Sunday, 1 May 2016

Why Dinosaur-Birds Evolved Beaks

Diet affects the evolution of birds -- ScienceDaily

One of the nicest things about studying biology, and particularly evolutionary biology, is the way it all dovetails neatly into place and fits in with, for example, climate change, geology, etc., to form a complete coherent picture that tells us not only why things are as they appear today but how.

For example, we it take for granted that there will be birds around. The world as we know it would be quite a different world without birds. Flowering trees might not have small berries; humans might not have colour vision because our simian ancestors would not have needed to recognise ripe fruit by the colour change that signals to birds that the fruit is ripe.

And now we learn how the reason we have birds at all may all be due to a fortuitous evolutionary direction taken by some of their dinosaur ancestors. This enabled them to survive the conditions which exterminated the other dinosaurs and it wasn't just the evolution of bipedalism, feather and the beginning of wings. According to a paper published in Current Biology published a few days ago by a team from the Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Canada, it might well have been that some of them had begun to evolved beaks.

Highlights
  • Tooth shape disparity in small maniraptoran dinosaurs is examined in the Cretaceous
  • Results show stability and sudden extinction in this guild at the end of the Cretaceous
  • Groups are mostly static in shape space except for larger size in the early Maastrichtian
  • Evolution of an edentulous beak and granivory may have been key to the survival of birds

Summary
The causes, rate, and selectivity of the end-Cretaceous mass extinction continue to be highly debated. Extinction patterns in small, feathered maniraptoran dinosaurs (including birds) are important for understanding extant biodiversity and present an enigma considering the survival of crown group birds (Neornithes) and the extinction of their close kin across the end-Cretaceous boundary. Because of the patchy Cretaceous fossil record of small maniraptorans, this important transition has not been closely examined in this group. Here, we test the hypothesis that morphological disparity in bird-like dinosaurs was decreasing leading up to the end-Cretaceous mass extinction, as has been hypothesized in some dinosaurs. To test this, we examined tooth morphology, an ecological indicator in fossil reptiles, from over 3,100 maniraptoran teeth from four groups (Troodontidae, Dromaeosauridae, Richardoestesia, and cf. Aves) across the last 18 million years of the Cretaceous. We demonstrate that tooth disparity, a proxy for variation in feeding ecology, shows no significant decline leading up to the extinction event within any of the groups. Tooth morphospace occupation also remains static over this time interval except for increased size during the early Maastrichtian. Our data provide strong support that extinction within this group occurred suddenly after a prolonged period of ecological stability. To explain this sudden extinction of toothed maniraptorans and the survival of Neornithes, we propose that diet may have been an extinction filter and suggest that granivory associated with an edentulous beak was a key ecological trait in the survival of some lineages.*
*Citation reference removed for clarity. See original paper.


Copyright © 2015 Elsevier Inc. (Open Access)

Because flowering plants had co-evolved with insects to produce seeds as their means of distribution, there was now a supply of seeds as a food source for anything which evolved the means to eat them, so a group of dinosaurs probably evolved this ability and evolved beaks as a more efficient mouth than teeth for picking up and cracking these seeds which would not then had needed chewing. Some of these would seem to have been the dinosaurs which had evolved feathers and a bipedal gait.

The small bird-like dinosaurs in the Cretaceous, the maniraptoran dinosaurs, are not a well-understood group. They're some of the closest relatives to modern birds, and at the end of the Cretaceous, many went extinct, including the toothed birds - but modern crown-group birds managed to survive the extinction. The question is, why did that difference occur when these groups were so similar?

Derek Larson. Lead author. Paleontologist at the Philip J. Currie Dinosaur Museum in Alberta
The most widely accepted theory for the extinction of most of the dinosaurs, which were the megafauna of their day, was that a comet strike on earth threw up do much dust into the atmosphere, maybe added to by volcanic activity, that Earth went into several years of permanent winter. This would have killed most of the plants, most of the animals which either couldn't survive without summer sun or who lost their source of food became extinct. But what food is best at surviving winter? Seeds of course!

So, we could have had a combination of feathered, beaked small dinosaurs already evolved to eat seeds, in an environment which needed them to have the ability to keep warm (feathers) and the ability to eat what may have been one of the few plant foods available and which those best able to eat it would be able to win the competition for it.

Here we have the essence of an explanation of why the dinosaurs which, with a little more evolution became what we now call birds, were the only ones to survive the mass extinction which killed off the rest of them. It would be interesting to speculate about what this world would now be like if this group had not taken this evolutionary direction and we no had no birds. There was, of course, no plan involved, just the operation of natural selection producing species best able to exploit their environment as their environment changed around them.

This, of course, is why life on Earth look as though it been designed to live here. It has been, by an entirely magic-free natural process. The great thing about science is how, because it's true, everything converges on a single truth in which all the geological, palaeontological, cladistic and genetic evidence all point at the same thing.

Compare this to religions which continually diverge and to creationism which can't get by without a lot of denialism of evidence, invocations of magic and evidence-free assertions, and even then different factions can't even agree on the timescale or come up with a testable mechanism. It's like looking in a book of fairytales expecting to find a biology text.

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