Pauline G. Farrell and Sydney Herbert James, in their 1979 review, based their decision to elevate the variety to species level on chromosomal and reproductive discontinuities.

Farrell and James reasoned that dysploid reduction (also called aneuploidy) in the genus Stylidium is frequent and probable in this case, meaning that S. calcaratum is virtually certain to be derived from S. ecorne.

This reduction is also evident from the floral spur, reduced number of ovules, and increased tolerance of other habitats, which the authors cited as derived conditions in S. calcaratum.

Crosses between S. ecorne and S. calcaratum yielded very few seeds, indicating that the taxa are well isolated from each other.

[1] S. ecorne is as widely distributed as S. calcaratum but occurs less frequently due to its restricted ecological habitat preference.