Re: Waimanu

[Michael Habib <mhabib5@jhmi.edu>]

On Sunday, May 21, 2006, at 12:57 AM, john bois wrote:

> "Yes...but perhaps the lack of diversity among small size classes (due 
> to
> avian competition/predation) placed the whole pterosaur taxon on a 
> ratchet
> of no return.  They were doomed to low diversity of size classes at 
> least.
> This is problematic from a taxon-wide standpoint.

In some ways it is, but I think calling the clade "doomed" is a bit 
strong.  By that reasoning, every large-bodied clade on earth is doomed 
to hit extinction soon.  Would you call elephants, artiodactyls, 
whales, ursids, etc all doomed?  Technically they are, because all 
clades go extinct eventually.  And it is true that large bodied animals 
have had trouble during several of the mass extinctions.  However, many 
large-bodied clades have had long durations.  This occurs, in part, 
because they tend to have large geographic ranges, which makes them 
robust against regional extinctions.

To take a more analogous example, pseudodontorns (which were discussed 
just the other day on the DML) became an entirely large-bodied clade 
early in their history (Paleocene).  However, they lasted most of the 
Cenozoic.  I would have hardly called the Paleocene forms 'doomed', 
given they had another 50+ million years of history to come.

It is worth noting that they had a worldwide distribution, despite 
having low taxon diversity.  This probably went a long way to keeping 
them around.  I suspect pterosaurs were similar in their range extents, 
by virtue of being large-bodied soarers.  This particular mode of life 
probably makes large-bodied soaring clades robust to local extinction 
events (and most 'background' types), but susceptible to global 
extinctions.  Because global phenomenon are rare, large-bodied 
supersoarers should be robust to extinction, not "doomed" to an early 
demise.

Sure enough, large pterosaurs lasted a long time, before finally 
hitting a wall at a major global-level extinction (ie. the K/T)

> My understanding is that large species (w/low reproduction rates, 
> etc.) are
> more prone to extinction.  This would seem to require a more or less
> constant refuelling from among the smaller species from the same taxon.
> This fuel may not have been present.for pterosaurs--if true they may 
> have
> been doomed, period."

They are more prone to global scale extinctions, as I mentioned above.  
Some large species, such as soarers, actually seem to be resistant to 
extinction otherwise.  They do not speciate rapidly,  however, probably 
owing to high levels of gene flow over huge distances.  Thus, they 
maintain long-lived clades with relatively small numbers of species.  
By contrast, many rapidly speciating, small-bodied groups are very 
likely to go extinct even under regional disturbances, because they 
occupy a very small area.  Geographic range is one of the best 
predictors of extinction risk (check out the literature by Gittleman, 
Cardillo, Purvis, Mace, and others.  I can supply refs and/or pdfs if 
required).

Personally, I think there are many misconceptions floating around about 
speciation and extinction, and their connections to clade traits.  I 
find that these misconceptions spawn most often from a feeling that 
there is some kind of ongoing 'war-like' battle for niche spaces, such 
that no clade could reduce in character diversity unless under duress.  
In reality, there are plenty of reasons why a clade might be under 
directed selection for large body size (or other traits), and they do 
not (on their own) imply that the clade is doing 'poorly'.

Cheers,

--Mike Habib