Re: Claws on deinonychosaurs

[Graydon <oak@uniserve.com>]

On Tue, Nov 01, 2005 at 08:17:40AM -0500, Thomas R. Holtz, Jr. scripsit:
> > From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On
> > Behalf Of
> >> Tim Williams 
> > Seriously... it does beg the question: if the _Deinonychus_
> > specimens were casualties of a desperate _Tenontosaurus_ fighting
> > for its life, isn't this is a fairly high mortality rate for a
> > _Deinonychus_ pack?  The species could not afford such losses every
> > time a pack went after large prey.
> >
> > Just playing Devil's advocate.
> 
> And playing Devil's advocate to your Devil's advocate...
> 
> Dinosaurs are not placentals, and Deinonychus packs (if such existed)
> are not wolf packs. Dinosaurs would necessarily have higher mortality
> rates than mammals, as they had much greater potential fecundity
> rates. That is to say, clutch size for dinosaurs are much greater than
> typical litter size for mammals. (This disparity is far greater at
> larger sizes, though).
> 
> So, all things being equal, the expectation is that far more
> Deinonychus would die per clutch than would Canis or Panthera per
> litter. Similarly, though, it would be easier to help maintain pack
> size (again, if such existed) even with high mortality.
> 
> All this is part of Holtz's Law of Mesozoic Terrestrial Paleoecology:
> In the Mesozoic, Life was Cheap!

Even if life wasn't quite _that_ cheap -- the extractable food value of
one Tenontosaurus wouldn't have produced 3 replacement adult pack
members -- it's relatively easy to argue for extreme circumstances; yes,
attacking this big thing is a bad idea, but there's a drought/late
spring/missed migration/cold snap/weeks of rain on and we're starving,
so what do we have to lose?

It's not even that difficult to postulate seasonal bad times, so that
there's this one fortnight every year where bringing down one big prey
animal hugely affects reproductive success, and the rest of the year is
spent living on lizards.  (This kind of thing happens with modern
predators; the Algonquin park wolves only go after moose when they must.
That 'must' is at least partly seasonal.)

I doubt the paleoecological information is available, but if it is,
there might be a correlation in small dromeosaurs between sickle claw
size and environmental extremes -- species in environments that don't
run out of lizards don't need to try to take down large prey, and have
smaller claws in consequence.