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Re: giant toothed bird-species or genus that lasts 50 million yrs?

 Yes, but what I am trying to ask is would an "Apatosaurus firstuvus"
 as a species last 10's of millions of years before "secundus" came
 around - or a species very closely related such as eastern and
 diamondback rattlers - not rattlesnakes as a whole or a very large
 genus. It would seem at first glance there would be a
 die-off/replacement of another species within the same genus?

First of all, genera don't exist outside our heads, and the term "species" unites lots of different things that have pretty much nothing in common.

Second, below a maximum set by the mutation rate, the rate of morphological evolution depends on natural selection, which is determined by the environment. If the environment changes so that a new phenotype has advantages over an old one, morphological evolution will be fast -- that's caused directional selection. If the environment does not change, morphological evolution will be very slow -- basically, an optimum phenotype will be approached or found, and all deviations of it will cause disadvantages or be neutral -- that's called stabilizing selection. Neutral changes can still spread through a population just by chance (that's called drift), but this is a slow process (except in small populations); and the stronger stabilizing selection is, the fewer phenotypic changes will be neutral.