Geoscience Reference
In-Depth Information
Whatwereecosystemslikeinthesameareaofthe
Troodon
nests?Muchstudy
on the sedimentary rocks there, including their geochemistry, revealed that this part
of Montana was probably warm and semi-arid 75 to 80 million years ago. Further-
more,
Maiasaura
and
Troodon
nested on river floodplains that were cut by me-
andering rivers and dotted with small lakes. Relatively little vegetation was in the
immediately surrounding landscape, based on how paleontologists have not found
many fossil trees or plant-root trace fossils directly associated with the nesting
grounds. Nonetheless, some root traces in nest rims show that at least a few plants
mayhavetakenrootintheseafterhavingbeenbuilt.Forestswerefartherawayfrom
the nesting grounds, as was an active volcano that occasionally erupted and helped
to preserve both the trees of these forests and dinosaur bones. Some sedimentary
deposits near, above, and below the
Troodon
and
Maiasaura
nests further indicate
periodic flooding, which along with other geological evidence suggests monsoonal
environments with annual dry and wet seasons. However, the area must have been
suitable enough for nesting to motivate
Troodon
mothers to keep coming back, as
paleontologistsfoundatleastteneggclutchesfromthreeseparatelevelsintherock.
I was reminded of this previous observation during a visit at Egg Mountain on
a field trip led by Varricchio and one of his colleagues, Frankie Jackson. On this
visit, I noted how the distinctive light-brown micrite composing the rim of the un-
doubted
Troodon
nest was also apparent in at least three horizons of the hillside
there. These may have been partially preserved nests, but ones not accompanied by
eggshells or bones: trace fossils in the purest sense, unadulterated by body fossils.
If so (although this requires more testing), this would be another way to find out
whether
Troodon
had what modern biologists have called
site fidelity
. This is a be-
havioraltraitsharedwithnestingseaturtles,crocodilians,andbirds,inwhichmoth-
ers come back repeatedly to the same nesting site for brooding, or their offspring
return to the same site where they hatched.
To better fill out this picture of the dinosaur nesting environments, and as an
example of how dinosaur trace fossils are best interpreted with trace fossils made
