When the Dinosaurs Died

The Cretaceous-Palaeogene (K-Pg) Mass Extinction, 65.5 million years ago, is probably the most famous biotic crisis, because it’s when the dinosaurs died. But, often forgotten are the other 40-76% of species that became extinct at this time. Despite extensive research, the cause of this mass extinction event remains contentious...

A mass extinction occurs when there is a significant reduction in organism abundance and diversity over a relatively short space of time. There have been 5 major mass extinctions through Earth's history (commonly referred to as the "Big 5"), which have interrupted the evolution of organisms through time. Mass extinctions are thought to “rest the evolutionary clock” as they alter the normal rules of Darwinian evolution that take place the rest of the time. It is therefore important to understand the trends that occur at mass extinctions in order to completely understand the evolutionary history of life and to predict the extinction and survival processes in organisms at the present day.

Although dinosaurs are the most famous organism to have become extinct at the K-Pg mass extinction, they are not necessarily the most useful group of animals for palaeontologists to study in order to understand the event because they are relatively rare in the fossil record. Many other groups also suffered significant declines: approximately 95% of planktic foraminifera became extinct; many marine invertebrates, such as echinoderms and bivalves, suffered severe extinctions too, and marine reptiles, such as plesiosaurs and mosasaurs, also became extinct. Ammonites were also completely wiped out. 

While a significant number of organisms clearly went extinct at this time, some groups appear to have been barely affected by the mass extinction. These include turtles, freshwater crocodiles and mammals. It is unclear whether these groups were purely lucky  - and survived by chance - or if selective extinction and survival occurred owing to particular traits possessed by these organisms.

The K-Pg boundary itself consists of a thin clay layer that is seen worldwide. This allows this moment in time to be accurately correlated between many localities across the world and indicates that the mass extinction was a global phenomenon. Abundant fossils are present in the Cretaceous (K) rock units that occur below this boundary. Above the boundary, Palaeogene (Pg) rocks are found, which contrast with those in the previous Cretaceous layers as they contain many fewer and less diverse fossils. In fact, the Cretaceous rocks show the last occurrences of 40-76% of species that were alive before the extinction event. The rock record clearly shows that something of significance happened between these two time periods that resulted in a significant faunal change.  

When the K-Pg boundary clay was investigated in detail in 1980, Luis Alvarez, a Nobel Prize-winning physicist, and his son, Walter Alvarez, discovered it contained a peak in iridium levels hundreds of times larger then normal. They hypothesised that this iridium came from an extra-terrestrial source – an asteroid or meteor some 10km in diameter – that impacted the Earth at this time and could be to blame for the coinciding mass extinction. It wasn’t until later that the matching 180-200km diameter crater was discovered: the Chicxulub Crater in the Yucatán Peninsula. This led to speculation that this meteorite impact was the cause of the mass extinction event: if the meteorite impact created a huge dust cloud that covered the world and blocked the sunlight from reaching our planet, Earth would experience a “nuclear winter”. In these circumstances, photosynthesis could not occur and would cause a collapse in the food chain. It has been suggested that the different feeding strategies of organisms at this time might have promoted survival or caused extinction, with those dependent on sunlight or primary producers most likely to suffer, and those able to feed on detritus or survive starvation fortunately well-adapted to survive the mass extinction event.

Whilst this theory is very widely accepted, as well as adding some Hollywood-esque drama to the story, it is still debated. Another proposed theory for the cause of the mass extinction relates to extensive volcanism that was occurring at around the same time. Sixty-eight million years ago extensive volcanic activity began in India. A few million years later, coinciding with the extinctions, these eruptions went beyond a scale of anything that we see today: huge quantities of lava erupted, with an area at least 11 times the size of England covered by lava. These lava deposits are known as the Deccan Traps, and we believe that these extensive eruptions may be to blame for the mass extinction. The dust emitted from the volcanoes could have also caused a “nuclear winter” scenario. Additionally, gases such as sulphur dioxide and carbon dioxide would have been released from the eruptions and significantly impacted life on the planet.

The Cretaceous world was very different to the one we live in today. The Earth was going through a greenhouse phase at the time - it would have been much warmer, with almost tropical conditions. There were fluctuations in this climate towards the end of the Cretaceous period. It is argued that some extinctions occurred before the actual K-Pg boundary and this might be a result of the associated environmental stresses that occurred due to these climatic fluctuations. 

Without a time machine and the ability to travel back 65.5 million years, though, it is more or less impossible to determine a sole cause for the K-Pg mass extinction event from the evidence we have available today. Of course, it is possible that a combination of events led to the dramatic extinction of so many organisms. There is little doubt that a meteorite impacted the Earth at this time or that huge-scale volcanism was occurring. Additionally, the iridium anomaly is found within the flood basalts of the Deccan Traps, indicating that these events were concurrent. The contentious issue, instead, is the level to which these events affected life on Earth at the time. There is evidence to suggest that extinctions were occurring before the K-Pg boundary, implying a more gradual extinction was underway already when these dramatic events took place. If the Late Cretaceous fauna were already stressed, it is probable that a dramatic meteorite impact and a burst of unbelievably large volcanic activity was the final straw, causing a mass extinction on the huge scale that we know about today...