The Asteroid that “Killed the Dinosaurs” was 6 Miles Wide

When your time is up, your time is up. For the dinosaurs the end came in the form of an asteroid, 65.5 million years ago. With a diameter of 10 km (6 miles), the asteroid created a crater 180 km (110 miles) wide off the coast of the Yucatan Peninsula. The result was a cloud of debris that encircled the Earth, plunging it in darkness and dropping its temperature. This killed the dinosaurs living on the warm, sunny globe at the time; it also allowed the subsequent rise of the mammals.

The Asteroid May Not Have Killed the Dinosaurs All Off

Until recently the asteroid theory has been the most popular one to explain the demise of the dinosaurs. A new publication in the journal ‘Nature Communications’ has, however, cast a different light on the original theory, by suggesting that not all dinosaurs would have been affected in the same way by the asteroid. Research scientists from the American Museum of Natural History in New York City showed that the well-known triceratops and the duck-billed dinosaurs may already have shown signs of decline before the asteroid finished them off. Finding the cause of their weakened state has given the researchers new methods for studying dinosaurs and their surrounds, while pointing to new insights into their ultimate fate.

The famous asteroid strike caused the K-T (Cretaceous-Tertiary) Extinction, an event that was always assumed to have had an equal impact on all species, and there was seemingly sufficient proof for this. The rise and fall of the dinosaurs was studied in the simple way of plotting the number of species present at any given time; i.e. when there were many species the lineage was doing well, but when species numbers dwindled – especially after the K-T – they were moving closer to extinction. This simple method has proved to be not quite reliable since palaeontologists tend to work and dig for fossil remains in a variety of different places. According to Steve Brusatte, this uneven sampling of the fossil record can lead to bias in the results of palaeontologists; Brusatte is a graduate student at Columbia University that contributed towards this new study. He explains that more fossil species are found in areas such as the Great Plains of America, where conditions are conducive to fossil formation, than in areas where fossilisation did not happen easily. In such areas one may find an under-representation of fossil species, even if many animal species lived there at some point in time.

Researchers Studied 150 Species to Determine what Killed the Dinosaurs

This potential flaw in research methodology led to an alternate approach to the problem.  The researchers from American Museum of Natural History, under leadership of Mark Norell, examined the biodiversity present within the various groups (or lineages) of dinosaurs. The theory is that a group that is doing well (the carnivores, for example) should be evolving more species than other groups that are battling to hang in there. The researchers studied a sample of 150 species across seven major dinosaur lineages, using this method, which showed a different and more varied picture of dinosaur well-being before the fateful asteroid strike killed the dinosaurs.

The study revealed that three groups spanning five lineages were doing well by either maintaining a steady state or increasing in species number. These are the Small Herbivores (ankylosaurs and pachycephalosaurs), the Largest Herbivores (sauropods) and the Carnivores (tyrannosaurs and coelurosaurs). The hadrosaurs and ceratopsids, on the other hand, were not faring so well. These Slightly Smaller Herbivores were bulk feeders, consuming a wide range of vegetation. They showed a steady decline in species number for at least 12 million years before the asteroid impact killed the dinosaurs off.

Dinosaurs Likely Evolved Very Differently

Richard Butler of the Ludwig Maximilian University, Munich, who was another collaborator in the study, notes that we tend to think of dinosaurs in monolithic terms, by using phrases such as “The dinosaurs did this, the dinosaurs did that”. We forget that the term dinosaur circumscribes a very diverse group of animals, with lineages that most likely evolved differently – a situation that is certainly confirmed by the present study.

The Sea and Changes in Ecosystems Could Have Killed the Dinosaurs

What, then, created the problem for the ceratopsid and hadrosaur lineages? It seems that geography accounts for some of it, since the bulk feeders were particularly prevalent in the continent of North America. North America was, at the time, divided in two by the Western Interior Seaway which formed a deep, wide water body from (what is currently) the Arctic Ocean to the current Gulf of Mexico. It is possible that changes in the width, depth and temperature of the sea could have adversely affected food supply for these herbivores; or that such changes caused ecosystems to alter in ways that killed the dinosaurs in question: ceratopsids and hadrosaurs. Likewise, tectonic collisions – such as those that formed the Rockies and other mountains in the west – may also have played a similar role.

Had the Asteroid not Killed the Dinosaurs of all Species, They Would have Died Anyway

Regardless of the cause of decline, there is no certainty that the end result for these two groups would have been extinction. They may have somehow managed a stable state if the asteroid impact had not intervened. In fact, dinosaur diversity was known to fluctuate a fair amount through the whole of the Mesozoic (a period from 250 million to 65 million years ago). According to Norell, the relative importance of small decreases and increases between two or three intervals of time may not be significant across the entire history of these groups, when seen in context.

Ultimately this debate is purely academic, since the asteroid and its devastating effects put an end to it all. This study does, however, leave us with a different view of the evolution of lineages within the dinosaurs, making us realize that things were possibly more difficult for some groups than for others. This does open up the debate a little more for what ultimately killed the dinosaurs though!