Retallack replies

My data on the stomatal density of fossil plants through time1 was made available to encourage refinements such as that now offered by Kerp. His corrections make no significant difference to my published curves, but future refinement should improve this palaeobotanical archive of atmospheric CO2 levels.

Kerp's characterization of stomatal distribution on Permian seed-fern leaves is similar to the situation in Lepidopteris stormbergensis, which has a highly variable stomatal index2,3. My rarefaction analysis of several fossil species, including those described by Kerp, shows L. stormbergensis to be the most variable taxon of my compilation; I therefore used this species to set the lower boundary for reliable analyses at 500 epidermal cells1. The stomatal index of living Ginkgo biloba can be determined reliably by counting as few as 50 cells; other species represented by the thousands of cells needed for rarefaction analysis fell between these extremes.

Differences between Kerp's taxonomic names and mine reflect a different view of palaeobotanical nomenclature. He gives fossil leaves the same name as reproductive structures that are considered on evidence of varying quality to have belonged to the same plant. This is risky, because few, if any, of the taxa studied have reproductive structures attached to leaves. In my compilation of fossil leaves1, I listed names as he cited them for ease of reference, but in quotes to indicate deviation from traditional palaeo-botanical form genera, such as the leaf genus Rhachiphyllum. Such taxonomic considerations do not affect the inferred CO2 curve, because each determination is made using a collection of leaves at a single locality that are thought to belong to the same species of the same geological age.

I welcome Kerp's comments on local stratigraphic relationships. He has indicated that there are problems with fossil plant cuticles that he previously labelled as being from Sobernheim4, Lebach4 and Saxony5 (pointing out that the cuticle from Saxony is not 'Autunia' conferta, and 'Autunia' conferta from 'Lebach' and 'Sobernheim' is really all from Langenthal, for which amended stomatal data are: stomatal index = 8.6 ± 1.1; Ne = 797, Ns = 80, Nf = 3). The best way forward is to count more cells from more fossils, at more localities tied to better-dated successions.