Geoscience Reference
In-Depth Information
watch melodic toe-tapping performances by bands such as Agent Orange, Black
Flag, and other colorfully named troubadours, we both took a class from Robert
(“Bob”)Frey,simplytitled
Trace Fossils
.Atthetime,Freywasoneoftheforemost
experts in ichnology, so we felt privileged to take a class from him. But it also res-
ulted in Dave and me sharing an ichnological worldview that stuck with both of us.
Thus I was not surprised to later read about his recognition of the
Troodon
nests,
whichheandhiscolleaguesdescribedquiteproperlyasdinosaurtracefossils.Dave
also dedicated the first paper about the
Troodon
nests to Frey, an acknowledgment
of how good mentorship can later contribute to discovery.
The first
Troodon
nest structure noted at Egg Mountain surrounded a clutch of
24asymmetricaleggs,which,likechickeneggs,werewideratoneendthantheoth-
er,butmoreelongatedandvoluminous,havingheldabout0.3liters(1.2cups)each.
Eggsintheclutchwereorientedalmostvertically,narrowendsdownbutalsopoint-
ing toward the center. They were unhatched and clustered in an oval space smaller
thanmostservingtrays,measuringonly45
3
56cm(18
3
22in).Theeggsdefineda
ring-like pattern, with what Dave and the other paleontologists called an “egg-free
space” in the middle. This clutch was surrounded by a semi-circular structure that
was about
1.6 to 1.7 m (5.3-5.5 ft.) wide, about the diameter of a small kiddy pool. The
“egg-free space,” egg clutch, and rim shared a north-south orientation, making con-
centric oval rings; this orientation was also apparent in another
Troodon
egg clutch
at the same site.
The rim of this structure sloped abruptly, and then more gradually inward to a
flattish area about a meter (3.3 ft) wide, where the egg clutch was centrally located.
The exterior of the nest was defined by a rim that was 10 cm (4 in) tall and about
twice as wide. The rim was composed of micrite, a hard, resistant, fine-grained
limestone; it and the egg clutch were buried under a mudstone. Unhappily, in re-
covering the egg clutch, the field crew had to destroy part of the nest to extract the
eggs. Still, they uncovered enough of the nest to realize what it was before taking
