Beyond the various questions about why certain forms of life either passed through this barrier or failed to do so, Alvarez was attracted to the conundrum of how long this thin slice of time represented. Perhaps, if he could find a proper timer, he could understand the speed of the disaster.

Together Alvarez, his physicist father Luis, Frank Asaro, and Helen Michel decided to track the amount of time the band of rock represented with an element called iridium. This platinum-like metal is rare in sedimentary rock on Earth, but is relatively common in the innumerable extraterrestrial bits of stone that fall to the planet every day. If these pieces spattered our planet at a near-constant rate, which the Alvarez team believed to be the case, then the amount of iridium in the clay K/Pg band could be used to come up with an estimate of how long the slice of rock took to be deposited, and, therefore, how long the end-Cretaceous extinction took to transpire.

The iridium levels the researchers gauged in the rock were exceedingly high. This was not an anomaly. Equivalent geologic seams between Cretaceous and Paleocene around the world showed a similar pattern. To the Alvarez team, this massive global influx of iridium pointed to an unexpected conclusion and what had previously seemed an unlikely cause of extinction – all that iridium was delivered by a massive asteroid impact. The Alvarez team published their new hypothesis for dinosaurian demise in 1980.

Paleontologists bridled at the hypothesis. Here was a team of geologists and physicists, none of whom were experts on prehistoric life, who had proposed a clean, simplistic explanation for one of the more unfathomable and complex events in our planet’s history. And still somewhat constrained by the intellectual straightjacket of strict uniformitarianism, researchers argued that there was no evidence that such impacts regularly happened after the “early bombardment” of meteors early in Earth history. Plus, the Alvarez team failed to provide an appropriately-large crater of the right age, or enormous chunk of space rock in terminal-Cretaceous rock to prove their point. The iridium band was the hint of an impact that was otherwise unseen.

The planet-scale bolide wound was soon found in what is now the Yucatan Peninsula. At 110 miles wide, and found at just the right rock layer, the crater was of the right size and age for an asteroid that would have rocked the planet. And once the site of impact was identified, researchers of various disciplines began to investigate and speculate about how the asteroid struck and the cataclysmic aftermath. The fact that the asteroid seemed to hit sulfur-laden sediments was especially important. After the localized impact and the bits of heated rock and glass speherules sent back into the atmosphere, gases released from the pulverized rock would have triggered global cooling and the fall of acid rain as the world became wrapped in a dust cloud, blocking the sun and setting off a quick degradation of life wherein plant life perished, robbing herbivores of sustenance before carnivores soon found themselves without sufficient food. Those species that were able to burrow underground, aestivate, or be flexible in their diets were those that survived. Or so the story goes.

The impact hypothesis immediately became the public face of dinosaur extinction. Dinosaurs died out simply because of cosmic bad luck. Only something as catastrophic as a giant asteroid striking the planet could have ended their reign. Quite a turnaround from the days of poor Jingo. But, as paleontologists pointed out, the confirmed asteroid strike was not the only major source of stress the Cretaceous world suffered. The global climate was changing, becoming cooler, as seaways and shorelines shifted. And for thousands of years, volcanoes called the Deccan Traps in what is now India poured out a truly fantastic amount of lava and greenhouse gases. These phenomena were already occurring when the asteroid struck. The question is which trigger – or combination of triggers – catalyzed the great Cretaceous die-off.

Even though paleontologists and geologists have identified the array of extinction triggers, we still know little about how these might have translated into mass deaths. And while the argument isn’t as high-profile as it was in the 1980s, academics are still tussling over the matter. In 2010, to celebrate the 30 year anniversary of the first impact hypothesis study, the journal Science ran a position paper carrying the names of 41 scientists reaffirming that asteroid strike was the primary cause of the end-Cretaceous extinction. The journal soon published a spate of responses from other researchers, with vertebrate paleontologists – many of whom are dinosaur experts – arguing that asteroid impact probably played a prominent role in the extinction, but was not the sole cause.

The popularity of the impact hypothesis has been difficult to beat. The concept is deceivingly simple, and is so far outside the realm of human experience that we can scarcely believe that a giant rock could strike the planet at speed and not cause disaster. The event seems like a celestial gunshot that deeply scarred the planet. And such an event might be repeated. When a meteorite broke up over Chelyabinsk, Russia, the same day that a large asteroid came within 17,100 miles of Earth, reporters were quick to jump on the connection between the day’s events and the fate of the dinosaurs. An asteroid striking the planet and rapidly changing the course of evolutionary history is a sexier idea than the thought of dinosaurs suffering a slow, suffocating death by climate change or other repeated pressure in the history of life on Earth. Despite this, though, researchers as yet know little about what actually happened in the aftermath of the impact. Our obsession with dinosaurs may even hinder our attempts to understand how such biological devastation unfolded.

