Why have birds survived when dinosaurs went extinct?

Did you know that more than 90% of all species that have ever existed on Earth are extinct? Our planet has already passed through five big mass extinction events. After each of these events, almost all living organisms vanished at once. The organisms that survived were able to occupy and re-colonize the new environment, creating an almost whole new biosphere. But why did some species survive, while many others perished in mass extinction events? What made their life viable even after such a catastrophic event? These questions lead some paleontologists’ researches.

The Cretaceous-Tertiary (K-T) mass extinction is the most studied and famous event (though not the most severe), and marked the end of the “age of the dinosaurs”. Nevertheless, one group of dinosaurs has survived the apocalypse, diversified and live until now: the birds! Research developed by Derek Larson and colleagues suggests that the secret for these animals to survive was in their mouth.

Let’s imagine this scenario: sixty-six million years ago, an asteroid about 10 km wide had smashed into the planet. The impact caused several effects: initially, there was global heating, earthquakes and tsunamis. The collision had also spewed up a lot of dust, which blocked sunlight, leading the planet to a prolonged winter and to the collapse of the previous ecosystem. Most species that had been fully successful until then, from Velociraptor to Tyrannosaurus, could not survive the rough change in the environment.

To understand the dynamic of species’ extinction during K-T event, Larson’s team has studied more than 3000 teeth of small carnivorous dinosaurs. Teeth can tell us an interesting story about their owner species, like indicating the kind of diet the organism had. If we know that, we can have insights about the type of environment it could occupy, for example. The team has found that before the K-T extinction, there was a huge diversity of dietary specializations of these dinosaurs – fresh-meat, fish and insect eaters are few of the examples. Larson’s group also figured out that this diversity has not changed through the Cretaceous, which means that there was no gradual decrease of dental variety. Instead, it seems that these small carnivorous dinosaurs suddenly disappeared at the end of Cretaceous. The event also wiped out many small feather-covered dinosaurs, that possibly behaved just like true birds.

Nevertheless, a few types of birds, the Neornithines group, had what it took to live on in the new environment. One of the few differences between these birds and others that could not survive was that Neornithines had replaced teeth with a keratinous beak (which truly is one of the traits that only modern birds share in common). The type of’ beak these birds exhibit was also correlated to a herbivorous diet –  they were probably seed-eaters.

birds_dinosaurs

Figure. The dinosaurs that got away. Family tree of dinosaurs and close relatives, showing victims (in red) and survivors (in green) of the end-Cretaceous mass extinction. Current Biology 26, R416, May 23, 2016.

So, do you remember how the environment was, after the asteroid impact? There was no sun light because of the dust – which means that there was no photosynthesis either. With no photosynthesis, many plants died. When living plants were going scarce, leaf-eaters and other living-plant dependent animals didn’t have food and died too. Nevertheless, seeds are a nutrient-rich resource and seed banks derived from plants can remain viable for more than 50 years! Needless to say, animals that could absorb nutrients out of these seed banks and from detritus had a great advantage in surviving. Well, that’s something birds with beaks could do. Therefore, dietary specialization toward seed-eating in some lineages of birds may have been one of the key factors in their survival through the end-Cretaceous mass extinction.

Like every step in Science, this may not be the final word on the subject. There may be other morphological or physiological characteristics of Neornithines that gave them better odds of surviving after the asteroid hit, and new ideas must be tested. So far, this research show us that even dinosaur teeth can tell us a great story. Besides, we can use the knowledge of what happens during mass extinction events to better understand our modern world, since Earth is undergoing huge changes in climate and temperature, ecosystems are collapsing and many species are going extinct.

By Bruna de Oliveira Cassetari.

Reference:

Larson et al., Current Biology 26, 1325–1333 May 23, 2016. http://dx.doi.org/10.1016/j.cub.2016.03.039.

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