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Re: Why did small dinos become extinct?

> Most accounts of the K-T extinction state that no purely terrestrial 
> animal larger than a cat survived (crocs and champsosaurs are / were 
> semi-aquatic). Cat-/chicken-sized non-flying predatory dinos are known 
> (most notably Compsognathus).

_No_ purely terrestrial animal larger than a cat survived, and _almost no_ 
purely terrestrial animal smaller than a cat survived.

> David Marjanovic suggests the whole terrestrial ecosystem was
> devastated too baldly to sustain viable populations of predatory dinos.

We have some direct evidence for this, in the form of paleosols, the fern 
spike, the fungal spike, and...

> But this is too simple:
> * Mammals survived.

As I just said: many mammals did _not_ survive. Let's take the metatherian 
example: Before the boundary, we had "pediomyids", stagodontids, peradectids, 
and maybe still deltatheridians. After, we had peradectids and 
notometatherians*, which is apparently descended from some "pediomyid" -- it is 
AFAIK enough to suppose that _two species_ of metatherians survived. The 
eutherian example is little different: before the boundary, we had true and 
false zhelestids, cimolestans, probably** asioryctitheres, *Deccanolestes*, 
probably** zalambdalestids, and leptictidans. After it, we had cimolestans, 
leptictids, and the clade composed of *Purgatorius*, *Protungulatum*, 
*Oxyprimus*, and the (other?) placentals in the strict sense, which is the 
sister-group of Leptictida; it is enough to suppose that _three species_ of 
eutherians survived. I know little about multituberculate phylogeny, but not 
long before the boundary there were djadochtatherian**, taeniolabidoid and 
ptilodontoid multis,
  of which only the latter two groups are known from after the boundary, and 
the known Cretaceous diversity of taeniolabidoids was very small, so it is 
likely that very few species of multis survived.

I also remember reading a little book from 1981 (by Archibald and someone else) 
that argued that 30 % of the North American lizard species died out at the 
boundary. Notably, the herbivorous polyglyphanodontids bit the dust.

* Contains the marsupials in the strict sense and the now extinct borhyaenoids.
** Known only from Mongolia, where the top of the Maastrichtian is missing.

> Why could small small non-flying predatory dinos not
> survive in similar ecological niches?

Because they, or most of them at least, did not have similar ecological niches. 
The surviving mammals and lizards mentioned above were for the most part 
granivores and insectivores; of the only dinosaurs in these niches, some 
survived -- all of them happened to be neornithean birds (and lithornithids, if 
those don't belong to Neornithes).

Incidentally, no flightless theropod the size of *Compsognathus* or smaller is 
currently known from the Maastrichtian, AFAIK.

> * The greatest devastation of plants was in N America, less in other 
> parts of the N hemisphere, and even less in the S hemisphere.

Are you sure? New Zealand burnt down as well. That's where the fungal spike 
(world going mouldy) was found for the first time.

> He also bases his response solely on the impact hypothesis. I've seen 
> suggestions that the N and S hemispheres had separate wind systems, as 
> they do now, so most of the fallout from Chixculub ( well N of the 
> tropics) would have affected only the N hemisphere, except for the CO2 
> emissions.

No. The gas emissions went all the way up to the mesosphere; the solid ejecta 
exited the atmosphere and reentered all over the globe. This is inevitable from 
an explosion of that size.

> If another factor (e.g. Deccan Traps) was responsible for most of the 
> extinction in the S hemisphere,

(The Deccan traps are out of the question anyway because the main episode of 
eruptions ended 100,000 years before the boundary. It had made the global 
average temperature rise by 3 °C, and when it was over, the temperature came 
back to normal. And that was it.)
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