The conclusion of the dinosaur story has often, and wrongly, been considered in isolation. We have been so entranced by their celebrity status that we can easily forget that the extinction of most dinosaurs was part of the last great mass extinction of all time – a devastatingly rapid event that eliminated 75% of known fossil species. Entire lineages of organisms vanished in a virtual instant. Giant, seagoing, and absolutely terrifying cousins of the Komodo dragon called mosasaurs; leathery-winged flying reptiles known as pterosaurs; coil-shelled cephalopods dubbed ammonites; and reef-building clams labeled rudists were among the groups to totally vanish. Even larger groups that survived lost many lineages – family trees that were not totally uprooted, but nonetheless pruned back. There were mass extinctions of lizards, snakes, birds, and even mammals.

In the tally of extinction, we often separate the evolutionary winners from the losers at the level of larger groups. We often say that dinosaurs, pterosaurs, mosasaurs, and other forms of Mesozoic life died out, while mammals and other scrappy little critters hung on. The truth is not nearly so simple.

Consider our own ancestors and cousins that witnessed the event. Mammalian life almost ended with that of the non-avian dinosaurs as various mammal lineages were stripped back. This happenstance fundamentally changed the course of evolution. Had the extinction not occurred, marsupials – pouch bearing mammals related to today’s kangaroos and opossums – might have remained the dominant form of mammal, therefore altering the evolution of our placental ancestors and closing off the possibility that humans would ever evolve. (To those who assert humanity is an inevitability, consider that anything that could be considered humanoid has only evolved once in the entire history of life on Earth, and even then we are the very last species of human a paltry six million years after the first hominins originated.)

Dinosaurs made the end-Cretaceous extinction famous, but in order to understand what happened 66 million years ago we need to take a broader view and consider the various other animals that suffered the same fate. The most informative clues about what the dinosaurs disappeared may not rest with the great ruling reptiles themselves, but with smaller creatures whose collected remains might record the pulse of extinction in higher resolution.

Still, dinosaurs remain the unquestioned, tragic stars of the event. And as part of that, the old image of dinosaurs as Mesozoic idiots deserving of extinction still looms over our imagination and can blind us to the truth of what transpired and what remains unknown. Beyond the fact that the event has been re-dated to 66 million years ago, and we know that one dinosaurian lineage survived in the form of birds, paleontologists have yet to agree about how asteroid impact, intense volcanic activity, and global climate change conspired together to trigger the extinction of populations, species, and entire groups of organisms. Since no human has ever observed such an impact on our own planet – thank the Flying Spaghetti Monster for that – the most we can do is create models and put forward conjecture that nevertheless leave us coming up short on determining why Tyrannosaurus, Triceratops, and so many other organisms died out.

We are really only just beginning to understand how the curtain fell on the Cretaceous. Paleontologists and geologists are trying to comprehend a global event, and most of what we know about who lived and who died comes from a relatively narrow swath of rock in western North America. Montana is not a microcosm of the entire planet. There is still a great deal of basic data-collection that needs to be done before we even start to understand how deep the death toll went. From there, we need ever-more refined ways of understanding dinosaurian biology to outline clues as to why the non-avian forms all perished, while birds managed to persist. What was the biological chink in the non-avian dinosaur armor that made them more susceptible to such intense ecological pressure? Given that many of the most basic questions about dinosaur biology are as-yet-unresolved, paleontologists still have a great deal of work to do in translating ancient bones into restorations of living animals. Yet, despite the uncertainty, a new vision of dinosaurs has emerged from our struggle to comprehend their fate.

Dinosaurs were not doomed to extinction from the start. Counting birds among their ranks, the group has thrived for over 245 million years. “Going the way of the dinosaur” should be abandoned as a metaphor of inevitable extinction due to inflexibility and small brain size, as is often used by politics and tech writers. No, the way of the dinosaur is survival through the luck of circumstance. Exactly what happened 66 million years ago is a persistent mystery that experts will continue to debate for decades to come, yet, as we argue, the remaining lineage of dinosaur continues to thrive in feathery magnificence. Dinosaurs may have earned their fame as prehistoric sideshow freaks, but they have traded that mantle of deserving doom for one of beautiful evolutionary success against all odds. Long live the dinosaur.

[This piece was originally written as a draft for Nautilus, and includes elements I previously wrote about in My Beloved Brontosaurus and a Slate piecea Slate piecea Slate piece on our revised view of the K/Pg extinction.]