Geoscience Reference
In-Depth Information
Expanding from a single toe to entire ecosystems, dinosaur tracks are also in-
credible tools for showing us exactly where dinosaurs lived, which seems to have
been nearly every lowland environment and with occasional forays into water bod-
ies. Their tracks are interpreted from geologic deposits formed in deserts, river and
delta floodplains, swamps, lakeshores, and tropical shorelines, and from formerly
equatorial regions to near the North and South Poles. Because of the preservation
problems of mountainous areas—their rates of erosion are too high to preserve
tracks very well—we don't know whether dinosaurs lived in alpine environments
or not. Still, the nearly ubiquitous occurrence of their tracks elsewhere implies that
they would not have been restricted from occupying higher-altitude places, either.
Dinosaurs, according to their tracks, covered the landscape.
The Last Steps of the Dinosaurs
So what about at the other end of dinosaur history, when they all died out? Do their
tracks help to answer the seemingly unanswerable? Well, sort of. As nearly every
child can tell you, non-avian dinosaurs all went out with a cosmically delivered
bang from a meteorite impact at the end of the Cretaceous Period and the start of
the Paleogene Period, around 65
mya
. This impact left an ash layer enriched with a
rare element—iridium—that is only found abundantly in meteors. It also left lots of
otherevidenceforitsimpact,notleastofwhichisahugecrateroftherightsizeand
age in the Gulf of Mexico next to the Yucatan peninsula of Mexico.
Tomake alongstoryshort,the effects ofthis impact likely caused what ecolo-
gists call a
trophic cascade
,inwhich the demise ofeach plant oranimal that played
a key role in an ecosystem led to further extinctions. Based on the impact scenario,
paleoecologists figured that dust caused by both the impact and widespread fires
blocked out most sunlight for several years, creating a Mesozoic version of a “nuc-
lear winter.” This effectively shut down photosynthesis in most land plants, thus
depriving herbivores of their basic foodstuff. Nearly anything big that required lots
of food to sustain its biomass, such as
Triceratops
or
Tyrannosaurus
, quickly went
extinct. Yet smaller dinosaurs went with the bigger ones, too. For some reason, and
