Determination of 2D strain from a fragmented single ammonoid

Planispiral ammonoids are used for strain analysis, as the pre-strain shapes of their shells are thought to be approximated by logarithmic spirals. Comparison of the pre- and observed post-strain spirals determines the strain. We show in this short note that the R_f/φ{symbol} strain analysis is applicable to the ammonoids, where the interspaces of a shell are used as strain markers. Unlike the previous methods that assumed the spiral, the present technique can use fragmented shells, provided that they have at least half windings and clear limbs. A strain ellipse and its 95% confidence limit were determined by the hyperbolic vector mean method from the ellipses that were fitted to the interspaces of a fragmented, deformed ammonite from the Kitakami Massif, Northeast Japan, as a worked problem for the present technique.