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diverse and unpredictable of dinosaur trace fossils. All of this points toward how
these so-called “ostrich mimics” were 70 million years ahead of ostriches in using
gastroliths, meaning these modern birds are actually the “mimics,” not their non-
avian predecessors. These gastroliths, combined with their toothless condition, also
imply that ornithomimids were herbivorous, lending to the still-radical concept of
vegetarian theropods.
Given this knowledge both old and new about gastroliths in theropods, what
about sauropods and gastroliths? The long-held assumption is that huge sauropods,
many of which only had puny, pencil-like teeth, used gastroliths to grind their food.
However, this idea is now seriously doubted. The biggest problem with the previ-
ous explanation is that these gastroliths, like those used for “buoyancy control” in
marine reptiles, are too few to have made any real difference in digestion. For saur-
opodstohaveactualfunctional“gastricmills,”theywouldhaveneededmanymore
gastroliths than the ones found in sauropod skeletons so far.
For instance, in modern ostriches and other birds that employ gastroliths to
help with their food, these rocks make up about 1% of their total body mass. The
EarlyCretaceoustheropod
Caudipteryx
matchesthisratio,implyingthattheserocks
served a similar purpose in its lifestyle. However, for sauropods, the proportion
between gastroliths and estimated body mass was about 10% that of birds. So if an
ostrich were scaled up to 50 tons (scary thought), then it would need about 500 kg
(1,100 lbs) of gastroliths to digest its food, which is about the weight of the largest
Harley-Davidson motorcycle. Yet the greatest mass of gastroliths described thus far
from a sauropod (
Diplodocus
) was only 15 kg (33 lbs), which is about the weight
of a Huffy bicycle. Also, gastroliths are relatively rare in sauropod skeletons, and
if used for something as essential as food processing, these trace fossils should be
much more common. This huge disparity between extant and extinct gastrolith-us-
ing animals led paleontologists to conclude that these stones surely were not used
for the same purposes, effectively pulverizing the “gastric mill” hypothesis. Altern-
atives may not be so exciting, but include: accidental ingestion, especially if rocks
