A new technique is devised with which to retrodeform two-dimensional images of tectonically deformed fossils. As opposed traditional methods, by which the strain ellipse is found directly, with the present method the two-by-two matrix that represents the retrodeformation is calculated first, using simple algebra. This method is widely applicable in the cases of various kinds of deformed fossil specimens, including isolated ones, as long as at least two sets of measurements, each comprising dimensions or angles that were equal to each other prior to tectonic deformation, are available. Applications of this method in the case of ichthyosaurian specimens from the Lower Triassic of British Columbia, formerly assigned to the genus Grippia, reveals that the fins of the specimens are wider than previously described, invalidating the ratios and angles the were used for taxonomic argument. It is not possible to assign specimens either to Grippia or Utatsusauru, based on available information.