Geoscience Reference
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by logs that dragged across the lake bottom during flooding of the stream and lake.
Once emergent, this shoreline would have served as a water source for local anim-
als, and its moist, silty, and sandy sediments may have registered tracks over the
course of several days or weeks. The tracks would have dried slightly underneath
the warmth of the Cretaceous sun, but not so long as to bake them and cause them
to crack around their edges. A minor flood from the lake or stream later covered
the tracks with a protective layer of sand, which, along with the track-bearing layer,
hardened and became part of the geologic record.
Atthetimeoftheirexcavation,theformsandsizesofthetrackspointedtoward
four species of bipedal dinosaurs as the track-makers: a medium-sized ornithopod,
about the same mass as a hefty heifer; a smaller ornithopod, close to the size of a
modern emu; a much smaller theropod, only slightly bigger than a chicken; and the
source of dramatic tension in the story, a large
Allosaurus
-scale carnivorous thero-
pod. Most of the tracks on the surface were from the small theropods and ornitho-
pods, and they were not lolling about in idyllic harmony at the lakeshore. Instead,
these dinosaurs were running flat-out, clearly motivated by something that caused
all of them to move in the same direction at high speed.
Drs. Thulborn and Wade came up with this scenario once they measured the
directions of the individual trackways and distances between tracks within each
trackway. Using the formula devised by R. McNeil Alexander for calculating di-
nosaur speed based on track length, hip height, and stride, they figured these di-
nosaurs were moving at about 12 to 16 kph (7.5-10 mph), which is quite fast for
short-leggeddinosaurs.Furthermore,thetracksmostlyshowedtoe-tipstouchingthe
ground instead of entire digits. Lastly, some tracks were greatly elongated, indic-
ating that slipping and sliding happened during their journeys. Very simply, when
small animals take big strides, get up on their toes, and lose their footing on moist
ground,theyarerunning.Tofindoutjusthowmanydinosaurswereinvolvedinthis
collectiveaction,theywalkedalineperpendiculartothedirectionofmovementand
