Sixty-five million years ago, give or take a few hundred thousand years, a meteor smashed into Mexico's Yucatan Peninsula, throwing up billowing clouds of ash and smoke that quickly spread, over the next few days and weeks, across the world's atmosphere. Blotted out, the sun could no longer nourish the earth's teeming ferns, forests, and flowers, and as these plants died, so did the animals that fed on them — first the herbivorous dinosaurs, and then the carnivorous dinosaurs whose populations these plant-eaters sustained.
A lot of food markets sell dinosaur shaped chicken nuggets but go the extra mile and arrange them on a platter like this dinosaur foodscape created by Michelle at Muffin Tin Mom. It's a great way to make the dish a bit more healthy by sneaking in some veggies. Dinosaur Trails Included with your admission to The Dinosaur Place, outdoor adventure park, encounter over 40 life-sized dinosaurs spread among 1.5 miles of easy to walk Nature Trails. Encompassing 60 acres of pristine Southeastern Connecticut woodlands this Jurassic era exploration will deliver a fun-filled day of unplugged education, discovery, and adventure for the entire family.
That, in a nutshell (or a meteor crater), is the story of the K/T Extinction Event. But some experts think this story is incomplete: it has a suitably thrilling climax, to be sure, but not enough attention has been paid to the events leading up to it. Specifically, evidence exists that the five million years leading up to the K/T Extinction witnessed a huge surge in volcanic activity — and that lung-choking, sun-blocking volcanic ash, every bit as much as meteor debris, may have weakened dinosaurs to such an extent that they were easy pickings for the Yucatan disaster.
Throughout its history, the earth has been geologically active — and during the late Cretaceous period, 70 million years ago, the most geologically active place on earth was northern India, near modern-day Mumbai. (This had nothing to do with the slow collision of India with the underside of Eurasia, which wouldn't occur for another ten million years, but stresses in the fast-moving subcontinental plate were certainly involved.) Specifically, the volcanoes of the 'Deccan Traps' spewed lava for tens of thousands of years on end; this lava eventually covered over 200,000 square miles of the subcontinent and reached a depth (in some locations) of over a mile!
As you can imagine, the Deccan Traps were bad news for local Indian and Asian wildlife, as terrestrial and marine animals were literally cooked alive and then buried beneath millions of tons of solidifying lava. But the traps may also have had a disastrous effect on the worldwide ecology since volcanoes are notorious for releasing high levels of sulfur and carbon dioxide — which would have both acidified the world's oceans and caused a rapid spate of global warming, even despite all the accompanying dust thrown up into atmosphere. (Carbon dioxide is a greenhouse gas, meaning it tends reflects heat from the earth back onto the surface, rather than allowing it to dissipate into outer space.)
What makes the volcano scenario hard to prove or disprove, vis-a-vis the meteor impact theory of dinosaur extinction, is that it depends on much of the same evidence. One key piece of data adduced by supporters of the Yucatan meteor impact is the characteristic layer of iridium, an element common in asteroids, in sediments laid down at the Cretaceous/Tertiary boundary. Unfortunately, iridium is also found in the molten rock under the earth's crust, which can be expelled by volcanoes! The same applies to shocked-quartz crystals, which can be caused by either meteor impacts or (at least according to some theories) intense volcanic eruptions.
What about the dinosaurs themselves, and their persistence — or lack of it — in the fossil record? We know that dinosaurs roamed the earth right up until the K/T boundary, 65 million years ago, whereas the Deccan Traps became active 70 million years ago. That's a very 'soft' boundary extinction of five million years, while it's clear that dinosaurs went extinct within a couple of hundred thousand years of the Yucatan meteor impact — a relatively 'hard' boundary extinction by geological standards. (On the other hand, there is some evidence that dinosaurs were dwindling in diversity during the last few million years of the Cretaceous period, which may or may not be attributable to volcanic activity.)
In the end, these two scenarios — death by volcano and death by meteor — are not inconsistent with one another. It may very well be the case that all terrestrial life on earth, including dinosaurs, was profoundly weakened by the Deccan Traps, and the Yucatan meteor delivered the proverbial coup de grace. In effect, a slow, painful extinction was followed by a fast, even more painful extinction (which brings to mind that old saying about how people go bankrupt: 'a little bit at a time, and then all at once.')
