The Rise and Fall of the Dinosaurs

by Steve Brusatte

  Counting the number of species or the abundance of individuals is easy. All you need are a good set of eyes and a calculator. But how to measure morphological disparity? How to take all the complexity of the animal body and turn it into a statistic? I followed the approach pioneered by the invertebrate paleontologists. It went something like this. I first came up with a list of all of the Triassic dinosaurs and pseudosuchians, as these were the animals that I wanted to compare. I then spent months studying the fossils of these species and made a list of hundreds of features of the skeleton in which they vary. Some have five toes, others have three. Some walk on four legs, others on two. Some have teeth, others do not. I encoded these features in a spreadsheet as zeros and ones, just as a computer programmer would. Herrerasaurus walks on two legs, state 0. Saurosuchus walks on four legs, state 1. At the end of nearly a year of work, I had a database with seventy-six Triassic species, each assessed for 470 features of the skeleton.

  With the long slog of data collection done, it was time for the math. The next step was to make what is called a distance matrix. It quantifies how different each species is from every other species, based on the database of anatomical characteristics. If two species share all features, then their distance score is 0. They are identical. If two other species share no characteristics, then their distance score is 1. They are completely different. For the in-between cases, let’s say that Herrerasaurus and Saurosuchus share 100 characteristics but differ in the 370 others. Their distance score would be 0.79: the 370 features they differ in divided by the 470 total features in the data set. The best way to envision this is to think of those tables in a road atlas, which give distances between different cities. Chicago is 180 miles from Indianapolis. Indianapolis is 1,700 miles from Phoenix. Phoenix is 1,800 miles from Chicago. That table is a distance matrix.

  Here’s the neat trick about a distance matrix in an atlas. You can take that table of road distances between cities, stick it into a statistics software program, run what is called a multivariate analysis, and the program will spit out a plot. Each city will be a point on that plot, and the points will be separated by distance, in perfect proportion. In other words, the plot is a map—a geographically correct map with all of the cities in the right places and distances relative to each other. So what happens if we instead input the distance matrix that encapsulates the skeletal differences of Triassic dinosaurs and pseudosuchians? The statistics program will also produce a plot in which each species is represented by a point, a plot that scientists call a morphospace. But really it is just a map. It visually shows the spread of anatomical diversity among the animals in question. Two species close together have very similar skeletons, just as Chicago and Indianapolis are comparatively near geographically. Two species at far corners of the graph have very different anatomies, like the longer distance between Chicago and Phoenix.

  This map of Triassic dinosaurs and pseudosuchians allows us to measure morphological disparity. We can group the animals in the plot by which great tribe they belong to—dinosaurs or pseudosuchians—and calculate which is occupying a larger swath of that map and therefore more anatomically diverse. In the same vein, we can further group the animals by time—Middle Triassic versus Late Triassic, let’s say—and see if dinosaurs or pseudosuchians were becoming more or less anatomically diverse as the Triassic progressed. We did that and came up with a startling result that we published in 2008 in a study that helped launch my career. All throughout the Triassic, the pseudosuchians were significantly more morphologically diverse than dinosaurs. They filled a larger spread of that map, meaning they had a greater range of anatomical features, which indicated that they were experimenting with more diets, more behaviors, more ways of making a living. Both groups were becoming more diverse as the Triassic unfolded, but the pseudosuchians were always outpacing the dinosaurs. Far from being superior warriors slaying their competitors, dinosaurs were being overshadowed by their crocodile-line rivals during the 30 million years they coexisted in the Triassic.

