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Re: The life of birds
Dann Pidgon wrote:
>My questions are thus:
>What of ratites such as the ostrich, emu, rhea and cassowarie?
>Did they develop on islands and eventually move to the mainland
>when they had reached a certain predator-resistant size, or is the
>flightlessness/non-predation relationship over simplifying the
First of all, I think we have to distinguish between two body forms among
flightless ground birds. One is the swift, lean runner, typified by the
ostrich, rhea, emu, and cassowary. The other is the heavy, graviportal
"waddler," demonstrated (rather poignantly) mostly by extinct birds such as
moas and elephant birds (and that Australian bird now being reclassified as
a giant flightless anseriform). At least, that's what some reconstructions
of moas and elephant birds portray them. I wouldn't know how to classify
the body form of the kiwi, diatrymids (runners?) and phororhascids (runners?).
>From what I know, traditional thinking has it that the ratites all evolved
from a common Gondwana ancestor and developed separately afterwards as the
southern continents parted. Africa, South America, and Australia all had
mammalian predators (placentals in the first, marsupials in the other two)
that must have been a factor in the evolution of fast runners. New Zealand
and Madagascar are relatively predator free (Madagascar never had large
predators, right?) and so there was no "pressure" to evolve into running
forms. I think, in the case of South America, thre might have been some
form of co-evolution between rheas and phororhascids. The latter must have
been an added "pressure" for the evolution of a swift-running rhea, as it
now had to contend not just with marsupial predators but now this monster
of a bird which could run as fast as it, equipped with a killer beak and
massive claws on its arms.
Now, if I'm not mistaken, the emu and cassowary, though related, occupy
different environments, right Dann. The former is an open field creature,
while the latter seeks the cover of forest. If the cassowary originally
evolved in the forest, would it have been a runner to begin with? Or does
this mean that the cassowary split off from the emu after the basic body
form was attained and then moved on to the forest?
I can't think of a scenario for the North American diatrymids. What were
they, runners or waddlers? North America had a plethora of placental
hunters (both carnivores and creodonts) that must have applied selective
pressure on the evolution of large ground birds.