Stomatal density and stomatal index of fossil Buxus from coprolites of extinct Myotragus balearicus Bate (Artiodactyla, Caprinae) as evidence of increased CO2 concentration during the late Holocene

Abstract

Stomatal density of plants may vary depending on environmental factors, such as CO2 concentration. Under the current atmospheric conditions, it is expected that leaves have different stomatal density than they had hundreds or thousands of years ago, due to the rise in CO2 in the atmosphere. Coprolites of the extinct Myotragus balearicus from Cova Estreta (Pollença, Mallorca), with a radiocarbon age of 3775–3640 cal. bc, have been used to study the diet of this bovid. A significant amount of epidermal fragments of Buxus was found in the faecal material. Three coprolites were used to estimate the stomatal density and stomatal index of Buxus epidermal fragments from this period. Samples of the endangered Buxus balearica, the sole species of Buxus currently present on Mallorca, and samples of the Buxus sempervirens, present in the nearest mainland, were also collected in different locations. Leaves were examined using microscopy to determine and compare the stomatal density and stomatal index between current plant material and coprolite material. The results indicated a higher value for stomatal index (12.71) and stomatal density (297.61 stomata/mm2) in leaves from the coprolites versus leaves of the living B. balearica and B. sempervirens species (7.99 and 227.77 stomata/mm2 respectively). These results could provide a palaeobotanic evidence of a change in atmospheric CO2 concentration since mid-Holocene in the Mediterranean basin.