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

Re: birds and pterosaurs (long)



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>