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Re: Eggs,Embryos,and Birds




Betty Cunningham wrote:

> KARI LYNN BAKER wrote:
> > How are scientists able to differentiate between the different egg laying?
>
> -Horner has a book, DINOSAUR BABIES, that covers much of the Egg
> Mountain finds.
> Maiasaur egg clutches are laid in a large pile and then covered by plant
> materials, thus separate clutches over time have plant materials and at
> least a years' worth of debris separating the layers.  Whether eggs
> (from a single clutch) were laid at one pass (like sea turtles) or over
> a period of a week (like most song birds) and THEN buried is not known
> from the fossils.   Troodons eggs are found TWO at a time along a sort
> of trail for about 10-15 feet.  I think it was Betty White working near
> Egg Mountain that suggested a dual ovipositor explanation for Troodon's
> odd egg distribution.  I don't know enough about egg-laying biology but
> it does seem a rather unusual method.

Dinosaurs Past and Present Volume 2 illustrates a paired line of six "bumpy"
eggs (not assigned to any taxon in this paper) on page 60.  Perhaps these are
the eggs you describe.  The same paper (_Ecological and Behavioral Implications
Derived from a Dinosaur Nesting Site_ by John R. Horner, circa 1986) also
depicts clutches of 10, 12, and 24 elongate Cartesian oval eggs which are
arranged with the "point" down and the "round end" angled slightly toward the
center of the nest (MOR 299).  These are referred to as "hypsilophodont" eggs,
which I believe were later dubbed "_Orodromeus_ eggs," based in part on the
embryonic remains which included tiny teeth.  These clutches were found among
remains of some very young and some juvenile _Orodromeus_ specimens, and were
also associated with _Troodon_ teeth.   It has since turned out -- on the basis
of a "nesting" _Troodon_ atop such a clutch -- that these "_Orodromeus_" eggs
were in fact _Troodon_ eggs.  Part of the confusion arose from similarities in
the teeth of the two types of dinosaur, in spite of their considerable
phylogenetic separation.  Horner was the epitome of humility when he recounted
the misidentification at the 1997 SVP meeting, and mentioned that the Museum of
the Rockies _Orodromeus_ diorama was going to be revamped to incorporate
_Troodon_ instead.

The recent study of the spiral _Troodon_ clutches put forth the opinion that
pairs of the eggs were positioned close to one another, suggesting that they had
been laid two at a time, and, furthermore, that there was a subtle linear gap
down the middle of each clutch arrangement, indicating that a parent might well
have brooded the eggs (a hypothesis later bolstered by the remains of an adult
found atop such a clutch).  The paired oviduct condition is plesiomorphic among
the Reptilia, and is the pattern found in extant lizards and crocodilians,
although it is less common in extant avian reptiles (for, as we know, birds are
reptiles).  The interpretation of the two elliptical shapes preserved within the
pelvis of one of the _Sinosauropteryx_ specimens being a pair of unlaid eggs is
consistent with such a reproductive anatomy, whereby the eggs would be laid two
at a time, and a clutch would be produced over a period of days.  It isn't that
unusual when you think of it; mammals have symmetrical reproductive organs too,
you know.  It is thought that the majority of volant birds have dispensed with
half of their "equipment" in order to lighten their payload at the expense of a
lower rate of egg production (laying only one egg at a time).

Among the Egg Mountain clutches, only those of _Troodon_, _Maiasaura_, and
_Hypacrosaurus_ are presently diagnosable with confidence; if the aforementioned
paired linear egg clutches have been positively identified, it's news to me.
Horner's previous characterization of the hypsilophodont hatchlings as being
precocial can now be applied to the troodonts.  Horner's _Dinosaur Lives_ gives
a good overview of some of these issues.

Many other dinosaur egg types have been attributed to particular genera,
sometimes due to an association of eggs with fragments of hatchlings (see "Eggs,
Eggshells. and Nests" by Mikhailov in Currie and Padian's _Encyclopedia of
Dinosaurs_ for a list), but few can be securely identified.  Among those
isolated eggs and nests that can be identified with confidence are oviraptorids
(the eggs formerly known as _Protoceratops_),  Asian therizinosaurs (genera
undetermined) and the recently described Patagonian titanosaurs.  Once a
positive ID is made, associating a particular egg with a particular genus of
dinosaur, other eggs may be likewise IDed on the basis not only of overall
shape, but of the distinctive microscopic cross-section, although these factors
alone could lead to misidentifications, too.

The May 1996 _National Geographic_ provides good photographs of a variety of egg
clutches; the December 1998 issue covers the titanosaur eggs (with embryo skin
impressions).  Some information can also be found on-line at
<www.nationalgeographic.com/dinoeggs/index.html>.

> I don't know in what arrangement sauropod eggs are found but I think
> it's supposed to be a sort of spiralled eggmound (that might be
> hadrosaurs I'm remembering).

The sauropod clutches I've seen are composed of cannon ball eggs tightly packed
in two or three straight parallel rows, arranged somewhat like the cells of a
honeycomb.

Regarding birds being descendants of dinosaurs, I say "Aye."  Either that, or
the fossil record is evidence of one heck of a conspiracy!  (No, do not start a
fossil conspiracy thread)!

Ralph Miller   <gbabcock@best.com>