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Your previous replies showed me that my idea was indeed quite reasonable.
So here is a possible reconstruction for Brontornis:
The "classical" interpretation for Brontornis is to regard it as a
slow-moving scavenger. But when I compared the Brontornis madible to those
of living scavenging birds (vultures), their was a striking difference.
All vultures are shipped with rather long, flat mandibles, and none of them
shows this strong upward curvature - obviously this is sufficient for a
scavenging way of life, even for those birds who could open bones.
The second idea came to me when i browsed a book about parrots - at least
some of the skeletal drawings there showed a jaw quite similar to
Brontornis. According to the descriptions, these birds were eating nuts
and fruits. So following the idea of functional analogy, we could
reasonably assume that Brontornis fed on fruits and nuts ....
But is is not so easy, of course. In all birds, food is processed in the
stomach, not in their mouth as in mammals. So a bird would only need a jaw
that is sufficient to TAKE UP the food - this is why even large vegetarian
birds like ostriches and moas, only have small heads: They don't need a
larger head for their kind of food. Of course this would still leave the
possibilty that Brontornis used to crack its dayly load of coconuts, but
so far I haven't heard of coconut trees in Miocene Patagonia.
This would leave us with the interpretation that Brontornis was indeed a
carnivorous bird, and that it used its jaw for killing prey. The mandible
is so strongly built that Brontornis must have used to apply immense
biting forces, or else this was a case of "over-design", which is not
favoured by evolution.
When hitting a prey animal, the jaw of Brontornis would have inflicted a
severe wound on the first strike. Helped by the strong upward curvature,
the bird would have bitten a large chunk of flesh out of its prey, which
would have been incapable of any further defense, then.
This is very similar to the way of life assumed for sabertooths like Smilodon
or Thylacosmilus. But while sabertooths must have avoided contact with
bone (or else they risked breaking their teeth), this limitation would not
apply to Brontornis. On attack, this bird could even have bitten into an
animal's ribcage without risk of injury. And of course it could have
crushed and eaten bones. Brontornis would perfectly fit the idea of a
top-predator of Miocene Argentina.
The longbones of Brontornis add another interesting observation. This bird
had a short tarsometatarsus, which makes it a rather slow runner. But it
must have been very strong and heavy (237 - 320 kg). I would not say that
Brontornis preyed on glyptodonts (too well armored even then), or ground
sloths (too much difference in body mass - and if the clawed forelimbs of
ground sloths were really adapted for stabbing, they may have easily
killed the bird).
So a prey candidate for Brontornis must have been of medium size
(cow-sized would be ideal), and rather slow in motion and not too
efficient in defence. Their is one group of Sout American animals which
would fit this requirement - the family Astrapotheriidae. Astrapotheres
were widespread and diverse in early Miocene times, so Brontornis might
have fed on them.
Of course the bird could not have chased down an astrapothere in the open
field - it would have relied on cover to approach near enough, one or two
dozen meters at least. Then it could have surprised its victim from
ambush. In early Miocene times, Patagonia ejoyed a rather humid climate,
and it certainly wasn't the strict arid grassland as later in the
Pliocene. When Brontornis was alive, their was plenty of cover for this
Things changed in the course of the Miocene - climate became increasingly
arid, and vegatation turned into a more uniform grassland. As a
consequence, herbivore diversity dropped markedly - and among the victims
were the astrapotheres, their entire family became extinct. In lack of its
favorite prey, Brontornis must have turned towards other resources, but
then it would have competed with its more agile phorusrhacid cousins. In a
country with less and less cover, this was of no success. After the
Mid-Miocene, no brontornithids have been shown up so far, indicating that
they indeed became extinct by then.
As some kind of compensation, the line of Phorusrhacinae may have
converged on brontornithids in some points - Onactornis showed very
similar foot bones, and it certainly could not have competed with
Andalgalornis for the same prey. But the "leader bird" was still agile
enough to cope with typical savannah inhabitants.
This would be my story, so far.