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How birds lost their teeth, a theory...





In a recent publication of a landmark study where 40 species of birds DNA were sequenced and cross compared some really illuminating findings about probable evolutionary reasons for specific traits and relationships were revealed. One of the most interesting things is what was not realized about the origins of the birds beak.

"Although birds are descended from dinosaurs, they have no teeth. Most previous studies have suggested the common ancestor of modern birds lacked teeth, but other work has disagreed. The new findings add support to the idea of a toothless common ancestor, because all birds sampled share some mutations that turn off five genes for building teeth.

Researchers estimate that teeth, or at least enamel-covered teeth, disappeared about 116 million years ago in the ancestry of birds. They suggest beaks replaced teeth in a two-step process. But it's still not clear why beaks took over for teeth, said study author Robert Meredith, a biologist at Montclair State University in New Jersey."


Indeed, this line immediately brought up an idea of how it might have worked based on what we know about evolution and the forces that cause adaptations to flourish or to disappear as lineages continue to specialize into constantly varying niches. I'll start by stating that it should seem obvious that the common ancestor for birds...that from which the line descended as prior that they were classed more accurately as dinosaurs with features than birds is what we know from dinosaurs that they had teeth.

Even if the progenitor of the Avian line didn't have teeth itself it still descended from some land living dinosaur this is pretty clear from the many fossil finds of early proto Avian forms among the dinosaurs which have been found in China where it appears the Avian transition took off from 150 million years or so ago. We know for a fact that this ancestor did indeed have teeth as all early dinosaurs did...so how and why did birds loose them?

I think conservation of energy in the context of lineages exploring new survival niches created by the combination of access to feathers and trees tells the likely tale. The theories regarding bird flight that are most popular today advocate for a top down approach to evolution of birds, the theory being that animals attempting to gain the upper hand (literally) on their pray would do so from rock faces, vines and trees and bushes...stalking with feathered wings could provide gliding advantages that would allow a striking animal to both increase the glide length of their descent to the pray as well as slow the rate of fall allowing them a measure of increased success while hunting relative to those that do not take such advantages. A likely temporally significant adaptation was the formation of increasingly ossified skeletons to reduce the carrying weight of the animal and thus again increase glide distance and slow descent rate.

The genes of these individuals then propagated and the strategy continue to be employed from higher and higher vantage points selecting from those lineages individuals with either longer feathers or other advantages to such a mode of attack.

Eventually creatures like Archeoptrix ruled the forests where they took advantage of their feather laden hands and high vantage point to thrive but as those hands became wings and the descendants become capable of powered flight from the ground as their bones ossified. Mean while the strategy of hunting from the vantage point was replete with dangers...as these animals became less dense the animals they could target as prey continued to be lethal land living therapod or mammalian forms, the task of hunting was both dangerous (throwing yourself down from a high point is not exactly an optimal survival behavior trait) time consuming.

However something became more true the greater their ability to ascend the trees and later fly between them was unleashed. The new lineages were able to find different food sources away from those that their ancestors prayed upon, the vast access to nuts, seeds and fruit evolved around the same time as the angiosperm radiation continued apace, I posit provided a huge niche for potential exploration as a food source that up to that point no other large animal (besides the super massive therapod dinosaurs) were able to mine efficiently.





The early therapod descended avians who transitioned to more fruit and nut eating could satisfy their dietary requirements (which were increasingly more efficient compared to land living thanks to that high metabolism and low bone density they'd evolved) using these sources while not engaging in the dangerous activity of attacking much more bone dense and potentially lethal pray forms on the ground.

With food sources waiting in the trees the need to have teeth at great energy cost no longer presented itself as a requirement, the efficiency of eating nuts and berries with a beak would provide an advantage to exploiting those resources and so the first mutations in lineages to reduce tooth profiles likely allowed those lineages to more efficiently consume nuts and fruit while avoiding increased likelihood of death from not attacking ground pray. Several hundred or thousand generations later and the beak had spread as an adaptation across lineages exploiting this new (angiosperm radiation hit its stride around 160 million years ago right in the midst of the therapod to avian evolutionary shift) and relatively untapped regime.

So a multi step process of the energy efficiency for continuing to attack downward led to an increase in efficiency for attacking the static and safe food in the trees now evolving around them an increasing fruit and nut carrying capability, they had ascended to meet this static bounty rather than the dynamic and dangerous food they had descended from.
We know that birds in fact form a critical piece of the seed distribution puzzle for the angiosperms and so a synergistic relationship now formed explains the radiation and expansion of the flowering plants along with the dinosaurs and the insects who also enabled and thrived as the angiosperm radiation took place.

Update:

In a related discussion on this thread at Facebook a friend noted the insect radiation that was on going about the same time as a likely driving food source in the trees for early avian forms to use to supplant land based quarry. I assert this was indeed a factor but a lesser factor as insects still need to be captured at energy cost relative to fruits and nuts which are simply sitting there.
So step by step:

1) (~200 - 180 mya) Small feathered dinosaurs learn to ambush from increasing heights.

2) (~180 - 160 mya) Feathers evolve to enable greater height ambush without damage and better glide times. At same time Angiosperm radiation goes into full effect as insect synergy begins.

3) (~160 - 140 mya) Bone density reduction happens in early avian forms allowing yet higher ambush falls and glide times. Opportunity costs for teeth starts to dwindle as attack cost for land predators increases.

4) (~140 - 120 mya) Angiosperm radiation quickens as fruits and nuts evolve in force, insects explode provide secondary air born sources of food. Early avian cost for attack on ground exceeds cost for taking either insects or static fruits and nuts.

5) (~110 mya) True birds evolve specializing in either fruits and nuts and insects.




Links:

http://en.wikipedia.org/wiki/Origin_of_birds 

http://en.wikipedia.org/wiki/Flowering_plant

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