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Dinosaurs are often thought of as kid’s stuff. In America, at least, going through a “dinosaur phase” is just another part of childhood, and somewhere along the way we’re expected to stop acting like walking encyclopedias to Mesozoic life. Yet this narrow view of dinosaurs as nothing more than pre-teen kitsch obscures the essential truths these animals can share with us about evolution, extinction, and survival.

As paleontologist Michael Novacek argues in the video above, the history of dinosaurs is also our history–our mammalian ancestors and relatives snuffled and scurried through a dinosaur-dominated world for more than 150 million years. We can’t understand where we came from without considering dinosaurs. And, says paleontologist Matt Bonnan, “Dinosaurs put our place in the world into perspective.” By asking questions about dinosaurs–when did they live and what was the world like then?–the history of life on Earth comes into focus, and the answers to these queries help us better understand the pervasive forces of evolution and extinction through time.

These critical aspects of nature can be difficult to detect on the timescales of our lives, but become much more apparent when we can peek into deep time by sifting through the remains of creatures that roamed the Earth long ago. An individual dinosaur discovery might not have any practical use or even significantly change our understanding of the past, but when considered together with the ever-growing body of research about dinosaurs, it can help us understand how we came to be on this planet and may even give us some clues about the future–how species emerge and decline, how creatures adapt, and how life evolves after catastrophic extinction events.

What do you think is the best case for the importance of studying dinosaurs?

For over a century, non-avian dinosaurs have been symbols of extinction. Our awe at their success, and our puzzlement at their ultimate demise, have made them perfect foils for our worries and fears. During World War I, for example, anti-war protestors cast dinosaurs as brutes who drove themselves into extinction by investing too much in their armor and weapons. Later, during the Cold War era, the asteroid strike that closed the Age of Dinosaurs was presented as a Mesozoic precursor to what mutual assured destruction might do to the planet. Not only have we looked to dinosaurs for lessons about what the future might hold, but we’ve also used them as icons of what might happen if we trade compassion for size and strength.

The 1967 Russian cartoon Mountain of Dinosaurs used extinction in a more specific and culturally subversive way. Rather than a literal lesson about dinosaurs–the fossil record doesn’t contain any hint that courting sauropods gave each other edible bouquets of ferns–the short warns about what happens if powerful stewards meant to care for individuals actually stifle those they are charged to protect. Dinosaurs didn’t die because of climate change, the short says, but because their eggs became so thick-shelled in response to colder temperatures that the baby dinosaurs couldn’t hatch. The shells (yes, the eggshells speak) mindlessly drone that they are doing their “duty,” but by growing thicker and thicker they kill the nascent sauropods. The scene is the saddest dinosaur cartoon I’ve ever seen, and it seems to be a metaphor for the Soviet government suppressing the rights of individual citizens. Indeed, the death of dinosaurs was not only used by Americans to issue dire warnings–they are an international symbol of extinction.

The end of the Cretaceous ended in one of the most catastrophic mass extinctions of all time. Among the various forms of life that were toppled were the non-avian dinosaurs. Triceratops and company didn’t exactly fall like dominoes, but, in this short video created by Flippycat.com, domino dinosaurs replay the epic destruction. And stay tuned for the behind-the-scenes video at the end. Just as the last non-avian dinosaurs had an evolutionary backstory stretching back millions and millions of years, it took a long time to set up the toy dinosaurs for their downfall.

Disease has often been blamed for the extinction of the last dinosaurs, such as this Edmontosaurus at the Natural History Museum of Los Angeles. Photo by the author.

There are more than 100 hypotheses for the extinction of the dinosaurs. Asteroid impact is the most famous, and the effects of volcanic eruptions, sea level change and climate fluctuations remain debated, but other fantastic and weird ideas have been tossed around. Many of the discarded notions, proposed before we knew an extraterrestrial bolide struck the Yucatán Peninsula, cited pathologies as the deciding factor. Cataracts, slipped discs, epidemics, glandular problems and even a loss of sex drive have all been proposed as the reason non-avian dinosaurs perished about 66 million years ago. In fact, pioneering paleopathologist Roy Moodie suggested that a startling number of accidents and injuries could have killed Triceratops and kin.

