By examining the fossilized leaves of any prehistoric plant it is possible to determine the climate in which that specimen grew because the size and shape of a leaf are unique to a given climate. Since the climate at a given location depends on the altitude at that location, it follows that the size and shape of a fossilized leaf also indicates the altitude at which the plant grew.
The reasoning in the argument is vulnerable to criticism on the grounds that it overlooks the possibility that locations at different altitudes can have the same climate.