Re: Preservational bias revisited

[John Bois <jbois@umd5.umd.edu>]

On Sat, 21 Jun 1997 jamolnar@juno.com wrote:

> 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.

My understanding of current dinosaur nest-siting knowledge is this: those
nests that have been discovered are all _in_ sediment.  They were not just
preserved in it.  This means nothing was carried to the nest.
The implication is that eggs need very specific substrates (especially if
they were buried) and dinosaurs sought such places just as modern
egg-layers do today, eg., Nile river crocodiles seek the sediment of the
river bank, turtles the sediment of the sandy shore. 
        Birds are different.  Most don't bury their eggs in heavy
sediment.  Generally speaking nest materials are light.

> >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.

I can't find a reference, sorry, but I'm sure I have read that dinos were
unlikely to have had the manual digging equipment to dig a burrow or a
den.  Are their any refs. to pre K/T burrows?  I am curious as to why
burrows don't preserve.  Some extant species dig in sediment don't
they (prarie dogs, I think, like to dig in sandy soil--at least the ones 
in the Balto Zoo do)?
 
> >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.

The molecular clock study some here have been talking about (the one that
has monotremes on the same branch as marsupials--Nature vol 387 pp. 549)
also shows the divergence of placental lineages.  It says that the
whale and cow diverged 60 mya ago (I know they were not whales and
cows then), the branch that now includes cats, seals, rhinocerus,
and horse before that, and going back in time respectively, rabbit,
primates, rodents, hedgehog, and finally, monotremes.

So, with respect to your comment, although the mammals of that time were
nothing like those of today, they were also nothing like those of the
Triassic and Jurassic.  They were new and different.  And they may well
have (as not) made life miserable for hollow-log layers, if any such
existed.

> You're asking too many confusing things in the same paragraph and I can't
> untangle it all.

You're right.  What I was saying was that if forest environments don't
preserve well, then maybe the dearth of small dinosaurs is due to that as
well.  In other words, there were plenty of close-cover small non-avian
dinos but they just didn't get preserved.  I am looking at a paper which
argues that tomorrow (if I can get away).

> 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").

I tried leafing through _The Dinosauria_ today to get this (I will pay
money to anyone who can give me a reference that lists sizes etc. of late
cret creatures),  Here's what I found.

Dromaeosuarids weighed an estimated 30-80 kg. (a chicken weighs 1 kg.)  
They were from 2-3 m long.

Hypsilophodontids were roughly 2.5 M long.

Protoceratops was a beefy 1 - 2.5 M (but these were all
but replaced by ceratopsids).  The only protoceratops I could find in the
Maastrichian was P. montane something or other (I forgot to write it
down).  And I couldn't find a size on it.

Troodon was probably 2 - 4 M long.

Elmisaurids were 35-65 kg. 

And that's about it.

Many of these we already know had nests in the open.  Regarding the
smallest creature on this list, protoceratops, I believe _P.
andrewsi_ in Mongolia laid in the red beds, i.e., sediment.  This was,
then, the primitive condition at least.  And I would expect nest substrate
to be highly conserved.

I don't think a den strategy is likely for dinosaurs.  But I don't want to
stretch the patience of the good folk on this list to argue why.  I'll do
that privately.