The extinction of the dinosaurs has long been debated. Now, researchers from around the world have joined forces to put all the pieces of the jigsaw in place. The results are presented in the latest issue of the journal Science and one of the authors of the article is Lund University researcher Vivi Vajda. “We have collected all the pieces of the jigsaw to create one large jigsaw, and fitted them together to form a complete picture”, says Vivi Vajda, a bedrock geologist and fossil researcher at Lund University.

Ms Vajda and the other researchers behind the article in Science are agreed that the reason the dinosaurs died out was a powerful asteroid crash off the Yucatan Peninsula in Mexico 65.5 million years ago.

In the collision, the more than 10-kilometre wide asteroid broke up and created a 2-kilometre deep and 200-kilometre wide crater. This caused vast amounts of dust to whirl up into the atmosphere. The disaster was a fact. The dust prevented sunlight from reaching the earth, which resulted in a global fall in temperature and caused ecosystems to collapse. Those organisms that depended on sunlight died, because they could no longer grow with the help of photosynthesis. The impact also triggered huge earthquakes and tsunamis, which had immediate devastating effects for life on earth.

Vivi Vajda has studied how vegetation changed in the area around New Zealand 65.5 million years ago. She uses plant fossils, mostly pollen, to study the flora of the time. She has discovered that the variety of plant species decreased drastically after the asteroid crash, and then increased again.

Ms Vajda’s colleague Pi Willumsen, who conducts research on plankton, has discovered the same phenomenon in the sea. ”Both environments produce results that show that photosynthesis could not function, which meant that there was no longer any food for the animals higher up in the food chain”, explains Vivi Vajda.

Most dinosaurs were large and required a lot of energy, hence they died first. Seeds, on the other hand, could survive for a long time in the earth and therefore vegetation did not die out in the same way. The reason why the variety of plant species increased after initially decreasing following the asteroid crash is that when so many species were wiped out, niches were created in which new species could establish themselves. Mammals and birds took over the niches left by the reptiles, while many plants were able to reclaim their old niches.

Iridium is an important jigsaw piece in the researchers’ detective work. This metal is rare on earth, but common in asteroids, and has been found in the clay layer formed from the dust that spread in the atmosphere 65.5 million years ago. The researchers have shown that the iridium layer becomes thicker the closer to the Yucatan Peninsula one comes. Iridium is also released in connection with powerful volcanic eruptions. However, the authors of the article in Science are agreed that the quantities of iridium found could not have been produced by volcanic activity alone.

For more information, please contact: Vivi Vajda, Department of Earth and Ecosystem Sciences, Lund University Vivi.Vajda@geol.lu.se , tel +46 (0)46 222 46 35, +46 (0)768 97 44 82 or Pi Willumsen, Department of Earth and Ecosystem Sciences, Lund University Pi.Willumsen@geol.lu.se , tel +45 22 30 21 32. Photo: Lund researchers Vivi Vajda and Pi Willumsen with a sample of Danish limestone showing the iridium layer. Website (English): www.lu.se/lund-university