Ironically, we do know one instance in which volcanoes had a major impact on dinosaurs — but it happened at the end of the Triassic period, not the Cretaceous. A new study makes the solid case that the end-Triassic extinction event, which doomed more than half of all terrestrial animals, was caused by volcanic eruptions accompanying the breakup of the supercontinent Pangaea. It was only after the dust had cleared that the earliest dinosaurs — which evolved during the middle Triassic period--were free to fill the open ecological niches left by their doomed relatives, and assert their dominance during the ensuing Jurassic and Cretaceous periods.
As dinosaurs go, stegosaurs are relatively easy to describe: these quadrupedal, small-to-medium-sized, small-brained herbivores were characterized by the double rows of plates and spikes along their backs and the sharp spikes on the ends of their tails. By far the most famous stegosaur (and the one that has lent its name to this entire family) is, of course, Stegosaurus, but there are at least a dozen other closely related genera, most of which are no less important from a historical perspective. (See a gallery of stegosaur pictures and profiles and Why Did Stegosaurus Have Plates on its Back?)
Evolutionarily speaking, stegosaurs are classified as ornithischian ('bird-hipped') dinosaurs. Their closest relatives were the armored dinosaurs known as ankylosaurs, and they were more distantly related to other four-footed plant-eaters like hadrosaurs (aka duck-billed dinosaurs) and ornithopods. In a crucial way, though, stegosaurs were less successful than these other dinosaurs: they only flourished toward the end of the Jurassic period (about 160 to 150 million years ago), with only a handful of species managing to survive into the ensuing Cretaceous.
Because they constituted such a small family of dinosaurs, it's relatively easy to distinguish among the various types of stegosaurs. The earlier, smaller stegosaurs of the middle to late Jurassic period are known as 'huayangosaurids,' typified by, you guessed it, Huayangosaurus and less well-known genera like the European Regnosaurus. The better-known 'stegosaurids' were larger, with more elaborate spikes and plates, and are best represented by the classic body plan of Stegosaurus.
As far as paleontologists can tell, the stegosaur family tree took root with the huayangosaurids of Asia, and grew larger and more ornate by the time Stegosaurus planted itself in North America. There are still some mysteries, though: for example, the tantalizingly named Gigantspinosaurus had two huge spikes protruding from its shoulders, making its exact classification within the stegosaur line (if it even belongs there) a matter of controversy. The last stegosaur to appear in the fossil record is the mid-Cretaceous Wuerhosaurus, though it's possible that some as-yet-undiscovered genus may have survived to the brink of the K/T Extinction 65 million years ago.
The most enduring mystery about stegosaurs is why they possessed those characteristic double rows of plates and spikes along their backs, and how these plates and spikes were arranged. To date, no stegosaur fossil has been unearthed with the plates still attached to its skeleton, leading some paleontologists to conclude that these scutes (as they're technically called) lay flat along the dinosaur's back, like the thick armor of ankylosaurs. However, most researchers still believe that these plates were arranged semi-vertically, as in popular reconstructions of Stegosaurus.
This leads naturally to the question: did these plates have a biological function, or were they strictly ornamental? Because scutes pack a large surface area into a small volume, it's possible that they helped to dissipate heat during the night and absorb it by day, and thus regulated their owner's presumably cold-blooded metabolism. But it's also possible that these plates evolved to deter predators, or to help differentiate males from females. The trouble with these latter two explanations is that a) it's hard to see how an upright array of blunt plates could possibly have intimidated a hungry Allosaurus, and b) there has been very little evidence to date of sexual dimorphism among stegosaurs.
The prevailing theory is a bit less exciting: the bulk of opinion today is that the plates and spikes of stegosaurs evolved as a way of differentiating individuals within the herd, along the same lines as the slightly varying black-and-white stripes of zebras (because they were well supplied with blood, these scutes may also have changed color with the seasons). No such controversy attaches to the sharp spikes at the end of most stegosaurs' tails, which were doubtless used for defensive purposes (and are often called thagomizers in tribute to a famous 'Far Side' cartoon by Gary Larson).