Re: How bats got off the ground (online article)

[Michael Habib <mhabib5@jhmi.edu>]

> It's interesting to note that today: Birds are the most diverse land
> vertebrate group. Bats are the most diverse mammal group. Flying
> insects are the most diverse animal group.
>
> Could have pterosaurs been *much* more diverse than the current known
> dozens of species? (Say, more diverse than the whole dinosaur group?)

In fact, mostly due to insects, MOST animals fly.  Certainly by species 
numbers, likely by individuals (though crustaceans may tilt that a 
bit).  In fact, about half of all  living tetrapods fly (depending a 
fair bit on which sources you use...I get values ranging from 39% to 
51%).

In any case, in terms of the pterosaur diversity question, I've thought 
about this question before myself.

There were probably many more pterosaurs than we give credit for, but I 
actually doubt that pterosaurs were as diverse as birds or bats.  In 
fact, they probably weren't even that close with regards to species 
numbers.

I make this hypothesis based in large part on patterns of species 
richness in the modern world, and on the processes of speciation.  If 
you look within birds, one group (Passerines) takes the biggest chunk 
by far (about 5,000 spp, or 50% of all birds).  In bats, 
vespertilionids hold that claim to fame (I think about 35% of bats; 
note that the latest taxonomy includes over 1,000 spp of bats).  I  
suggest that the major reason for this is dispersal patterns.  In both 
cases, the species in question have a tendency to isolate, because they 
are largely short-range fliers (migration 'doesn't count' for these 
purposes, as I'll explain below*).

Songbirds and vesper bats both have the ability to occasionally seed 
far-strung colonies (because they fly), but once this happens they stay 
put.  A storm may blow a few sparrows to a new area, but they're pretty 
unlikely to backtrack to the mainland (and then naturalists come along 
and are astounded by their beak diversity).  In other words, birds and 
bats are dominated, in terms of SPECIES NUMBERS by large numbers of 
restricted range taxa.

Both groups, and birds especially, also have a few clades that are 
small, but widespread.  Seabirds and raptors often have enormous home 
ranges, and huge species ranges (ie. 'geographic range').  They cross 
many barriers easily, and are not prone to gene flow separation.  Yet 
they have very significant ecological impacts, because there are a lot 
of individuals.  Some seabirds and raptorial birds (especially things 
like tytonid owls) do form isolates, that then form subspecies and 
daughter species, but overall clades with hyper-mobile individuals 
don't seem to isolate easily, and probably tend to speciate slowly.  
They also probably stick around a long time, because having a huge 
range makes a taxon robust versus some extinction threats.

*Migration doesn't count because migrating species go to and from 
distinct areas, and don't generally breed while migrating.  The effect 
is that such species simply have two ranges, for the purposes of the 
model here.  A migrating species with two small ranges will still 
isolate easily compared to a species with a single huge range.


So, on to pterosaurs: the vast majority seem to be high-efficiency, 
long-range fliers.  Most appear to have been marine, and even inland 
species were likely to cover vast distances.  Isolating pterosaurs 
should have been about as easy as isolating peregrine falcons.  Thus, I 
hypothesize that pterosaurs comprised a clade with low species 
diversity, long average sp. durations, and that most species probably 
had enormous ranges.

Seriously, I'm done now.  Honest.

--Mike