  PUT YOURSELF BACK in the tiny furry feet of our Triassic mammalian ancestors, surveying the Pangean scene as the Triassic was drawing to a close 201 million years ago. You would be seeing dinosaurs, but you wouldn’t be surrounded by them. Depending on where you were, you might not have noticed them at all. They were relatively diverse in the humid regions, where protosauropods got as large as giraffes and were the most abundant plant-eaters, but there carnivorous theropods and herbivorous to omnivorous ornithischians were considerably smaller and less common. In the more arid zones, there were only small meat-eaters, the herbivores and larger species being unable to tolerate the hyperseasonal weather and megamonsoons. There were no dinosaurs remotely approaching a Brontosaurus or T. rex in size, and all across the supercontinent they were living under the thumb of their much more diverse, much more successful pseudosuchian adversaries. You would probably consider the dinosaurs a fairly marginal group. They were doing OK, but so were many other newly evolved types of animals. If you were of a gambling persuasion, you would probably have bet on one of these other groups, most likely those pesky crocodile-line archosaurs, as the ones that would eventually become dominant, grow to massive sizes, and conquer the world.

  Some 30 million years after they originated, the dinosaurs had yet to mount a global revolution.

  3

  Dinosaurs Become Dominant

  Scottish sauropod

  Chapter Title art by Todd Marshall

  SOME TIME AROUND 240 MILLION YEARS ago, the Earth began to crack. True dinosaurs hadn’t quite evolved yet, but their cat-size dinosauromorph ancestors would have been there to experience the cracking—except there wasn’t much to experience, at least not yet. There may have been some minor earthquakes, but these probably wouldn’t have even registered with the dinosauromorphs, who were busy with more important things like fending off the supersalamanders and surviving the megamonsoons. As these dinosauromorphs gave rise to dinosaurs, the fracturing continued, many thousands of feet underground. Imperceptible on the surface, these fissures were slowly moving, growing, merging together, a hidden danger lurking under the feet of Herrerasaurus, Eoraptor, and the other first dinosaurs.

  The very foundation of Pangea was splitting, and with the blissful ignorance of homeowners who don’t realize there’s a creeping crack in their basement until their house comes tumbling down, the dinosaurs had no inkling that their world was going to dramatically change.

  As these earliest dinosaurs were evolving in fits and starts during the final 30 million years of the Triassic, great geological forces were tugging on Pangea from both the east and west. These forces—a planet-scale cocktail of gravity, heat, and pressure—are strong enough to make continents move over time. Because the pull was coming from two opposite directions, Pangea began to stretch and gradually become thinner, each small earthquake causing another tear. Imagine Pangea as a giant pizza, being torn apart by two hungry friends at opposite ends of the table: the crust becomes thinner until there is a rupture and it breaks into two. The same thing happened with the supercontinent. After a few tens of millions of years of slow and steady tug-of-war, east versus west, the cracks reached the surface, and the giant landmass began to unzip down its middle.

  It’s because of that ancient divorce between east and west Pangea that the seaboard of North America is separated from western Europe and South America sits apart from Africa. It’s why there is now an Atlantic Ocean, which didn’t exist until seawater rushed in to fill the gap between the separating tracts of land. Those forces and fractures over 200 million years ago shaped our modern geography. But there was more to it than that, because continents don’t just split up and call it a day. As with human relationships, things can get really nasty when a continent breaks up. And the dinosaurs and other animals growing up on Pangea were about to be changed forever by the aftereffects of their home being ripped in two.

  The problem boils down to this: as a continent tears, it bleeds lava. It’s nothing m
ore than basic physics. The Earth’s outer crust is pulled apart and thins, decreasing pressure on the deeper parts of the Earth. As pressure lessens, magma from the deeper Earth rises to the surface and erupts through volcanoes. If there is only a little rip in the crust—two small bits of a continent separating from each other, let’s say—then the effects aren’t too bad. You might get a few volcanoes, some lava and ash, some local destruction, and then eventually it stops. That kind of thing is happening in eastern Africa today, and it’s far from catastrophic. But if you’re slashing apart an entire supercontinent, then you approach the realm of apocalypse.

