Geoscience Reference
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certain: we know that at least a few dinosaurs left land and got into the water. One
grouping of swim tracks, made by seven separate theropods, is preserved in Early
Cretaceous rocks (110 million years ago) of Spain. Many more swim tracks are
in Early Jurassic rocks from about 190 million years ago in southwestern Utah.
The latter site has shattered any doubts about dinosaurs swimming, with more than
a thousand such tracks, linked to theropods and ornithopods, showing how they
paddled against, with, and across currents. Until lately, swimming was thought of
as an extremely rare behavior in dinosaurs. Hence, these tracks have impelled pa-
leontologists to reexamine their presumptions, and they are now looking for more
evidencethatsomedinosaurswerecomfortableinwater,oreventhattheymayhave
occasionally gone fishing and taken advantage of the plethora of food waiting for
them beneath the water's surface.
Dinosaur digging, whether used for making burrows in which they lived, to
acquire underground prey, or to make ground nests, is yet another newly diagnosed
behavior in dinosaurs, and one based mostly on their trace fossils. In 2007, I helped
two other paleontologists document the first known burrowing dinosaur (
Orycto-
dromeus cubicularis
, a small ornithopod) from Cretaceous rocks (95 million years
old) in Montana. Incredibly, this dinosaur was found in its burrow with two partly
grown juveniles of the same species. Two years later, I interpreted similar bur-
rows in older Cretaceous rocks (105 million years old) of Victoria, Australia. Why
would small dinosaurs burrow? Some of the reasons were proposed in the story,
suchasprotectionofyoungandmaintainingacontrolledundergroundenvironment,
although these are still subject to debate.
Adifferenttypeofdiggingbyotherdinosaursalsohasbeeninspiredbyunusu-
al trace fossils found in Late Cretaceous rocks (about 75 million years ago) of Utah
in 2010. These are interpreted as claw marks made by predatory theropods. The
closeassociationofthesemarkswithunderlyingfossilburrows,inferredasthoseof
mammals, adds another previously unconsidered dimension to dinosaur behavior,
which was their preying on small subterranean mammals. Sediment-rimmed nests
