Geoscience Reference
In-Depth Information
these paleontologists' descriptions of it or their critics' guffaws. In 2002, my col-
league Steve Henderson, a bunch of our eager undergraduate students, and I vis-
ited the Purgatoire tracksite. Steve and I were co-teaching a dinosaur field course
and swung by La Junta to see the incredible dinosaur trackways south of town. Our
guide—U.S. Forest Service paleontologist Bruce Schumacher—happily showed us
the hundreds of dinosaur tracks there, wowing students and instructors alike. He
then detoured our group to the suspected urolite to talk about it as an example of
“science in progress.”
Although I'm still skeptical of its proposed identity, it nonetheless provided
an enjoyable experience for our students, in which we asked basic scientific ques-
tions such as “What evidence supports the hypothesis?” “How would you test this
hypothesis?” and more specifically “How much pee would be needed and from
what height would it have been delivered to make something like this?” Urolite or
not, this enigmatic structure presented us with a fine educational opportunity, and a
memorable one.
Given the controversy over the Morrison urolite, I was much relieved to later
find out that paleontologists Marcelo Fernandes, Luciana Fernandes, and Paulo
Souto had also interpreted dinosaur urolites in the Late Jurassic-Early Cretaceous
Botucatu Formation of southern Brazil. Although they found only two such trace
fossils, both were beautiful textbook examples of what urolites should look like,
bearingallofthemarksofsteadybutfocusedstreamsofliquidhittingdrysand.Pre-
served in sandstones, these trace fossils are teardrop-shaped craters connected dir-
ectly to streamlines. The structures were likely made in sand dunes, with the craters
havingformedbyerosionofuppersandlayers—causedbyurinaryimpact—andthe
streamlines from liquids trickling downslope on a dune surface.
The craters and streamlines of the two specimens were nearly identical in size
and form, measuring about 2 cm (<1 in) deep, 16 to 19 cm (6.3-7.4 in) long, and
11 to 13 cm (4.3-5.1 in) wide, about the size of a gravy boat. The craters would
have held about 300 to 400 cc of liquid (a cup and a half), but some of this would
