How the dinosaurs died is maybe the most well-known story in all paleontology, but the dinosaurs' origins are more obscure. Dinosaurs might well have gotten their chance to rule the planet from the very thing that later killed them off.
The two most famous extinction events are the Cretaceous-Tertiary extinction event 65 million years ago, which of course killed off the dinosaurs, and the Permian-Triassic extinction event 251.4 million years ago, which is often called the Great Dying and wiped out 96% of marine life and 70% of animal life.
Sandwiched between them is the lesser-known Triassic-Jurassic extinction event, which occurred 199.6 million years ago at a time when dinosaurs were present on Earth but not yet ascendant. This extinction event wiped out many of the other large reptiles and amphibians that had until then shared the planet with the dinosaurs. With all these old rivals extinct, dinosaurs assumed their ecological niches and became the dominant form of life for the next 135 million years.
The question then is exactly what caused this mass extinction. Many theories have been put forward, including climate change, volcanic eruptions, and an asteroid impact. Since the extinction event took place over a relatively rapid 10,000 years, and marine life in particular went extinct very suddenly, the more gradual process of climate change is generally considered the least likely of these explanations. There's some good isotopic evidence to support the volcanoes explanation, but the evidence we have isn't definitive, and again the hundred-thousand year run of volcanic mega-eruptions doesn't quite sync with the much more rapid rate of extinctions.
So what about the asteroid impact hypothesis? It's a perfectly valid explanation, but it's missing one rather vital piece of evidence: the asteroid itself. Without an impact crater that can be dated to the same time period as the Triassic-Jurassic extinction, the giant impact explanation can't really be taken seriously. That's where the recent analysis of a crater near Rochechouart, France enters the picture. Originally dated at 214 million years old - well outside the range of this extinction event - the new data pushes the impact to between 199 and 203 million years ago, which overlaps nicely with the Triassic-Jurassic extinction.
Now, geoscientists Paul Olsen and Dennis Kent are in Wales, digging through sedimentary rock dating back to the extinction event. They're doing this in order to find the chemical signature left behind by the asteroid impact - if it really did hit at the same time as the extinction event, its debris would leave unique chemical traces in rocks that formed during that period, and these could be used to establish a more definitive link.
Still, the researchers say there is one rather big problem with the Rochechouart crater: its size. Compared to 110-mile-wide crater in Chicxulub, Mexico that is left from the Cretaceous-Tertiary impact, this crater is tiny, only 25 to 30 miles wide. An asteroid that size could still do some serious damage - it would have caused an 11.5 magnitude earthquake, 100 times worse than any experienced in human history - but it probably wouldn't have had the power to kill off the dinosaurs all by itself.
Instead, the researchers suggest that this asteroid is part of a larger story that also includes the massive volcanic eruptions. If the asteroid hit before or at the beginning of the eruptions, it could have weakened existing ecosystems so that they couldn't survive the coming chaos. Or, if it hit towards the end of the eruptions, it might have delivered a final blow to an already battered, weakened planet.
Either way, if evidence can be found linking the Rochechouart crater to the Triassic-Jurassic extinction event, then that means giant asteroids both began and ended the age of dinosaurs. As far as geology goes, that's downright poetic.