The Eocene–Oligocene transitional strata of the Hampshire Basin, Isle of Wight, provide a unique opportunity to study evolution and biotic turnover in the continental realm in relation to the global change occurring at this time. Gyrogonites of the charophyte genus Harrisichara vary through the late Paleogene and their abundance has enabled a rigorous morphometric study. Morphometric methodologies (including eigenshape analysis) have been used to best quantify evolution within the genus. Thirteen horizons from six Isle of Wight localities representing a time period of circa 3.5 million years have been used in this study. Results show a sharp morphological shift occurring within the genus Harrisichara between the Headon Hill Formation and the Bembridge Limestone Formation. The morphological evolution constitutes a doubling in the volume of the gyrogonites. The observed morphological shift within the genus Harrisichara is argued to be a functional response to the changing global climate and the associated increase in seasonality. The results of the morphological shift have the potential effect of increasing both establishment and persistence rates within the genus Harrisichara. The timing of the observed shift coincides with mammal faunal turnover and this implies a wider biotic response to the inferred climate change at this time. However, Harrisichara does not show evolutionary change across the ‘Grande Coupure’ (the earliest Oligocene major mammalian faunal turnover event). The timing of the morphological change is also coincident with the zonal boundary between the Harrisichara vasiformis-tuberculata Zone and the Harrisichara tuberculata Superzone and the morphometric approach would represent a much more robust definition of the boundary. Biostratigraphy based on multiple morphometric studies of assemblages of individual charophyte taxa will facilitate superior correlation and should be more widely applied in charophyte biostratigraphy.