Geoscience Reference
In-Depth Information
Let'ssayyou'reapaleontologist andyou'vebeenaskedtostudyageological form-
ation dated from the time when dinosaurs ruled the earth, or at least when they co-
ruleditwithinsects.Alloftherocktypes,sedimentarystructures,invertebratetrace
fossils, andgeochemical data inthis formation showit wasformed inenvironments
that should have been inhabited by dinosaurs, such as river valleys, lakeshores, or
seashores. Yet after weeks in the field spent scrutinizing these strata, you realize
they not only lack dinosaur bones, but also are devoid of tracks, nests, eggs, tooth-
marks, coprolites, or other obvious evidence of dinosaurs.
Theonlypossiblesignofaformerdinosaurpresencearesomeannoyinglyreg-
ular, rounded, polished quartz pebbles and cobbles, which are locally abundant in
some sedimentary layers. With a great sigh, knowing that your colleagues will cas-
tigate you, you are forced to consider that these rocks might be gastroliths, the only
clues that dinosaurs were present. With much resignation, you write “Gastroliths?”
in your field notebook, draw a little sad face next to this entry, and wonder what to
do next.
Along those lines, in an article published by Robert Weems, Michelle Culp,
and Oliver Wings in 2007, they argued that the presence of many unusual, rounded,
polishedrocksintheBullRunFormationofnortheasternVirginiademonstratedthat
dinosaurs lived there about 200
mya
. They came to this conclusion despite a com-
plete lack of dinosaur bones, tracks, nests, eggs, coprolites, and other fossil eviden-
ceofdinosaursintheBullRunFormation.Thismeantthesegastroliths,whichwere
also actual examples of exoliths, constituted the only signs that dinosaurs had been
thereandthen.Itwasanichnologicallywonderfulstudy,onethatdemonstratedhow
more than a hundred years of cumulative knowledge about dinosaur gastroliths can
be applied to quite reasonably state “dinosaurs were here” on the basis of a pile of
rocks.
ThepaucityofdinosaurfossilsotherthangastrolithsintheBullRunFormation
was frustrating, because it was the right age (Late Triassic) for preserving evidence
of early dinosaurs in North America. Unfortunately for dinosaur hunters who
