Re: Preservational bias revisited

[jamolnar@juno.com]

On Tue, 17 Jun 1997 12:44:00 -0400 (EDT) John Bois <jbois@umd5.umd.edu>
writes:
>
>
>On Mon, 16 Jun 1997 jamolnar@juno.com wrote:
>> The rate
>> varies from very quickly in a rain forest to leisurely in a northern
>> woodland, but the recycling happens.  This makes for lousy 
>preservation,
>> unless it gets so acidic it becomes bog-like and suppresses the
>> decomposing bacteria.
>
>If memory serves, one of the points from the earlier thread on
>preservational bias was that occasionally there are events such as 
>ash
>falls , etc. which nevertheless inter forest dinosaurs.  So why not 
>any
>eggs which may have been present?  I understand that this is a rare
>occurrence but why not _any_?

I don't know!  Maybe just the physical presence of trees make it
difficult to actually cover anything to a depth for proper preservation?

>       Also, on e would think that forested riparian environments 
>might have been
>subjected to preserving deposition.   I'm just now reading Horner's
>_Digging Dinosaurs_.  He describes an island surrounded by trees, but 
>the
>nests are in the clear, not among the cover!  They would have been
>preserved, though, if they were in the cover.

Only if the water got up that high. Frequent flooding usually eliminates
forests, because seedlings and saplings need stable environments to
become established.
There are forest ecosystems that establish themselves in flooded
conditions (bald cypress, tupelo, mangrove), but the depositional
environment in these places are not very conducive to much preservation.
>
>> 3) The ground in a wooded area is not easy to mound up or dig into, 
>due
>> to the dense packing of roots from forbs, shrubs, ferns and trees.  
>The
>> mound-like nests found for dinosaurs so far are easier to build from
>> other (drier sedimentary?) materials.  This is not to say that 
>dinosaurs
>> couldn't take mud from a forested riverbank or stream and build up a
>> mounded nest in the woods,...
>
>
>How would they move it.  Some of those nests have quite a large volume 
>of
>nest material?

If they can move mud or other materials to make nests on dry land, why
couldn't they transport mud up out of a riverbank to a nest in a riparian
forest a few dozen yards away?  I don't really understand the question. 
Birds and other nest building species move huge amounts of material in
all possible ways, mostly in their mouths.  I'm sure dinosaurs did the
same.

>
>> ...but most mud under those circumstances would  have a lot of 
>organic
>> matter in it, and therefore be more likely to
>> decompose after use.  Just as in birds, dinosaurs would not reuse a 
>nest
>> the next season, or the parasite load would get intolerable.
>
>Horner believes some dinosaurs at least revisited nest sites year 
>after
>year.

Revisiting a _site_ is different than _reusing_ a nest.  Colonial nesting
birds that choose the same beach or cliff to nest on year after year
don't nest on the _exact_ same ledge or spot as they did the year before.
 If they build a nest at all, they start fresh.  Some birds of prey are
exceptions, but they add another layer onto the nest to distance
themselves from any leftover parasites.  The birds don't use the nest
except in the nesting season, so the parasites must go into some dormant
stage or else die.  Songbirds never reuse a nest built in the previous
year, and will not come back to a bird box unless it is cleaned out.
>
>> 4) If dinosaurs in woodlands were more secretive, due to a larger
>> diversity of potential predators being present, they might choose to 
>nest
>> in hollow logs or tree bases, or even dig a den of some sort.  Those
>> types of nests are not easily preserved either.
>
>Again, if memory serves, dinosaurs were unlikely to be den diggers due 
>to
>morphology. 

What morphology specifically?  They were too big?  Or they were incapable
of digging?  If the trees were huge, they wouldn't have to dig for a den.


 Also, mammals, lizards, and, toward the end of the 
>cretaceous
>at least, snakes may have made laying eggs in hollow logs and tree 
>bases
>rather difficult.  

No more difficult than it always was.  Mammals and lizards were around in
the Triassic and Jurassic too, I believe.

I know I asked a general question but, to focus on 
>the
>late cretaceous for a minute, there were no really small dinosaurs
>(actually, if one argues that there _were_ close-cover nests but they 
>have
>been simply obliterated by some of the forces you mention, couldn't 
>one
>also argue that there were small close-cover dinos which suffered the 
>same
>fate), chicken-sized and above would be less likely to lay in these 
>micro
>nesting sites--or am I wrong about that?
>
>
>
You're asking too many confusing things in the same paragraph and I can't
untangle it all.
Can someone else answer the size question?  I haven't had a chance to
look up sizes in all the late Cretaceous dinosaur fauna to answer.  My
first impulse is to guess that the dinosaur fauna was low in diversity,
but it did include small forms (and I guess we should define "small"
somehow.  Since bears dig dens, let's call it "grizzly- or
polar-bear-sized or smaller").

Judy Molnar
Education Associate, Virginia Living Museum
vlmed@juno.com
jamolnar@juno.com
All questions are valid; all answers are tentative.