[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Island-dwelling dinosaurs (was Re: Gargantuavis neck vertebra)

Rescued from truncation, my additions below:

In response to John Bois:

>  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.
>  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


I'd like to add:
-- "(except Cassowary)", you write -- yes, what about the cassowaries? How do they (3 species last time I checked) keep their nests from being plundered by goannas too often?
-- "Hunt down" isn't a term I'd apply to "nesting sites".
-- The timing of the extinction of Sparassodonta is not actually known to fit the Great American Interchange, AFAIK. That leaves thylacine vs. dingo as a sample of 1, and even there I have no idea what effect it had that people have been burning much of Australia down every year for some 50,000 years now.