Geoscience Reference
In-Depth Information
we're still not sure why, birds were the only dinosaurs to survive this global disas-
ter.
Seeminglyeveryfewyears,though,paleontologistswhointensivelysearchthe
boundary between Cretaceous and Paleogene rocks end up uncovering a few bits
and pieces of dinosaurs from above the boundary, suggesting dinosaurs were still
living 63 to 64
mya
. If these paleontologists publish a peer-reviewed article report-
ing such a find, the peer review does not end then and there. Post-Cretaceous dino-
saur bones are always contentious, and in nearly every instance are explained as re-
worked older material that was deposited again, into a younger formation. In short,
there's just no way to avoid an argument about the veracity of post-Cretaceous di-
nosaurs if your claim is based on bones.
If only we had some foolproof way of knowing whether any dinosaurs lived
into the Paleogene Period. What evidence would convince even the most hardcore,
pessimisticpaleo-curmudgeon?Youguessedit:dinosaurtracks.Trackswouldshow
notonlydinosaursactively behaving,butalsowouldbeinthesameplacewheredi-
nosaurs walked. In contrast, a transported, out-of-place and hence out-of-time track
is very easy to identify. Tracks found in situ might even be linked with a general
cladeofdinosaurs,suchasnon-aviantheropods,hadrosaurs,orceratopsians,whose
bones are in strata just below.
Sowiththisgoalinmind,severalplaceshavewhatareamongthelastdinosaur
tracks, coming from just below the Cretaceous-Paleogene boundary. One such site
in Utah has hundreds of deeply impressed tracks preserved in strata at multiple
levels, one of which is only a few meters below the boundary. Although the authors
couldn't identify the specific dinosaurs that made these tracks, their huge sizes and
numbers pointed toward herding herbivores, such as ceratopsians or hadrosaurs. A
single—but huge—theropod track from New Mexico, attributed to
Tyrannosaur-
us rex
, also came from a layer just below the boundary. In Spain, abundant dino-
saur tracks, some ascribed to hadrosaurs and sauropods, are preserved only a few
meters below strata that preserved fossils of Paleogene fish and mammals. At one
