October 5, 2012
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.
June 15, 2012
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.
May 1, 2012
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.
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.
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
April 18, 2012
How did dinosaurs come to rule the Mesozoic world? No one knows for sure, but the way dinosaurs reproduced probably had something to do with it. Dinosaurs grew fast, started mating before they hit skeletal maturity, and laid clutches of multiple eggs—a life history that may have allowed dinosaurs to rapidly proliferate and diversify. And egg laying itself may have been critical to why many dinosaurs were able to attain gigantic sizes. By laying clutches of small eggs, dinosaurs may have been able to sidestep biological constraints that have limited the size of mammals.
But there was a catch. Consider a large dinosaur, such as Diplodocus. Infant Diplodocus hatched out of eggs roughly the size of a large grapefruit, and if they were lucky, the dinosaurs grew to be more than 80 feet long as adults. And the little sauropods were not just small copies of adults. Like many other dinosaurs, individual Diplodocus changed drastically during their lives, and young dinosaurs may have preferred different habitats and food sources from those of more mature individuals. As outlined by Daryl Codron and co-authors in a new Biology Letters paper, this peculiar life history may have been a consequence of laying eggs.
Codron’s group created a virtual dinosaur assemblage to see how intensely dinosaurs might have competed with one another as they grew. If all dinosaurs started off relatively small, then the largest species had to pass through a series of size classes and change their ecological role as they matured. This ramped up the pressure on young dinosaurs. Juvenile dinosaurs had to contend with other juveniles as well as dinosaurs that topped out at smaller sizes. In a diverse Late Jurassic ecosystem, for example, young Allosaurus, Torvosaurus and Ceratosaurus not only had to compete with one another, but also with smaller carnivores like Ornitholestes, Coelurus, Marshosaurus and Stokesosaurus. Dinosaurs would have faced the most competition at small size classes, and this may have driven some dinosaur lineages to become large.
The new paper also suggests that dinosaur life history may have played a role in the demise of the non-avian species. Competition at smaller size classes, Codron and colleagues suggest, drove dinosaurs to become bigger and bigger, and this created a lack of species that were small at maturity. Mammals and avian dinosaurs occupied those niches. This could have made dinosaurs more vulnerable to the intense pressures of the end-Cretaceous extinction. If the catastrophe targeted large animals, but was less severe among small animals, then non-avian dinosaurs would have been doomed. The big dinosaurs disappeared, and there were no small non-avian dinosaurs left to quickly proliferate in the aftermath.
As John Hutchinson pointed out in a Nature news story about this research, however, we’re going to need a lot more testing to see if this hypothesis holds up. The conclusion is based on a virtual model of ecosystems that we can’t study directly, and mass extinctions are frustratingly complicated phenomena.
Of course, a new dinosaur extinction scenario is irresistible journalist bait. Various news sources picked up the extinction hook (promoted in the paper’s press release) and pointed to the fact that dinosaurs laid eggs as the seeds of their undoing. But this isn’t quite right. After all, turtles, crocodylians and birds all laid eggs, too, and they survived. And mammals did not survive the end-Cretaceous extinction unscathed—several mammalian lineages disappeared or took major hits during the catastrophe. Likewise, not all dinosaurs alive during the final days of the Cretaceous were huge. Titans like Tyrannosaurus, Triceratops and Edmontosaurus are the most famous end-Cretaceous dinosaurs, but in western North America alone, there were also relatively small ceratopians, oviraptorosaurs and troodontid dinosaurs that topped out at about six feet in length. Were these dinosaurs still too big to survive? Was the threshold even lower? If it was, then the reason why medium-sized animals such as crocodylians survived, and why some mammals disappeared, becomes even more complicated. Why non-avian dinosaurs perished, and why so many other lineages survived, remains a mystery.
Codron, D., Carbone, C., Muller, D., & Clauss, M. (2012). Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates Biology Letters DOI: 10.1098/rsbl.2012.0240
April 11, 2012
Somewhere, out in the interstellar void, there may be a planet inhabited by hyper-advanced dinosaurs. At least, that’s what a new paper by Columbia University chemist Ronald Breslow says.
This morning, friend and fellow science writer David Dobbs forwarded me an American Chemical Society press release titled “Could ‘advanced’ dinosaurs rule other planets?” Since I was still a little bleary-eyed at the early hour, I thought I had read that wrong. But I saw it right the first time. “New scientific research raises the possibility that advanced versions of T. rex and other dinosaurs—monstrous creatures with the intelligence and cunning of humans—may be the life forms that evolved on other planets in the universe,” the item explained.
I couldn’t help but wonder if the pronouncement was inspired Planet of Dinosaurs—the awful 1978 film about a futuristic space crew stranded on a planet stuck in the dinosaurian heyday of the Mesozoic. But the paper itself suggests a different origin for what is ultimately a fossil-based non sequitur.
