Geoscience Reference
In-Depth Information
pods, and ceratopsians produced certain types of eggs or egg clutches, initial iden-
tifications are occasionally tested by the discovery of embryonic bones in an egg.
Moreover, as of this writing, no one has yet identified an undoubted stegosaur, an-
kylosaur,nodosaur,orpachycephalosauregg,whichmakeforsurprisinglybigholes
in our knowledge of the life cycles for these dinosaurs.
Given that eggs are body fossils, is there any way to elevate their importance
further by somehow making trace fossils out of them, too? The answer is yes. At
least two species of dinosaurs,
Citipati osmolskae
and
Troodon formosus
, provide
evidence of post-laying movement of eggs in a nest, an action that must have been
done by one or both dinosaur parents. This sort of arrangement constitutes a trace
fossil of that parent's behavior like how a flower arrangement reflects the handi-
work of a florist.
In the case of
Troodon
nests, distribution patterns and orientations of the eggs
provided marvelous clues about female
Troodon
reproductive anatomy, as well as
what she did with the eggs after they exited her body. Paleontologists who studied
Troodon
egg clutches were surprised to notice that not only were eggs paired but
also aligned vertically. Based on their statistically significant pairing, paleontolo-
gists surmised that this pattern must have been caused by the mother laying eggs
twoatatime.Secondly,theeggswerelongerthantheywerewide(elliptical),which
meant they naturally should have rolled onto their sides once deposited onto the
ground surface. As a result, their vertical orientation means they were righted after
laying.
To best accomplish this feat, the mother or father
Troodon
likely would have
used their hands to turn the eggs upright, then buried the lower end of each egg so
that they wouldn't just roll onto their sides again. No one knows whether
Troodon
mothers did this with each pair of eggs—two at a time—or whether they waited
until the entire clutch was laid before turning them all sunny-side up. In an evolu-
tionary sense, the turning-two-at-a-time scenario seems more likely than the let's-
