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Re: Why did small dinos become extinct?

David Marjanovic wrote:

> As far as we currently know, there were no burrowing predatory dinosaurs ever.
> While I obviously can't exclude future surprises of the sort that *Drinker* 
> and *Oryctodromeus* have been, using the forelimbs to dig must be difficult 
> when the forearms cannot be rotated -- the palms constantly faced each other, 
> unless the arms were spread very wide. 

The hands of alvarezsaurids faced ventrally, not medially, based on Senter's 
2005 study on _Mononykus_'s forelimbs (Paleobiology 31: 373-381).  Senter 
suggests that the forelimbs could have been used for both scratch-digging and 
hook-and-pull motions, with alvarezsaurids using their claws to tear open 
insect nests (as do modern anteaters and pangolins).  But Senter specifically 
ruled out burrowing behavior in alvarezsaurids (wrong body proportions).  Also, 
having a diet consisting largely of insects doesn't really qualify as 

I also like the alternative idea of alvarezsaurids being professional 
scavengers - using their stubby clawed forelimbs to rip into carcasses, 
gobbling up the yummy bits, and using their long legs to run away when a bigger 
predator came along.  Having only a single big claw on each forelimb, sheathed 
in keratin, might also have decreased the risk of infection from a putrefying 

> That only leaves the feet and the mouth, and the latter is not recommended in 
> the absence of gnawing teeth or a beak.

Small ornithischians may have used their heads to burrow; Varricchio et al. 
(2007) suggested that the fused premaxillae may have helped to loosen or move 
dirt.  This "head-lift digging" is known for some Recent mammals (e.g., 
notoryctids, chrysochlorids, spalacids), all of which are specialist 
burrowers/diggers.  The extinct rodent _Ceratogaulus_ (the "horned gophers", 
Mylagaulidae) were once thought to have used their prominent horns in digging, 
but this idea no longer has much support.  But the large mylagaulids also had 
thick, bony bosses at the front of the skull (= the anterior ends of the 
nasals) that may have been used in digging.  Many ornithischians had jugal 
bosses (e.g., _Orodromeus_, _Zephyrosaurus_, _Changchunsaurus_, 
heterodontosaurids, marginocephalians) which may have served to shovel aside 
loose dirt (at least in the smaller taxa).



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