(asteroid soaring) - [Host] A gigantic asteroid, six miles in diameter collided violently with earth.

(asteroid exploding) Forests burned in volcanic ash, soon filled the skies blocking out the sun.

This was an extreme survival situation, one that 75% of earth's species weren't ready for.

Among the mammals the smallest, some weighing less than one pound managed to escape the catastrophe, as well as some reptiles like turtles and crocodiles.

Birds were also hit hard.

Only the smallest, such as Asteriornis survived the chaos.

How is it that all but one group of birds were wiped out?

What did they have that no other archaic bird and no other dinosaur had?

(steps plodding) Janavis is a bird that did not make it.

Its fossil dates back 67 million years to a time just before the asteroid hit.

Close analysis is helping us to better understand the reasons for its disappearance.

- Here you can see the skeleton of Janavis.

We have some of the bones, particularly the arm bones, and part of the peripheral column.

- [Host] Equipped with teeth Janavis has an imposing stature.

Is this why it didn't survive the asteroid strike?

The key difference between Janavis and Asteriornis is size.

Asteriornis is much smaller, a trait that was key to its survival post-asteroid strike.

- So larger bodied animals through this mass extinction event didn't do very well at all.

So a really big bird like Janavis probably would've been at a major disadvantage compared to Asteriornis after the extinction event.

Getting a meal for an animal the size of Janavis probably would not have been a very easy thing to do.

- They also had very different ecologies.

Janavis was basically a seabird like we will have been fishing and eating fish and squid.

Asteriornis was more like a generalist, ground dwelling, bird eating whatever he found in the beach.

- [Host] With the trees gone, a large number of tree dwelling birds could not survive.

Instead, it was ground dwellers like Asteriornis that held the advantage.

Archaic birds, although more numerous, were devastated by the asteroid.

Their ecosystems were destroyed.

In this hostile environment, their imposing size life in the trees and specialized diets led to their extinction.

Only modern ground nesting birds with more flexible diets were resilient enough to survive a decimated landscape.

(soft music)