THAT the dinosaurs went out with a bang is well known. About 66m years ago a large space rock hit what is now southern Mexico. As a consequence, and with the assistance of some enormous volcanic eruptions on the other side of the planet, the terrible lizards were consigned to history. That left the world open for the rise of mammals. What is less well known is that the dinosaurs themselves rose in circumstances similar to those that felled them. The animals’ long reign through the Jurassic and Cretaceous periods was enabled by another, albeit smaller, period of mass extinction, which happened between 234m and 232m years ago during the Triassic period.
This extinction is thought to have been caused by a period of unstable climate called the Carnian Pluvial Episode (CPE), in which the climate went from dry, to wet, to dry again four times over the course of 2m years. As is often the case in matters palaeontological, the effects of such changes are easiest to see at sea, because most sedimentary rocks (the sort that the bulk of fossils are found in) are marine, and also because the composition of such rocks reflects that of seawater at the time, which in turn reflects matters such as temperature, rainfall and carbon-dioxide levels. Marine rocks laid down at this time show a huge turnover of species, and that this coincides with the CPE.
Connecting events on land to the CPE is harder, though. And it is this that the authors of a study just published in Nature Communications have done. Massimo Bernardi of the Museum of Sciences in Trento, Italy, and his colleagues looked in particular at fossil animal tracks (see picture above) in the Dolomite mountains, a part of the Alps to the north-east of Trento. This part of the world is rich in such tracks, and Dr Bernardi has been able to use them, together with details of the Dolomites’ geology, to paint a precise picture of the dinosaurs’ emergence.
Footprints in the mud of time
Dinosaurs were one of many groups that evolved after the greatest mass extinction in history, which happened 252m years ago between the Permian and Triassic periods. This extinction emptied both the land and the oceans, leaving a blank canvas for evolution to work on.
The earliest fossil remains that are widely (though not universally) agreed to be those of a dinosaur come from Tanzania and date from about 245m years ago. For the next 10m years or so, however, dinosaurs are nothing special. In assemblages of the bones of land animals they form about 5% of the total. But then the fraction starts to rise and within 5m years or less (the dates are a bit uncertain) they dominate such assemblages, while other, better-established groups of creatures, such as dicynodonts and rhynchosaurs, dwindle and eventually vanish.
The bone evidence, however, is restricted mainly to what are now Argentina and Brazil. Dr Bernardi and his colleagues sought to corroborate it elsewhere, and with a different type of evidence—not animals’ bones but the tracks they left behind.
Dr Bernardi and his colleagues identified more than 20 sets of such tracks, called ichno-associations, in Dolomite rocks laid down during the relevant part of the Triassic. They studied descriptions of them by previous researchers and also visited several of the sites themselves to make their own observations. The Dolomite ichno-associations, they found, fall into three groups. The oldest show no signs of dinosaur tracks. In those of middling age, about 40% of the tracks have been laid down by dinosaurs. In the most recent, that proportion is 90%.
The relative ages of rock strata are easy to see. Unless a rock formation has been turned upside down by movement of Earth’s crust, young layers will be at the top and old ones at the bottom. That is how Dr Bernardi knew which tracks were older and which younger. Working out absolute ages, though, is harder. But the Alps are well studied, and he and his colleagues were able to assemble a chronology for their ichno-associations by looking at research that had been carried out on the layers in between them.
The Dolomites’ rocks were formed at a time when the sea’s level, relative to the land, was going up and down like a yo-yo. This was partly because of rises and falls in the sea itself, and partly because of the land rising and falling in response to tectonic shifts. Such transgressions and regressions mean that marine and terrestrial sediments are interleaved in these mountains, and the marine sediments provided the information needed.
Most obviously, those sediments show how far, at the time they were laid down, the turnover of species had proceeded at sea. On top of this, some of them contain layers of volcanic ash that can be dated from radioactive isotopes within. And some of them also preserve evidence of the reversals in Earth’s magnetic field that happen from time to time, and which act as useful date markers.
The upshot was that Dr Bernardi and his colleagues were able to establish accurate dates for the various ichno-associations they had catalogued, confirming, as they had suspected, that those associations precisely straddle the Carnian Pluvial Episode, thus matching the bone evidence from South America. The oldest tracks date from 236m years ago, before the CPE began. The middling ones were laid down 234m years ago, during it. The youngest are 230m years old, dating from after it was over. That is clear evidence the dinosaurs were indeed the beneficiaries of the CPE, though why they did better than other groups is not yet understood.
As to what caused this climatic hiatus, the best guess is that it was a consequence of vast volcanic eruptions, similar to those at the end of the Cretaceous, that were going on at the time in what is now Alaska and western Canada. And that, in itself, is interesting. The extinction at the end of the Permian also happened at the same time as a huge outpouring of lava (in what is now Siberia). Although the Cretaceous extinction is usually blamed on the extraterrestrial collision that left its mark in the Yucatan, with the coincidental eruptions (which happened in what is now India) relegated to a supporting role, some geologists wonder if the importance of those roles should be reversed.
There is no reason, of course, why all mass extinctions should have the same cause. Besides cosmic collisions, both nearby stellar explosions and climate-changing burps of methane released from the ocean depths have been suggested as possible biosphere-killers. But the geological record shows that sub-continental-sized eruptions do happen quite frequently. It is humanity’s luck not, thus far at least, to be living at the same time as one.