Figure 19.7   Experimental ichnology: (a) graduate student Jesper Milàn, trying to persuade an emu to walk where he wants it to walk, and (b) the tracks and undertracks of the emu – results of an experiment where an emu stepped on a package of alternating layers of concrete and sand. After the concrete hardened, the sand was fl ushed out and replaced with silicone rubber. The top print (left) made an impression on several layers below, shown as undertracks at depths of up to 40 mm. Notice how the impressions of the digits become wider and less well-defi ned along each subjacent horizon. (Courtesy of J. Milàn.)

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