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Re: Pterosaurs and birds, was Re: birds and pterosaurs

Some comments inserted below
JimC

----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <jorgedich@yahoo.com

Are you thinking there is a metabolic difference then?

Although I think pterosaurs are endothermic, it should be noted that except for travel at altitude, their flight style does not require endothermy. There could have been a substantial metabolic difference, though I personally doubt that there was.

It is not exactly physical constraints (body mass,
wing size etc.). It is ecological constraints
(like most biomechanical problems). Bird which
feathers grow for too long cannot fly "well enough to
feed". What is "well enough to feed" ultimately
depends from food in birds native ecosystem.

I am not really sure what you mean by this; I'm assuming that you're implying that birds of very large size would be poor flyers.

No, I think he's assuming a slow growth rate of feathers that would limit size. I do think that mechanical limits on practicable feather size would limit span in birds, but if I understand him correctly, I think that feather growth rate in the big birds was faster than he is assuming.

However, large size is an advantage in soaring flight in many ways, 

Indeed so.

Judging that 7-m wingspan birds are sporadic in fossil
record, they need extremely food-rich conditions to
evolve. 7-m pterosaurs were more common - ergo they
were more efficent, they ecological niche was less
restricted.

Actually, pseudodontorns were pretty common. Granted, there were more large pterosaurs for a longer period of time. However, I suspect the reason for the abundance of large pterosaurs was that most Cretaceous forms were marine soaring birds,

I'm not aware of any large soaring birds that overlapped the pterosaurs. I thought the big pseudodontorns developed after the demise of the pterosaurs. In fact, I don't know of any soaring birds of any size that overlapped the pterosaurs (there probably were some, a few -- but I suspect that the more efficient pterosaurs kept their numbers down).

And yes, mechanical constraints do seem to differ such that pterosaurs generated more large-bodied flyers than birds,

For a given span, pterosaurs tended to have smaller bodies (torsos) than birds, but larger necks and heads.

but this is not the same as being 'more efficient' or 'less restricted' (I suspect it was partially to do with the relatively lower wing loadings of pterosaurs per unit mass.)

In the bigger sizes, pterosaurs didn't have lower wingloadings per unit mass. Pterosaurs had relatively more muscle in the inner arms, and they tended to have larger necks and heads than birds. For example, a 5 meter span Anhanguera piscator could be expected to mass about 18-19 Kg, with only a minimal fat load. A 4.8 meter span Quetzalcoatlus species could be expected to mass well over 20 Kg.

One problem with the food-rich conditions hypothesis for large soarers is that while large soaring birds do need a large total allotment of food, they should also be better at covering large distances to find it. That is, a large soarer is actually well adapted to low food density (though total food biomass must be reasonably high).

Note that the geographic area that can be searched for food per unit time increases as the as the animals' weight increases. Weight can be an advantage for a soaring predator.