Taper of some Early Palaeozoic plants

Abstract

A new method was used to measure the degree of taper of Devonian plant axes: the width w of each compression fossil (measured at distance x along the length L of each axis) was normalized with respect to the basal width w₀ of each axis w/w₀ and plotted against normalized axial length x/L. the degree of taper was taken as the slope m of the simple linear regression of (w/w₀)³ versus x/L. Comparisons among m showed that different levels of branching on some specimens were statistically distinguishable, as were sterile and fertile axes from the same specimen. In contrast, sterile axes of similar rank in the hierarchy of branching from different species were found to have statistically indistinguishable degrees of taper. The degree of axial taper was observed to slightly increase over geological time, although the limited number of taxa examined make any definite conclusion unwise. Provided fragmented plant axes can be identified as belonging to a particular species, the allometry of axial taper could provide a useful method distinguishing different levels of branching in a plant body. Additionally, because the method used to quantity the magnitude of axial taper was derived from engineering theory, it may ultimately provide quantitative insight into evolutionary changes of the “mechanical design” of columnar and lateral cantilevered axes.