Dinosaur disappearance: hybrid theory gains added support

Although the debate surrounding the extinction of dinosaurs will no doubt continue for many years to come, a hybrid theory, volcanism combined with an asteroid strike, seems to answer most questions surrounding the disappearance of the “terrible lizards” – at least for the time being.

An international team of scientists used radiometric dating analysis of rock and ash samples to establish that the dinosaurs died out around 66 038 000 years ago, a more accurate date than was previously believed.

The findings were published in the February 8 issue of Science journal by a research team headed by Paul Renne, and comprising scientists from Berkeley Geochronology Center and the University of California at Berkeley in the US, Glasgow University in the UK, and Vrije Universiteit Amsterdam in the Netherlands.

The latest extinction date aligns with the theory that an asteroid that hit Mexico killed off all dinosaurs except birds.

However, the hybrid theory suggests that it was in fact a combination of climate change prompted by volcanic eruptions, and culminating in the asteroid strike, that led to the extinction.

The new paper provides precise dates that confirm the “impact theory” of dinosaur extinction – namely that the asteroid hit Earth 300 000 years before the dinosaurs became extinct.

Renne et al have established that the impact in fact took place within 30 000 years of the dinosaur extinction, leading them to add their voices to the hybrid theory.

There is evidence that animals that roamed the Earth at the time were stressed by several unseasonal cold snaps towards the end of the Cretaceous Period.

Although his field of expertise is dinosaurs and bird evolution rather than extinction theory, Wits Senior Researcher in dinosaur palaeontology Dr Jonah Choiniere says “the great iridium anomaly”, as it is known in scientific circles, is a well-known phenomenon.

The reference relates to the discovery of iridium in the 1980s by father-and-son team Luis and Walter Alvarez. They established that a layer of clay found throughout the world which marks the end of the Cretaceous period, is iridium-enriched. Iridium is a common component of space rocks, which led to their theory that an asteroid impact had wiped out the dinosaurs.

The Chicxulub crater, formed by the impact of an estimated 10km-diameter asteroid, would have resulted in a sunlight-blocking dust cloud that wiped out plant life and reduced global temperature. The force of the Chicxulub impact would have been equal to the simultaneous explosion of two million hydrogen bombs.

However, the asteroid impact may still not be the whole story in the end-Cretaceous extinction. Volcanism, as well as climate change, could have played a powerful catalytic role.

“Evidence of volcanism in India which pre-dates dinosaur extinction has been seen in the Deccan Traps, one of the Earth’s largest volcanic features,” says Choiniere.

The Deccan Trap evidence dates back to between 60- and 68-million years ago, towards the end of the Cretaceous period, which pre-dates the extinction of dinosaurs by a few million years, and continues long after their disappearance.

More recently, volcanoes have caused dramatic changes in the world’s climate. With an explosive force 10 times greater than the famous eruption of Mt Vesuvius (which buried the ancient city of Pompeii in 79AD), the 1815 eruption of Mt Tambora in Indonesia blasted 100 cubic kilometres of rock into the atmosphere.

The climatic effects of the eruption of Tambora were so extreme, they’ve become part of rural folklore in Choiniere’s hometown in the north-eastern United States.

“The year Tambora erupted was often referred to as ‘the year with no summer’ where I grew up, because we experienced frost nearly every day in the US,” he recalls.

Geological evidence also tells us that the volcanic activity in India happened on a much larger scale than that of Indonesia, so there is no doubt that weather patterns changed rapidly in response.

“But I don’t think volcanism alone was enough to cause extinction. It’s more likely that volcanism was a contributing factor to a change in the world’s environment,” says Choiniere.

In fact, Choiniere says: “We also know that there were changes in sea level around the same time, possibly due to a decrease in global temperatures as a result of climate change or due to one of the cyclical events experienced by planet Earth.”

Following the volcanism in India and the Late Cretaceous cold snaps, the Chicxulub asteroid collided with Earth, striking Mexico’s Yucatán Peninsula.

“Since we lack sufficient fossil record resolution, we cannot substantiate current theories. Ideally, we need clear fossil records before and after the event from many places on Earth to contextualise the demise of dinosaurs,” Choiniere explains.

We know from history that smaller, more adaptable animals – such as burrowing creatures, for example – survive extinction far better than any others, and we do know that dinosaurs were definitely in decline before the asteroid hit.

“There are so many factors to consider in terms of formulating a theory, but the hybrid theory definitely has some merit,” he says.

Adding a final word on the extinction debate, Choiniere adds: “Actually, it’s not really scientifically accurate to say that dinosaurs became extinct, because modern birds evolved from a group of small, agile, long-limbed, meat-eating dinosaurs called Paraves.”