In response to John Bois:
>=20
> Two things worth considering:
> 1. Some continental habitats are like islands inasmuch as nesting
> large flightless birds can enjoy low predator density. Highest
> concentration of ostriches occurs in the more arid sections of savanna
> grasslands. Indeed=2C all continental large flightless birds (except
> Cassowary) depend on expanse of grassland for nesting.
>=20
> 2. Not all predators are created equal. A still-open hypothesis is
> that placentals have a predatory edge over marsupials. The latter are
> handicapped by their ontogeny particularly in the sensory and social
> domains=2C i.e.=2C their brains have less time to develop before basic
> structures must be laid down in order to make trek to pouch. So niches
> that worked before presence of placentals may have been islands
> (excuse the phrase) of low predator ability. Without doubt=2C thylacines
> were fearsome animals. However=2C they may not have been as clever at
> hunting down large flightless bird nesting sites as their placental
> counterparts.
Those points are valid but I don't know if they tell us very much in terms =
of fitness of flightless birds vs. predatory mammals. The first argument is=
a no-brainer: It's only logical that prey species should or could be more =
common in environments with low predator density. Ostriches are extremely o=
mnivorous=2C tough and very fertile=2C so it seems natural that they might =
grow much more common when predation stress is more limited. I think that p=
oint is moot. Regarding cassowaries=2C predators may not have been quite as=
limited before the Holocene. That certainly wasn't the case for emus or rh=
eas. Nor was it for brontornithids=2C dromornithids=2C phorusrhacids and es=
pecially eogruids. The latter were ostrich-analogues that thrived alongside=
giant hyaenodonts=2C mesonychids=2C entelodonts and whatever *Andrewsarchu=
s* was. Later on=2C after the demise of these creatures=2C they would have =
had to contend with a more modern fauna of placental predators. Perhaps sma=
ller=2C but presumably more flexible and adaptable. Gastornithids=2C *Eremo=
pezus* are comparable cases in that they too coexisted with more archaic pr=
edators. That was true of *Remiornis*=2C *Paleotis* and so on as well. They=
may well have evolved in semi-insular conditions=2C but they survived in c=
ontinental ones.
Your second point reeks to me of either 'placentals are inherently superior=
to marsupials/metatherian'. This may be true=2C but I don't think the Ceno=
zoic crop of marsupial/methatherian predators are good proof of that. It's =
true that they rapidly declined and disappeared in the face of placental en=
croachment upon their habitat but your own argument of 'some continental en=
vironments are functionally islands' might apply here: Generally=2C island =
faunas fare poorly when continental or 'more continental' invaders show up.=
This tells us more about the fragility of island ecosystems than of any in=
herent inferiority of the islanders' lineage. Your argument amounts to 'If =
the sparassodonts had been Old World creatures and the carnivorans were Sou=
th American=2C sparassodonts would still have bought it during the Great Am=
erican Interchange on account of their reproductive methods'. You might be =
right=2C but I have serious doubts about it and it can't be proven. I think=
the case for 'methatherian predators are largely extinct because they had =
the disadvantage of evolving in less demanding ecosystems' is much=2C much =
stronger than for 'metatherian predators can only kick the bucket once plac=
ental predators show up'.=20
Brian