  At the very end of the Triassic, 201 million years ago, the world was violently remade. For 40 million years, Pangea had been gradually splintering apart, and magma had been welling underground. Now that the supercontinent had finally cracked, the magma had somewhere to go. Like a hot-air balloon rising through the sky, the liquid-rock reservoir rushed upward, broke through the shattered surface of Pangea, and gushed out onto the land. As with the volcanoes that had erupted at the end of the Permian Period some 50 million years earlier, causing the extinction that allowed dinosaurs and their archosaur cousins to get their start, these end-Triassic eruptions were different from any that humans have ever witnessed. We’re not talking Pinatubo here, with hot clouds of ash bursting into the sky. Instead, over a period of some six hundred thousand years, there were four big pulses of drama, when enormous amounts of lava would surge out of the Pangean rift zone like tsunamis from hell. I’m hardly exaggerating: some of the flows were, added up together, up to three thousand feet thick; they could have buried the Empire State Building twice over. In all, some three million square miles of central Pangea were drowned in lava.

  It goes without saying that this was a bad time to be a dinosaur, or for that matter, any other type of animal. These were some of the largest volcanic eruptions in Earth history. Not only did lava smother the land, but noxious gases that rode up with the lava poisoned the atmosphere and caused runaway global warming. These things triggered one of the biggest mass extinctions in the history of life, a mass die-off that claimed over 30 percent of all species and maybe much more. Paradoxically, however, it was also a mass extinction that helped dinosaurs break out of their early-life slump and become the enormous, dominant animals that stoke our imaginations.

  IF YOU’RE WALKING down Broadway in New York City and happen to catch a gap between the skyscrapers, you can see straight across the Hudson River to New Jersey. You’ll notice that the Jersey side of the river is defined by a steep cliff of drab brown rock, about a hundred feet high, studded with vertical cracks. Locals refer to it as the Palisades. During the summer it can be almost unrecognizable, engulfed by a dense forest of trees and bushes that somehow cling to the sheer slopes. Commuter towns like Jersey City and Fort Lee are perched on top of the cliff, and the western end of the George Washington Bridge is built deep into it, an ideal anchor for the world’s busiest overwater crossing. If you wanted to, you could walk along the Palisades for about fifty miles, from where it begins in Staten Island and extends along the Hudson to where it juts into upstate New York.

  Millions of people look at this cliff every week. Hundreds of thousands of people live on it. Few realize that it is a remnant of those ancient volcanic eruptions that tore apart Pangea and ushered in the Age of Dinosaurs.

  The Palisades is what geologists refer to as a sill—an intrusion of magma that pokes its way in between two layers of rock far underground, but then hardens into stone before it can erupt as lava. Sills are part of the internal plumbing system of volcanoes. Before they harden into rock, they are pipes, which transport magma underground. Sometimes they are conduits that bring magma to the surface; other times they are dead-end extensions of the volcanic system, cul-de-sacs that magma can’t escape from. The Palisades sill formed at the end of the Triassic, as Pangea was rupturing along what would become the eastern coast of North America, just a few miles from what is now New York City. It formed from those very magmas that were coursing up from the deep earth as the supercontinent broke into two.

  The magma that became the Palisades sill never made it to the surface. It never got to be part of those three-thousand-foot-thick lava sheets flushed out of the Pangean rift, the ones that engulfed ecosystems and belched out the carbon dioxide that would doom much of the planet. About twenty miles to the west the magmas did erupt, however, and the basalt rocks that formed from them can be seen in a low range of hills called the Watchung Mountains in northern New Jersey. Calling them mountains is generous—they’re just a few hundred feet high, and they cover a tiny area about forty miles north to south—but they are a beloved oasis of natural beauty within one of the most urbanized parts of the world.

  In the middle of the mountains is Livingston, a bedroom community of about thirty thousand people. In 1968 some folks discovered dinosaur footprints a couple of miles north of the town, in an abandoned quarry where red shales, formed in rivers and lakes near the ancient volcanoes, were being mined. There was a blurb in the local newspaper, which caught the eye of a mother, who told her fourteen-year-old-son, Paul Olsen, who was gobsmacked to learn that dinosaurs once lived so close to his home. He rounded up his friend, Tony Lessa, and they hopped on their bicycles and sped to the old quarry. It was no more than an overgrown, rock-strewn hole in the ground, but the discovery had caused a local sensation and several amateur collectors were already there, on the hunt for more tracks. Olsen and Lessa befriended some of the amateurs, who taught them the basics of fossil collecting: how to identify dinosaur footprints, how to remove them from the rock, and how to study them.

