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by other animals living with the dinosaurs, I later did a study with Varricchio on
fossil insect nests near the
Troodon
nesting sites. Over years of field work there, he
and other paleontologists in the area had noticed that several strata contained con-
centrated zones of fossil insect cocoons and burrows. I was astounded the first time
I visited these sites, as the fossil cocoons were so abundant that handfuls of them
couldbescoopedupfromthegroundwheretheyerodedandfelloutoftheoutcrops.
The cocoons were also sometimes directly associated with dinosaur nests, having
been reported previously by Horner, Varricchio, Jackson, and others. In fact, during
several subsequent visits to this area, I noticed cocoons embedded in the rims of a
few of the
Troodon
nests, showing that insects may have been nesting at the same
time and same place as the dinosaurs.
The cocoons were exquisitely preserved, some with impressions of spiraled
silk weaves still apparent, and a few burrows bore scratch marks from the legs of
their makers. These trace fossils were apparently preserved through filling of the
burrows and cocoons by fine-grained sediment, followed by microscopic precipita-
tion of micrite. This process was further aided by a semi-arid climate, which would
havecausedrapidevaporationofwaterinbetweensediments,formingcalcium-rich
soils called
calcisols
.
What we also eventually figured out about these insect trace fossils was a
little surprising. Based on the appearances of the cocoons and burrows, we sur-
mised that burrowing wasps probably made most of them, with perhaps only a few
made by beetles or other insects. Because modern ground-nesting wasps also bur-
row into and pupate in well-drained soils, we presumed that these trace fossils were
madeinsimilarsoils.Theseinsecttracefossilsthusaffirmedthat
Troodon
probably
picked its nesting locations for the same ecological reasons as the wasps and oth-
er insects. Horner originally conjectured that these cocoons, which were also near
the
Maiasaura
nest sites, were those of carrion beetles, which he imagined fed on
hatched eggs, or dead eggs and hatchlings. In the light of the new interpretation of
thecocoons,hismoregruesomescenarioisnowdoubted.Incontrast,dinosaursand
