Geoscience Reference
In-Depth Information
last, added up to a big mystery. Why even have these rocks in their guts in the first
place?
One possibility for these gastroliths would have been accidental ingestion of
rocks from the seafloor. This might seem like a bizarre idea, but is actually backed
up by even more ichnological evidence, which means it must be taken seriously.
These spectacular trace fossils, evident as long, wide grooves (“gutters”) preserved
in Jurassic strata of Switzerland and Spain, are interpreted as gouge marks made
by plesiosaurs as they swam along a sea bottom and scooped up seafood. Paleonto-
logists concluded these were trace fossils based on the sizes of the grooves: some
were as much as 60 cm (24 in) wide and 9 m (about 30 ft) long. Trace fossils this
big could only have been made by marine vertebrates with large noses, and plesio-
saurs were the only vertebrates living then with such massive snouts.
Based on this information, paleontologists interpreted them as a result of ple-
siosaurs diving to the bottom, inserting their muzzles into the bottom sediment, and
snatching up hapless invertebrates hiding in that sediment that had no idea death
would come from above. Any rocks on the sea bottom would have been accident-
ally consumed with the seafloor as a sort of by-catch. Nowadays, some whales and
walruses make similar gutters while feeding, so this behavior is not completely un-
known in modern animals, either.
Of course, the alternative to the “accidental” hypothesis is the opposite one,
which is that these marine reptiles ate rocks on purpose. But if so, why? How about
a synthesis of two previously competing explanations, in which gastroliths were
used for both buoyancy control—but for balance more than ballast—
and
helped to
digest their food? This idea got more support when two Early Cretaceous plesio-
saursdescribedin2005fromQueensland,Australianotonlyhadgastrolithsbutalso
stomach contents. These contents included bits of clams, snails, crustaceans, and
other invertebrate animals that lived on the ocean bottoms during the Cretaceous.
This evidence came as a big surprise, because paleontologists had always assumed
