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Re: Phorusrhacids killing large mammals in National Geographic Channel



"I do not know which one was the largest pre-Pliocene South American
mammal, but pre-Pliocene Borhyaena was considered as larger than human
on the basis of the skull, but approximately 30 kg. on the basis of
the whole skeleton. The head of these beasts seem to have been very
large compared to placentals, and if a few 30 kg. African wild dog can
kill a zebra, I bet the much more massive head and musculature of
Borhyaena may have permitted this beast to prey upon large and not so
fast items (there was plenty of sloth species in South America,
although these would put a fight with their massive arms, claws and
large caniniforms)."

My cursory check of Wikipedia claims that *Borhyaena* reached 100 kg,
which is about the size of a large cougar. This is compared with
(again Wikipedia, I don't have time for a proper reference search
currently) *Kelenken*, which was roughly contemporanous with
*Borhyaena* - and which had a 71-cm skull, is estimated to have been
3+ m tall, and could perhaps have swallowed "dog-sized" prey *whole.*
(http://en.wikipedia.org/wiki/File:Comparo.jpg) If that's the kind of
competition *Borhyaena* was up against, I'm still not sure we can say
that the pouchies were near the top rungs of the predator ladder.

Sloths are slow, true, but they're also armored in osteoderm
chainmail, besides the weaponry you mentioned. I'm guessing that
critters like *Borhyaena* hunted more conventional prey, and that they
were the predominant predators in regions with heavy cover, where they
could hunt from ambush. They were short-legged and fully plantigrade,
IIRC, which would have made them quite slow compared to modern
quasi-cursorial hunters like cats, or even bears - which are only
plantigrade on the hindpaws - while more open terrain was dominated by
phorusrhacids. It's sort of a leopard/lion duality, with habitat
structure rather than direct competitorship being the more important
consideration. That's my guess, anyway.

Incidentally, my check of Wikipedia also suggests that there is no
evidence of *Titanis* in North America after 1.8 mya, way, way before
the Pleistocene/Holocene extinction pulse (albeit long after the
initial interchange).

"Seriemas do not perform long flights, however. They are said to be
helped climbing by the laterally compressed claws, which are also
present in Phorusrhacids. So perhaps they were not very different to
the smaller phorusrhacids in habits."

This was replied to (below), but I'll tack on some thoughts:

"A major advantage of flight in many (if not most) birds is that it
increases the number and quality of nesting sites. I wonder where, in
relation to its usual foraging habitat, the Seriemas nest. I know they
nest in trees (but, apparently within climbing distance)."

I'dd add that, specifically, nesting higher up makes a nest harder to
detect, even if an otherwise ground-based predator is capable of
easily reaching it. The ability to climb means little if the would-be
predator doesn't know the target is there. Phorusrhacids would have
been worse off due to their nests being more exposed to a greater
diversity of predators, large *and* small.

I'm not sure how the day/night problem would relate to marsupial
predators, which I assume would have had schedules similar to modern
predatory mammals in general. I think things like reproductive rates,
omnivory, and generally the things that may have given placental
predators an edge against their pouched counterparts need to be
considered in this context as well - since taking over ecospace from
those forms would probably have been a necessary first step in
becoming abundant enough to threaten the birds. (Of course, IIRC, the
interchange seems to have been preceded by an independently-caused
extinction that cleared the predator ecospace beforehand, but you get
the idea.)

We can't rule out big cursorial hunters simply eating the adults at
night, either. Dire wolves and sabertooths would have been all too
eager to take advantage of any big stubborn bird that had bedded down
in the open and was adapted for fight rather than flight.


"Elephants do so, and it is
difficult to me to find other adaptive explanation (assuming there is
one) for their tusks (ignoring intraspecific combat/sexual selection)."

Elephants also use their tusks to dig for groundwater in the dry
season, and for vital minerals where the conditions permit. But sexual
selection does seem to be the main factor, AFAIK.

"Regarding the question of whether the downcurved tip of the upper jaw
of phorusrhacids implyied carnivory: are not the most prominent of
these beak tips present in macaws?"

Macaw beaks have a strong curvature all along their length for maximum
strength/leverage against large, thick-shelled nuts. Raptors have
beaks that tend to be straighter proximally and only really curved
distally, at the "business end" so to speak, which is the pattern we
see in phorusrchacids. Even in keas, the only parrots to eat meat, the
curvature is weaker than in nut-cracking parrots and somewhat closer
to the condition seen in raptors. Macaws also have very short, robust
lower jaws, again not a good match for phorusrhacids (although raptors
aren't exactly perfect either, as you've mentioned). I would assume
that a big herbivorous ground bird would be mainly
frugivorous/folivorous anyway, rather than granivorous, so we wouldn't
expect to see a strong curve in those forms (as is true for elephant
birds, moas, takahes, etc; the kakapo is an exception, but that's just
a plesiomorphy from its parrot heritage).