Breslow’s paper is primarily concerned with why the biochemical signature of life on earth is so consistent. Molecules such as amino acids, sugars, DNA and RNA exist in one of two possible orientations, left-handed or right-handed. Instead of showing a mixture of both forms, biomolecules typically come in only one form: Most sugars have a right-handed orientation, while most amino acids exhibit a left-handed orientation. Why life on earth should exhibit these particular arrangements and not the other possible orientations is a mystery that goes back to the origin of life itself.
One idea, favored by Breslow, is that meteorites carried specific types of amino acids and other organic flotsam to earth around 4 billion years ago. This is an extension of the idea that life here was “seeded” by comets, asteroids or meteorites. The origin and subsequent evolution of our planet’s flora and fauna would be constrained by the characteristics of the biomolecules that gave life a jump-start.
None of this has anything to do with dinosaurs. (The first dinosaurs, as far as we know, originated a scant 230 million years ago.) Yet, in closing, Breslow briefly speculates on what alien creatures might look like—perhaps possessing the opposite biochemical orientations of life on earth. “Such life forms could well be advanced versions of dinosaurs,” Breslow writes, “if mammals did not have the good fortune to have the dinosaurs wiped out by an asteroidal collision.” Whatever such space dinosaurs might look like, though, “We would be better off not meeting them,” Breslow warns.
As much as I’m charmed by the idea of alien dinosaurs, Breslow’s conjecture makes my brain ache. Our planet’s fossil record has intricately detailed the fact that evolution is not a linear march of progress from one predestined waypoint to another. Dinosaurs were never destined to be. The history of life on earth has been greatly influenced by chance and contingency, and dinosaurs are a perfect example of this fact.
Prior to 250 million years ago, the synapsids—our ancestors and relatives—were the dominant creatures on land. But the apocalyptic extinction at the end of the Permian Period eliminated most synapsid lineages, in addition to many other forms of life. This clearing of the ecological slate is what allowed a different group of creatures to proliferate. Early archosaurs, or “ruling reptiles,” included the archaic forerunners of crocodiles, pterosaurs and dinosaurs, in addition to various groups now extinct, and these creatures dominated the Triassic.
Despite what has been traditionally told, though, the dinosaurian branch of the greater archosaur family tree didn’t immediately out-compete its neighbors. Eoraptor and Herrerasaurus were not the Triassic terrors they were cast as during the mid-1990s. For the most part, Triassic dinosaurs were small, rare, marginal parts of the ecosystems they inhabited. It was only after another mass extinction at the end of the Triassic, around 200 million years ago, that the competitors of early dinosaurs were removed and the reign of the dinosaurs truly began. “[T]here was nothing predestined or superior about dinosaurs when they first arose,” paleontologist Stephen Brusatte and colleagues wrote in a massive review of dinosaur origins, “and without the contingency of various earth-history events during the early Mesozoic, the Age of Dinosaurs might have never happened.”
Even if we ignore all the major evolutionary events prior to 250 million years ago, the fossil record demonstrates that the origin and rise of the dinosaurs were heavily influenced by two catastrophic extinction events. Had the Permian or Triassic extinctions not happened, there is no indication that dinosaurs would have evolved or come to rule the world—unforeseen events drastically shaped evolutionary history. Why on earth would we expect such patterns to be played out in just the right sequence on another planet? To say that there are dinosaurs on alien worlds presupposes that there is an irresistible direction that all life follows, and that dinosaurs are an inevitable actors in the drawn-out drama. There is no evidence that this is so.
The strange thing is that Breslow acknowledges the role of mass extinctions in evolutionary history. His speculative space dinosaurs are supposedly “advanced” creatures which were spared from oblivion. Other writers have toyed with this concept before, the most famous example being Dougal Dixon’s The New Dinosaurs. Sadly, though, Breslow did not include any illustrations or offer specific details about the sort of uber-dinosaurs he has in mind.
Yet, what we know of the history of life on earth dispenses with the need to imagine such fantastic, alien creatures. Dinosaurs still exist—birds are a surviving dinosaur lineage that has exploded into a beautifully array of disparate forms. And some birds, such as ravens, are quite intelligent, so we don’t have to wonder about what an especially smart dinosaur would have looked like. The reign of the dinosaurs may have ended 66 million years ago, but their 230-million-year-old legacy continues to this day. A simple shift in our understanding of dinosaur evolution has rescued the beloved creatures from extinction. I deeply doubt that there are dinosaurs in space, but I am glad that at least one variety of feathered dinosaur remains with us here.
Breslow, R. (2012). Evidence for the Likely Origin of Homochirality in Amino Acids, Sugars, and Nucleosides on Prebiotic Earth Journal of the American Chemical Society DOI: 10.1021/ja3012897
Brusatte, S., Nesbitt, S., Irmis, R., Butler, R., Benton, M., & Norell, M. (2010). The origin and early radiation of dinosaurs Earth-Science Reviews, 101 (1-2), 68-100 DOI: 10.1016/j.earscirev.2010.04.001