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... vs energy deficient gigantothermy (boo hiss)

<<Time alone is not enough, of course. The biggest known mammal of all first
165 or so million years of mammal history was the 1 m long platypus
*Steropodon galmani*. For becoming gigantic a taxon needs to dominate an
ecological niche that allows for gigantism. This has rarely been the case
for birds. Biggest were the carnivores (Gastornithidae = "Diatrymidae",
Phorusrhacidae, Dromornithidae), that lived off rather small prey -- there
is no need for *T. rex* sizes if no hadrosaur-sized prey is available...>>

First of all, there are large herbivores that would indeed be big enough for
a large predator...ie Elephants.  I think the reason for lack of gigantic
mammals, predatory or not, is that as mammals get larger, they produce
significantly less offspring per year.  Archosaurs, on the otherhand, can
produce large clutches of eggs, sometimes several clutches a year, at any
size.  I suggest you  check out:

Janis, Christine M. and Matthew Carrano. 1992. Scaling of reproductive
turnover in archosaurs and mammals: why are large terrestrial mammals so
rare?. Annales Zoologici Fennici 28(3-4): 201-216

Randall I.