Re: birds and pterosaurs (long)

[Michael Habib <mhabib5@jhmi.edu>]

The Wang et al. paper is interesting (though, as all Nature papers are, 
a bit too brief to get much data from).  I agree with Phil that 
taphonomy may be playing a role, but it is not true that the long bones 
of pterosaurs are universally thin-walled.  The larger pterodactyloids 
(such as Pteranodon sp.) indeed had very thin walled bones.  However, 
smaller species, especially more basal forms, had much thicker cortices 
in their long bones (take a look at the figures in the histology papers 
published by Padian et al. on small pterosaurs and dinosaurs).

The overall aspect ratio, wing loading, and other wing shape parameters 
(such as the tapered wing profile) of the large-bodied, Cretaceous 
pterodactyloids were probably most similar (when compared to extant 
flyers) to frigate birds (albatrosses, etc. have rather higher wing 
loadings but are also somewhat similar).  The location of fossils and 
wing shape, along with the relatively large body size, implies that 
most Cretaceous pterosaurs were accomplished marine soarers.  With 
their high aspect ratios, but reasonably low wing loadings, they may 
have been capable of both dynamic soaring and riding marine thermals 
(frigates do both, but otherwise convective soaring is much more common 
amongst terrestrial birds today).

In general, though taphonomy could be playing a role, the Cretaceous 
pterosaurs currently known (especially the large species) seem to have 
physical characteristics that would indeed be associated with pelagic 
lifestyles.  On the other hand, the lack of many terrestrial pterosaur 
fossils obscures whether there was a diversity of pterosaurs better 
built for terrestrial existence.  In theory, large-bodied pterosaurs 
flying above the land would have thin cortices as well (Quetzalcoatlus 
may, in fact, be such an animal), but that is not certain, which makes 
the taphonomy argument more complicated.

The greatest diversity of extant birds is still (with regards to total 
species) among semiarboreal and semiterrestrial animals (ie. 
Passeriformes).  In fact, it may be that semiarboreal habitat 
preferences actual promote speciation (by increasing the chances of 
small-scale isolation).  For this reason, the comparison of species 
numbers may generally be biased towards groups of flyers with a 
semiarboreal mode of life, especially small-bodied forest dwellers.  I 
am also somewhat hesitant to assume that the diversity of seabirds was 
low, given the large range now attributed to hesperornithiforms (in 
time and space), and the fact that the secondary flightless nature of 
those animals implies the presence of earlier, volant, loon-like forms. 
 (that being said, pterosaurs certainly seem to have had the market 
cornered on large-bodied pelagic soarers pre-K/T).

--Mike


On Saturday, October 8, 2005, at 08:00 AM, Phil Bigelow wrote:

> I think taphonomic processes are partly responsible for this 
> observation.
>  Inland environments tend to be brutal on delicate fossils.  And 
> although
> birds have relatively delicate bones compared to other vertebrates, my
> observation is that the bones of pterosaurs are even more delicate.  
> The
> long bones of pterosaurs have a thickness (measured from the exterior
> surface to the medulary cavity) of two chicken egg shells.
>
> In contrast, coastal plain environments and marine environments have a
> lot of standing water which tends to protect the fossils prior to their
> burial.  The exceptions to this are beach environments.
>
> The Hell Creek Formation is a coastal plain unit that contains a large
> number of fragments of birds, while the first pterosaur from the
> Formation was reported only a few years ago.  Terror Lizards are by no
> means common in the Hell Creek Fm. (although this may also be related 
> to
> their declining diversity and population numbers during the latest
> Maastrictian).
>
> <pb>