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Extinctions



 
So, if I've got this right...
 
Among the sea reptiles, plesiosaurs, ichthyosaurs, and mosasaurs were 
all extinct by the beginning of the Tertiary.  Ichthyosaurs (I'm 
not sure here) and the marine crocodilians were extinct well before 
the K/T boundary, plesiosaurs were in decline, but mosasaurs were 
still going strong.
 
Dinosaurs (the non-avian ones) were in decline (at least in 
N.America) in the late Maastrichtian.  But, as for the pterosaurs 
(which were in decline in the later Cretaceous) and the
plesiosaurs above, a decline need not inexorably lead to natural 
extinction - they could just have been in a slump. 
 
Among other diapsids, squamates (lizards, snakes), sphenodonts 
(tuatara), crocodilians, choristoderes (those weird crocodile-like 
things), all survived.  Except for the big crocs.  But, no one's sure 
if those that survived did so unscathed, or whether they crashed 
as well at the K/T boundary, then recovered in the early Palaeogene.
 
For mammals, I'll wave the white flag.  Monotremes, marsupials, and
placentals obviously survived, and so did the multituberculates.
Marsupials were apparently hard hit (at least in N.America) - but I
don't think the Maastrichtian record is good enough to be sure if
their survival in the other continents (like their Caenozoic
strongolds of S.America & Australia) followed a K/T crash.  Were
placentals affected at all?

I've also heard that marsupials were more common than the placentals 
in the later Cretaceous, and multituberculates were more common than 
either. 

As for the birds, the neornithines were the only K/T survivors.  I 
have the impression that it was the enantiornithines that were the 
dominant avians during the later Cretaceous, and neornithines were 
relatively rare.  Alvarezsaurids (_Mononykus_ and friends) went 
extinct, and so did those "one-off" lineages like gobipterygids and 
patagopterygids.  Hesperornithiforms and ichthyornithiforms seem to 
have gone extinct by the end of the Cretaceous.