Geoscience Reference
In-Depth Information
ers of mud and silt. What might this clue tell us about the cause of
death? These layers represented silt and clay particles that were car-
ried over the banks of the streams when they flooded and inundated
the adjacent lowlands. Similar events are common in our modern
world; every year, heavy rains in many parts of the globe unleash
torrential floods that inundate major river valleys, often destroying
populous cities and leaving a blanket of mud wherever the flood
passes. Objects as large as cars and houses can be transported or
buried by the onslaught. Although the streams on the primeval
Patagonian floodplain were not large enough to generate such enor-
mous floods, they nonetheless overflowed their banks from time to
time, carrying a blanket of mud and silt to be deposited as currents
slowed, away from the main channels. Eighty million years ago, no
cities stood in the way of these floods—only the vegetation that
grew on the plain and occasionally a nesting ground of sauropods.
It appears that the dinosaurs intentionally looked for places away
from the streams in safer areas of the floodplain to lay their eggs.
Eggshell is actually pretty durable, and fragments can be carried
over substantial distances without being completely destroyed. How-
ever, we didn't find any fragments of eggshell in the sandstone formed
from the bars in the channels that were active at the time of the nest-
ing, and the eggs did not appear to have been broken or even trans-
ported by the floodwaters. So, the currents depositing the mud could
not have been too strong, and the eggs in the nesting ground could
not have been very close to active channels. Why did the dinosaurs
apparently avoid the areas near the active stream channels? Perhaps
it was just chance, but it may also have been because they had some
sense that the streams could destroy their nests. In any case, at times
this strategy, if there was one, didn't work. For the floods, although
not very powerful, nonetheless brought a wave of sudden death to the
nesting ground.
Since the fossil eggs and embryos that we found in 1997 were
exclusively buried in the layers of mudstone, not the sandstone, we
concluded that the eggs and embryos had quickly been buried when
the streams flooded. (Years later, we would find eggs that had been
laid in sand rather than mud. But the sand formed the bed of a
long-abandoned channel at the time that the dinosaurs laid their
eggs, and the eggs had later been buried by mud during a flood.)
