Why did dinosaurs become extinct?

By Leanne Gunn

From the day dinosaur fossils were first discovered they captured the heart and imagination of children and adults alike. Perhaps this is why discoveries of these massive bones were once thought of as proof of dragons, giants and other mythological creatures. 

'Dippy' the diplodocus at the Natural History Museum, London.  CC-BY-Drow Male

‘Dippy’ the diplodocus at the Natural History Museum, London. CC-BY-Drow Male

Despite now being recognised as their own group of diverse, real-life animals, our love of dinosaurs has not dwindled. Since 1979 visitors of the Natural History Museum in London have been greeted by the cast of a 292 bone diplodocus skeleton, affectionately known as ‘Dippy’. His almost smiling skull, long neck and tail, sweeping high over the heads of passers-by is often considered a highlight of any museum visit.

Earlier this year the museum announced their new plans for this iconic spot. Plans which involve moving Dippy and replacing him with the bones of an even bigger creature: a blue whale.

All this talk about Dippy leaving the Hintze Hall of the Natural History Museum got me thinking about the events that led to dinosaurs leaving us on a more permanent basis. Their extinction.

The life and death of the dinosaurs

By investigating the fossil record paleontologists can work out when, where and how dinosaurs lived. The oldest known dinosaur fossils date back to 234 million years ago. Over time they evolved and diversified until they became the dominant land-dwelling vertebrates on Earth.

Sauropod sizes CC-BY-Matt Martyniuk

We often think of dinosaurs as being huge. Like these Sauropods.
CC-BY-Matt Martyniuk

Not all dinosaurs were huge. Like this tiny microraptor.  CC-BY- Matt Martyniuk

Not all dinosaurs were huge. Like this tiny microraptor.
CC-BY- Matt Martyniuk

We often think of dinosaurs as huge animals, however, dinosaurs actually came in a variety of shapes and sizes. The largest dinosaurs were sauropods. These were slow moving plant eaters with long necks and tails allowing them to reach up to 40 m in length. In contrast, microraptors were some of the smallest dinosaurs. These four-winged dinosaurs were less than 1 m in length.

Despite such a successful start the existence of dinosaurs on Earth was not a roaring success. 66 million years ago something disastrous happened to Earth leading to the death of not just the dinosaurs, but almost all life on Earth. It led to the extinction of three quarters of Earth’s plant and animal species including plesiosaurs (marine reptiles), pterosaurs (flying reptiles) and ammonites (a group of marine invertebrates similar to a squid or nautilus).

Pterosaurs (CC-BY-Smeira), plesiosaurs (CC-By-Adam Stuart Smith) and ammonites  (CC-BY-Liftarn) also became extinct at this time.

Pterosaurs (CC-BY-Smeira), plesiosaurs (CC-By-Adam Stuart Smith) and ammonites (CC-BY-Liftarn) also became extinct at this time.

Earth 66 million years ago

Picture a scene from ‘Jurassic Park’ and then throw in some scenes from ‘Dantes Peak’ and ‘Deep Impact’ and you are left with something similar to the Earth 66 million years ago.

This was a time of extreme volcanic activity in west central India forming the one of the largest volcanic features on Earth, the Deccan Traps. The volcanic eruption lasted thousands of years covering an area half the size of modern India with lava, and pumping vast amounts of volcanic gases into the atmosphere. These gases led to a global cooling effect on Earth.

Sea life was being threatened by an entirely different problem. Many marine creatures and plants live in the warm, nutrient rich, shallow waters off the coast of the Earths continents. However 66 million years ago the Earth was experiencing global sea level fall at a catastrophic rate! As a result the amount of these shallow marine environments were in decline and sea life previously living there were losing their habitats.

CC-BY-Don Davis (NASA)

CC-BY-Don Davis (NASA)

Just to add insult to injury, a 10 km wide asteroid also collided with the Earth at around this time.

Such a sudden and powerful impact would have send the Earth into darkness by kicking up rock dust and sulphur trioxide vapour. While the dust would have blocked the sun for about a year, the vapour would have quickly combined with water forming sulphuric acid aerosols working to reduce the level of sunlight reaching the Earth by up to 20 % for a further 10 years.

Such low levels of sunlight has huge implications for plant life which is dependent on sunlight to produce their food through photosynthesis. In other words, whilst the volcanic activity polluted the land and sea level fall reduced marine environments, the asteroid impact would have caused the base of the food chain to collapse completely.

 A series of unfortunate events

Unraveling the effects of each individual event that occurred at this time is difficult and it is unclear which event was responsible for the death of the dinosaurs. It is possible that such extreme loss of life on Earth was a result of all three of these events taking place at the same time. The asteroid impact was the final straw, it pushed an already stressed system over the edge into irreparable decline.

A world where dinosaurs survived?

What if the asteroid had hit a few million years later or earlier? What if the dinosaurs had survived? Would they still be here today? 

While it is tempting to imagine a world where humans and dinosaurs live together, this is not necessarily the case. In fact, it is entirely possible that without the dinosaur extinction we would not exist at all. The death of the dinosaurs may have actually cleared the way for mammal evolution and allowed humans to stake our claim on Earth. For how long? We just don’t know.

CC-BY-science made simple

CC-BY-science made simple

Much like dinosaurs becoming extinct, Dippy moving from the Hintze Hall of the Natural History Museum is the end of an era. But as with any era that ends, a new one begins. Just like losing the dinosaurs allowed mammals to thrive, Dippy will leave allowing a mammal to take the limelight.


 

 

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Posted in Biology, Geology
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