Moodie wrote an initial report, Studies in Paleopathology, in 1917 and followed with a full book called Paleopathology in 1923. The books are surveys of fractures, infections, arthritis and other pathologies visible in fossils. And after examining these cases, he created a graph of injury and ailment incidence over time. Dinosaurs and their reptilian neighbors seemed to have a rough time. Bone breaks, infections and other pathologies “reached a maximum of development among the dinosaurs, mosasaurs, crocodiles, plesiosaurs and turtles,” and the curve dropped off only when the Mesozoic “Age of Reptiles” ended. The increasing occurrence of pathologies may have driven dinosaurs into extinction. “It seems quite probable,” Moodie wrote, “that many of the diseases which afflicted the dinosaurs and their associates became extinct with them.”

Dinosaurs really did suffer from a variety of ailments. Dinosaurs scratched at parasites, endured bone infections, and even developed cancer. But we now know that there wasn’t a dramatic uptick in dinosaur sickness between the Triassic and Cretaceous. There is no sign that pathologies did in the dinosaurs, and this hypothesis doesn’t explain why so many other creatures—from the seagoing lizards known as mosasaurs to coil-shelled ammonites—disappeared at the same time. Focusing on dinosaurs too narrowly hides the true pattern of extinction. Exactly what happened at the close of the Cretaceous will remain hotly debated for decades to come, but dinosaur disease no longer figures into the discussion.

Small coelurosaurs like this Troodon appear to have maintained stable levels of disparity during the last 12 million years of the Cretaceous. Image courtesy AMNH/J. Brougham.

Why did the non-avian dinosaurs become extinct? There’s no shortage of ideas, but no one really knows. And even though paleontologists have narrowed them down to a short list of extinction triggers—including an asteroid strike, massive volcanic outpouring, sea level changes and climate alterations—how these events translated into the extinction of entire clades of organisms remains hotly debated.

One of the most contentious questions is whether dinosaurs thrived right until the end of the Cretaceous, or whether they were already declining before the lights went out. Based on species counts, mostly from the roughly 66-million-year-old rock of western North America’s Hell Creek Formation, it might seem that dinosaurs were not quite as diverse as they were in the same area 10 million years earlier. But detecting this decline depends on how species are identified and counted—a quirk affected by how we distinguish dinosaurs and other organisms known only from fossils. If we recognize that Triceratops and Torosaurus were separate dinosaur genera, for example, there were two big ceratopsids present in western North America at the end of the Cretaceous. But if we start from the position that the dinosaurs we call Torosaurus were really the skeletally mature form of Triceratops, then ceratopsid diversity is cut in half. And even the best circumstances, the fossil record is an imperfect catalog of prehistoric life that we are only sampling a few pieces from. Determining diversity by taking species counts is not as simple as it sounds.

In a Nature Communications paper published today, paleontologists Stephen Brusatte, Richard Butler, Albert Prieto-Márquez and Mark Norell take a different approach. Rather than track species and genera, the researchers followed trends in morphological disparity—how the forms of dinosaurs varied across seven major groups, both globally and regionally. Differences in form translate to differences in lifestyle and behavior, mostly avoiding tangled taxonomic arguments, and this technique gauges how many forms of dinosaurs were present at a given time. This is a proxy to detect which groups of dinosaurs might have been thriving and which were declining over time.

