About 250 million years ago there occurred the single greatest extinction event the earth has ever witnessed. Some 96% of marine species and over two thirds of terrestrial species were wiped out. Think about what that must have been like for a moment. This extinction event delineates the boundary between the Permian and the Triassic eras of geological time.
No one can say with any degree of certainty what caused the catastrophe; it’s just too long ago for definitive evidence to have survived. The leading theories are the usual suspects of meteor impact and/or vulcanism or perhaps a massive and catastrophic removal of oxygen from the oceans. The reality is likely to be a mixture of several events.
Not everything died, though, animals that we call dinosaur precursors struggled on. Mostly therapsids and archosaurs (crocodiles and birds being the only two examples of archosaurs still alive today), these were the animals that would go on to be the archetypal monsters of the Jurassic period that come to mind when we think of dinosaurs.
The devastation on the earth was of such an extent that it took about 50 million years for biodiversity to recover. Conventional wisdom says that the mass die off opened up the playing field for the precursor dinosaurs to fill by the Middle Triassic. However, a new paper published in the Proceedings of the National Academy of Science has conducted a new, thorough and more precise measure of the rock strata that contain the precursor dinosaurs and the dinosaurs themselves.
As radioactive uranium atoms decay they turn into lead which, in turn, is incorporated into crystals of zircon. By analysing the relative ratios of the uranium and lead they were able to refine the range at which dinosaur precursors begin to show up in the fossil record. The new range makes them 10 million years more recent than previously thought pushing them firmly into the Late Triassic. This new date means that they were not a part of the initial bounce back after the mass extinctions as previously thought.
It also bunches up the time between the dinosaurs and their precursors, more than halving the time between them to less than 5 million years. The evolution of dinosaurs, then, was a relatively rapid event and was not as a direct result of a huge change in the environment that opened up new evolutionary niches to them.
Like all good science the new data throws up as many questions as it answers. What did cause the speciation event that led to dinosaurs? What, if anything, changed during the Late Triassic? It is incredibly difficult to look back in time that far with a high degree of confidence but with a lot of hard work, and a little luck, we may just get there.