Geoscience Reference
In-Depth Information
rots—found that these birds were also attracted to soils with high sodium content.
Hence,consumptionofthesesoilsmayprovidetraceelementsandmineralsinthese
birds' diets, but while also incorporating some gastroliths along the way.
Everything mentioned so far applies to geo-gastroliths in land vertebrates.
What about aquatic ones? It turns out that gastroliths are indeed used in a variety
of swimming vertebrates, such as pin-nipeds—marine mammals such as sea lions,
seals, and walruses—penguins, and crocodilians. For pinnipeds and penguins, rock
swallowing may be therapeutic in the same way it is for raptors, in which these
stones are used to clean stomachs, then regurgitated. Yet, considering how all of
these animals swim, these gastroliths also might help with buoyancy control in all
three groups of animals. This is not necessarily like a scuba diver wearing a weight
belt, though, and the “buoyancy control”
vis-á-vis
gastroliths in aquatic vertebrates
was questioned in a 2003 study on alligators, which demonstrated that gastroliths
only added 1 to 2% to their body weights. In other words, these rocks had a negli-
gible effect on alligator buoyancy. As a result, the explanation was adjusted to state
that these gastroliths were used more for stabilizing bodies while swimming. In-
stead of weight belts, think of the tiny weights placed on car tires when these are
balanced and rotated, helping the tires to wear more evenly.
Oddly enough, gastroliths are completely absent in whales and sea turtles,
whichintheirevolutionaryhistoriesmanagedtofindotherwaystomovefromdeep
to shallow water and back again in a balanced way without the use of gastroliths.
Yet these trace fossils show up as concentrated masses of rocks in the skeletons of
big, non-dinosaurian Mesozoic marine reptiles, such as plesiosaurs. Paleontologists
who found these gastroliths were at first perplexed by them, as digestion was pre-
sumedtohavebeenanunlikelyreasonfortheirpresence.Afterall,thesevertebrates
had seafood-only diets, much of which should not have required as much grinding
as, say, plants from terrestrial environments. This supposition, combined with the
negligible amount of weight these rocks added to a multi-ton marine reptile as bal-