  The two teenagers became obsessed. They kept coming back to the quarry, and before long they were working late into the night, removing slabs of dinosaur footprints by firelight, even in the dead of winter. They had to go to school during the day, so the night was their only option. For over a year they toiled, outlasting the other rockhounds, who began to trickle away once the excitement of the discovery died down. The boys collected hundreds of tracks left by all kinds of creatures, including meat-eating dinosaurs similar to Coelophysis from Ghost Ranch, plant-eating dinosaurs, and some of the scaly and furry creatures that lived alongside. But the more they collected, the more they became dismayed: during their nighttime excavations, they were constantly interrupted by trucks illegally dumping trash, and while they were at school, unscrupulous collectors would often sneak into the quarry and poach footprints the boys hadn’t yet been able to remove.

  So what’s a 1960s teenager to do when his favorite fossil site is being destroyed? Paul Olsen skipped the middlemen and went right to the top. He began writing letters to Richard Nixon, the newly elected president who had yet to disgrace himself. Lots of letters. He begged Nixon to use his presidential powers to get the quarry preserved as a protected park, and even sent a fiberglass cast of a theropod track to the White House. Olsen led a media campaign, too, and was profiled in an article in Life magazine. His brazen persistence paid off: in 1970 the company that owned the quarry donated the land to the county, which made it into a dinosaur park called the Riker Hill Fossil Site. The next year, the site was granted official national landmark status and Olsen received a presidential commendation for his work. Little did he know it, but he was also an inch away from a White House visit. Some of Nixon’s image-conscious aides thought a photo op with a young science enthusiast would be great PR for the jowly president, but it was killed at the last minute by Nixon’s advisor John Ehrlichman, later one of the key villains of Watergate.

  It was a great accomplishment for a kid—collecting a haul of dinosaur tracks, getting his site preserved for posterity, becoming pen pals with the president. But Paul Olsen didn’t stop. He went to college to study geology and paleontology, completed a PhD at Yale, and was hired as a professor at Columbia University, across the Hudson from Riker Hill. He became one of the leading academic paleontologists in the world and was elected to the National Academy of Sciences, one
of the greatest honors for any American scientist. He also had the burden of being a member of my PhD committee, a far lesser honor, when I did my doctorate, in New York. During that time, he became one of my most trusted mentors, a brilliant sounding board for whatever crazy research ideas I had. For two years, I assisted him as he taught his popular undergraduate course on dinosaurs at Columbia, always oversubscribed by nonmajor students, who were seduced by the eminent scientist with a white Geraldo moustache prancing around with the enthusiasm that comes from several preclass energy drinks. Much of my ebullient, wildly animated lecturing style comes from watching Paul.

  Paul Olsen made his career by continuing what he started as a teenager. Much of his work has focused on those events that were occurring around the time dinosaurs were leaving footprints in New Jersey: the breakup of Pangea at the very end of the Triassic, the unimaginable volcanic eruptions, the mass extinction, and the rise of dinosaurs to global dominance as the Triassic transitioned into the subsequent Jurassic Period.

  Although he had no idea when he first cycled up to that quarry as a kid, Paul grew up in the best place in the world for studying the Late Triassic and Early Jurassic. His boyhood stomping grounds are within a geological structure called the Newark Basin, a bowl-like depression filled with Triassic and Jurassic rocks. It is one of many such structures—called rift basins, because they formed as Pangea rifted apart—extending for over a thousand miles down the eastern coast of North America. The Bay of Fundy, up north in Canada, laps onto one of these basins. Farther south is the Hartford Basin, which cuts through much of central Connecticut and Massachusetts. Then the Newark Basin, followed by the Gettysburg Basin, site of the famous Civil War battle, the topography of the rocks so instrumental in shaping military strategy that depended on securing bits of high ground. South of Gettysburg are many smaller basins that pepper the backcountry of Virginia and North Carolina, finally culminating in the huge Deep River Basin of the Carolina interior.