Re: Re-emergence of lost features.

[John Bois <jbois@umd5.umd.edu>]

On Fri, 13 Mar 1998, Thomas R. Holtz, Jr. wrote:
> We don't have any good evidence that they "lost out" to carnivorous mammals,
> since a) they coincided for much of their history with the carnivorous
> mammals of South America and b) they overlapped in time and space with very
> modern style carnivorans during and after the Great American Interchange.
> One could argue that their extinction was due to competition with, for
> example, sabrecats, but one could argue on pretty much the same evidence
> that the extinction of the sabrecats was due to competition with the last
> phorusrhacoids.

Marsupials have smaller brain cases and lack the more acute sensory
abilities of placentals.  As such, they likely had greater ability in
locating phorusrhacoid nests.  In extant large-bodied birds, ostriches
at least, the great bulk of mortality derives not from competition, but
from predation on eggs and juveniles (ref. if needed).  Adult forms are
relatively immune from predation.
My point here is that although we cannot know what did in the
phorusrhacoids, relevant extant examples ought to carry some weight in the
discussion.  I would also argue that post-interchange-time-to-extinction
was short enough to allow for placental predation hypotheses.