Geoscience Reference
In-Depth Information
other organisms that have ever been found. Famous fossil sites such
as the Cretaceous Liaoning lake sediments in northeastern China and
the Tertiary Green River lake beds in Wyoming represent good exam-
ples of these ancient, fossil-rich environments, and similar rocks
would yield critical clues about what dinosaurs preyed on the
sauropods at Auca Mahuevo.
Finally, after the river flows into the sea, any remaining sand and silt
are deposited near the shoreline, but finer-grained silt and mud can be
carried farther out into the ocean basin. Out in deep ocean waters, the
shells of microscopic, single-celled plankton settle to the bottom
after the organisms die to form layers of limy mud on the bottom that
are eventually compacted into limestone.
Given this brief introduction, let's once again look at the rocks con-
taining the sauropod eggs and embryos at Auca Mahuevo. The eggs
and embryos were entombed in rusty brown layers of silt and mud,
which were mixed in with coarser layers of greenish and reddish
brown sandstone. The alternation of layers containing sandstone
and mudstone closely resembled the kinds of sediments that are
deposited across floodplains by streams and small rivers. Thus, evi-
dence from the rocks indicated that between 79 million and 83 mil-
lion years ago, the dinosaurs at Auca Mahuevo lived on a broad,
gently sloping floodplain, crisscrossed by shallow streams and rivers.
This floodplain formed as South America drifted away from Africa due
to the enormous forces generated deep within the earth as the result
of plate tectonics, the geological process that drives the continents
across our planet's surface. Thin layers of sandstone were the geologic
clues that documented the ancient presence of shallow stream chan-
nels and their sandbars. The sand was not too coarse, and few pebbles
were present, which suggests that the streams were not as swift and
turbulent as the ones that deposited the coarse gravel found in the
ridges behind Doiia Dora's
puesto.
Clearly, there were no large hills or
mountains nearby to provide steep stream gradients and large pebbles
or boulders. In addition, the thickest layers of sandstone were about
three to four feet thick, suggesting that the streams were not terribly
deep. Over time, these streams migrated back and forth across the
floodplain, cutting and then filling in the channels with sandbars, but
the eggs were not found in these layers of sandstone.
Fossils of the eggs and embryos were found in the finer-grained lay-
