Re: Preservational bias revisited

[jamolnar@juno.com]

On Sat, 14 Jun 1997 16:25:45 -0400 (EDT) John Bois <jbois@umd5.umd.edu>
writes:
>Here is my question:  In my search of the literature I have been 
>unable to
>find a reference that reports vegetable matter _around_ dino nests (I 
>know
>there is matter _in_ some nests).  In fact, most nests seem to be 
>sited in
>sedimentary deposits.  First of all, is this true?  (I would greatly
>appreciate any refs. contra this!)  Secondly, if it is true, is there 
>any
>reason why nests in, say, wooded areas would be any less well 
>preserved?

I can think of several reasons:
1) A wooded area has a "fast" turnover rate of organic matter.  Leaves or
needles fall, decompose, and get recycled relatively quickly, because of
the shady, moist and humid conditions on the forest floor.  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.

2) Have you ever tried to find a ground-nesting bird's nest on the ground
in the woods in the fall?  Their nests are made of vegetation and they
either decompose rapidly once the birds are gone, or other critters make
off with the materials to make their own nests (squirrels, wasps, mice,
etc.).  You can find nests, but extremely few compared to the number of
bird nests that are actually there in the spring and summer.  This
pirating of materials also happens in grasslands, where many of the nests
are disassembled and used to line underground burrows of mice and other
rodents.  I don't think this dispersion is greater in a woodland, but
with the faster recycling of nutrients on top of it, I'd say little
evidence of nesting would be preserved  in a wooded area.  

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, 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.

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.

My two cents

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