[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: back to science



Dave <cadams@hh.gpz.org> writes:
> First, here are some facts that I take as givens.  The European sauropod
> Hypselosaurus was about 40 feet long and produced eggs about a foot in
> diameter.  Trackways show some sauropods moving in groups which included
> fairly small individuals.  --SNIP--
> The eggs
> developed quickly, like bird eggs, the baby sauropods hatching in a
fairly
> precocial state.   --SNIP--
>   The fact
> that they laid such small eggs is due to their inability to put much
energy
> into reproduction; attempting to store energy would only have compounded
> their mass problem and made them less efficient.

Quoting the _Dinosaur Eggs_ chapter by Karl F. Hirsch and Darla K.
Zelenitsky from _The Complete Dinosaur_:
"The eggshell must be strong enough to support the weight of brooding
parents or the overlying burden of nesting material, and at the same time
weak enough to allow the embryo to hatch...  It must permit the diffusion
of gases and water vapor, and in some cases allow the absorption of liquid
water (Rahn et al. 1979)."

So among the reasons that no terrestrial egg has ever gotten very big is
the fact that the baby has to be able to break through it (although the
parent can pitch in if necessary) and that the embryo must be able to
breathe through it and absorb moisture through the shell.  And a very large
egg shell would require a considerable amount of structural integrity to
support the pressure and weight of its contents, or it would be too fragile
to be viable (especially in the company of adult sauropods).

I don't know what clutch count would be typical for sauropods, but I did
photograph a clutch of presumed sauropod eggs from Mongolia (at Dinofest),
and here we see three rows of 6" diameter eggs, amounting to 14 eggs in
all.

I don't have data on the ontogeny of _Hypselosaurus_.  Have embryos or
hatchlings been studied to determine whether the precocial model applies in
this case?  I will offer what information and ideas I can, but I eagerly
await answers from others on the list.

_The Encyclopedia of Dinosaurs_ features an article entitled _Growth and
Embryology_ by David J. Varricchio, which touches on questions of altricial
versus precocial dinosaur hatchlings.  The concluding paragraph states:

"The degree of ossification at hatching varies in modern archosaurs and
correlates with behavior and growth (O'Connor, 1984).  Poorly ossified
altricial chicks remain nest-bound, rely heavily on parental care, and grow
rapidly.  More completely ossified precocial young of birds and crocodiles
can move and feed on their own shortly after hatching, require less
parental care, and grow more slowly.  The degree of ossification observed
in _Orodromeus_ and oviraptorid embryos suggests that these dinosaurs
hatched precociallly.  In contrast, bone development in _Maiasaura_,
Hypacrosaurus_, and possibly _Camptosaurus_ may indicate altriciality
(Horner and Weishampel, 1988; Horner and Currie, 1994; Chure et al., 1994),
although this has been questioned by some (Geist and Jones, 1996)."

Clearly, the passage does not spell out the suspected mode for sauropod
dinosaurs (perhaps in lieu of some hard evidence), but if you are proposing
rapid growth in the sauropod chicks, wouldn't this more closely match the
altricial model?  Certainly large size was accorded high value among the
sauropod clade, and the prospect of slow growing sauropod chicks among a
family of titans would appear perilous, would it not?  (On the other hand,
you might argue that, being precocial, at least the little nippers would
have enough sense to get out of the way).  It seems to me that there is a
correlation between altricial young, rapid growth, and the large size of
the adults in the groups named in Varricchio's article.  Then again, I'm
not aware of bowl-shaped nests in sauropods (the Mesozoic version of the
playpen), so perhaps the altricial lifestyle was not practiced by sauropod
young.

Perhaps someone like Gregory S. Paul will weigh in on the subject, and
offer a more informed viewpoint.        

> Yet they were so
> much larger than their eggs they could not have used their bodies to
> incubate them.

No argument there!

-- Ralph Miller III     gbabcock@best.com