competition

[KELL00BELL@aol.com]

    There is no doubt that Tomistomines gradually radiated, and supplanted 
the dyrosaurs.  Both taxa probably frequented estuaries and would have 
competed.  Occasionally, floods could have swept dyrosaurs out to sea where 
they were buried, just like some near-coastal nodosaurs.  Had the dyrosaurs 
been truly pelagic, they would have vanished with the mosasaurs when marine 
productivity collapsed.
    Like many other forms, Tomistomines were initially successful only to be 
supplanted in their turn.  The generalized Nile crocodile was more adaptable 
than the specialized Tomistomines.  Drier conditions or periodic droughts 
more seriously affected crocs which relied exclusively on fish.  So, perhaps, 
did avian competitors for that niche.  In contrast, the Nile crocodile could 
eat a wide variety of prey, and scavenge carcasses.  Specialized crocs 
persist in some environments, but their decline is not surprising.
    Of course there were no modern whales when Basilosaurus lived.  In a 
scenario of competitive replacement, the less adapted forms are superceded by 
better adapted ones (e.g. whales with increasingly vestigial hindlimbs, 
equines with fewer toes, hominids with bigger brains) essentially as soon as 
the latter arise.  There is usually no discernible overlap, just a succession 
of more advanced forms.  The fact that this occurred in many lineages of 
higher vertebrates indicates that competition is the key agency of extinction 
and turnover.  Random physical factors would not have favored progress so 
consistently.