Geoscience Reference
In-Depth Information
in the 1850s, it was framed like a work of art (which it is). It has two slightly offset
pairs of feet and rear “heel” (metatarsal) impressions, and between those, an oval,
apple-sized impression from a svelte part of its rear end. The detail associated with
these traces is incredible, accompanied by wrinkle structures formed as the thero-
pod shifted its weight from one side to the other when sitting down and getting up.
In2004,Istudiedthisspecimenintensivelywithacolleague,EmmaRainforth,
in which we tried to figure out the sequence of movements made by the theropod
that would have produced such a trace fossil. We were also testing an audacious
idea that some of the wrinkle marks near the edge of the leg impressions were ac-
tually from feathers. This was an extraordinary claim at the time because feathered
theropods, although then-recently discovered in Early Cretaceous rocks of China,
werecompletely unknownfromtheEarly Jurassic anywhere intheworld.Yetother
paleontologists who had examined the trace fossil just a few years before us con-
cluded that the oddwrinkle marks were “feathers.” The surface preserving the trace
fossil also had little pockmarks, which had been interpreted as “raindrop impres-
sions” imparted by a Jurassic shower.
We wanted these structures to be feathers, too; but in science, reality does not
always live up to our wishes. Once we looked at the trace fossil more carefully, we
realized that a thin algal film covered the original muddy (but firm) surface. This
film acted like plastic wrap covering a dish: any pressure exerted laterally against it
caused the film to deform and wrinkle. Debunking further, we also concluded that
supposed “raindrop impressions” were more likely gas-bubble escape structures.
These were made when the theropod stepped onto the surface and pressed its full
body weight on the mud when it sat down; this in turn caused trapped gas in the
mud from underneath to bubble up to the surface. In short, this specimen records a
full sequence of movement by the theropod and how it altered the ground beneath
it, recorded in exquisite detail because it was preserved under the right conditions.
As fascinating as these dinosaur-sitting traces might be, though, we are still
puzzling over why they only seem to be in Early Jurassic rocks. Did dinosaurs just