Disparity trends in four dinosaur groups during the final 12 million years of the Cretaceous (North American species only). Time (from 77-65 million years ago) is shown on the x axis. The y axis shows the disparity metric: sum of variances derived from anatomical character databases. The error bars indicate whether comparisons between time intervals are significant or not (overlap of error bars means non-significance, no overlap means significance). Overall, the large-bodied bulk-feeding ceratopsids and hadrosauroids underwent a marked long-term decline, but the carnivorous coelurosaurs and small herbivorous pachycephalosaurs were stable. (AMNH/S. Brusatte)

Brusatte and co-authors tracked disparity trends among ankylosaurs, sauropods, hadrosauroids, ceratopsids, pachycephalosaurids, tyrannosauroids and non-avian coelurosaurs during the last 12 million years of the Cretaceous (from the Late Campanian age to the Maastrichtian). There was no simple pattern that held true for all dinosaurs—some groups stayed the same while others declined. The heavily armored ankylosaurs, dome-headed pachycephalosaurs, formidable tyrannosaurs and small, feathery coelurosaurs didn’t seem to show any major changes in disparity over this span. And the massive, long-necked sauropods showed a very slight increase in disparity from the Campanian to the Maastrichtian. Both locally and globally, these dinosaur groups were not dwindling away.

The shovel-beaked hadrosaurs and horned ceratopsids showed different trends. Horned dinosaurs suffered a significant drop in disparity between the Campanian and the Maastrichtian, at least partially attributable to the disappearance of an entire ceratopsid subgroup. During the Campanian, both centrosaurines (like Centrosaurus) and chasmosaurines (like Chasmosaurus) roamed North America, but by the Maastrichtian, only the chasmosaurines were left. And while hadrosaur disparity dipped slightly from a global perspective, the pattern differed between continents. In Asia, hadrosaurs appear to show very slight increases in disparity, but North American hadrosaurs suffered a sharp decline across the 12-million-year study range. What was true for North American dinosaurs was not necessarily true for the rest of the world.

“Compared with previous studies that focused on species richness or faunal abundance,” Brusatte and colleagues write, “these disparity calculations paint a more nuanced picture of the final 12 million years of dinosaur history.” The idea that dinosaurs, as a whole, were either thriving or declining is a false dichotomy. The last twelve million years were clearly a time of flux—especially in North America, where some dinosaur groups stayed stable but the largest, most abundant herbivores were not as varied as their predecessors had been.

That sauropod dinosaurs increased in disparity at the end of the Cretaceous is especially noteworthy. When I was a kid, sauropods were often cast as Jurassic titans that were replaced by dinosaurs with superior plant-shearing abilities, such as certaopsids and hadrosaurs. Yet sauropods hung on, and as the horned and shovel-beaked dinosaurs declined, sauropods might have again been expanding. We will never know what would have happened had the Cretaceous extinction been canceled. Although, if the non-avian dinosaurs had been given a reprieve from extinction, we almost certainly wouldn’t have evolved to ponder what happened so long ago.

As this study points out, it is a mistake to think of dinosaurs as a monolithic group. The pressures behind dinosaur evolution, and the reasons for their extinction, varied from group to group and place to place. The more we learn about them, the more complex their history becomes. And there’s still much we don’t know. To date, most of what we think we understand about the extinction of the non-avian dinosaurs comes from western North America—relatively accessible sites that record the transition from the last days of the dinosaurs to a world dominated by mammals. These sites, no matter how well we study them, can only be a small part of what was a global extinction, and what we find in North America may not be representative of the rest of the planet. “It may be,” Brusatte and collaborators write, “that the North American record represents a local anomaly,” with “extreme fluctuations of the inland Western Interior Sea, mountain building, and proposed biogeographic provincialism” influencing dinosaur evolution in a unique way not seen on other continents.

If we want to understand the evolution and extinction of the last dinosaurs, we need to take a more refined, localized approach and not think of dinosaurs as a uniform group. For as much ink has been spilled about dinosaur evolution and extinction, we are still only beginning to piece together a picture of what the final days of the Cretaceous were like.

Reference:

Brusatte, S., Butler, R., Prieto-Márquez, A., & Norell, M. (2012). Dinosaur morphological diversity and the end-Cretaceous extinction Nature Communications, 3 DOI: 10.1038/ncomms1815