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Foliar adaptations of Rhus asymmetrica sp. nov. from the Oligocene of Cervera (Catalonia, Spain). Palaeoclimatic implications

Tosal, Aixa, Sanjuan, Josep, Martín-Closas, Carles
Review of palaeobotany and palynology 2019 v.261 pp. 67-80
Acer, Oligocene epoch, Rhus aromatica, Toxicodendron, evapotranspiration, habitats, holotypes, leaf morphology, leaves, morphs, new species, teeth, woodlands, Spain
Rhus asymmetrica sp. nov. from the lower Oligocene of Cervera (Catalonia, Spain) is characterized by a trifoliate leaf with a symmetric sessile apical leaflet and asymmetric lateral leaflets. The apical leaflet displays a serrate margin and decurrent base with pinnate primary venation, craspedodromous secondary venation and an irregular reticulate tertiary framework. The lateral leaflets show a rounded base with a secondary basal vein in the distal part of the lamina, while the proximal part is straight or concave. Formerly, these specimens were accommodated within Rhus pyrrhae Unger. However, detailed study of the R.pyrrhae holotype has led us to emend its diagnosis to include its brochidodromous secondary venation pattern, which differs clearly from the venation pattern of the new species. Additionally, a number of leaf morphotypes from the Cervera palaeobotanical site that were erroneously attributed to Rhus by former authors have been segregated and related to Acer and Toxicodendron. The extant species R. aromatica displays the closest foliar structure to R. asymmetrica sp. nov. However, differences in the leaf morphology suggest that the new species grew under distinct palaeoclimatic conditions: (1) R. asymmetrica sp. nov. bears glands at the tips of the teeth of the lateral leaflets while R. aromatica is devoid of these glands. This character would help to prevent excessive evapotranspiration; (2) lateral leaflets of R. asymmetrica sp. nov. are asymmetric while in R. aromatica they are symmetric, suggesting that R. asymmetrica sp. nov. grew under a greater environmental stress than R. aromatica; (3) R. asymmetrica sp. nov. displays fewer teeth with a larger tooth-area than R. aromatica. This would indicate growth under a warm climate with low seasonal contrast. These features are compatible with an open woodland habitat under subtropical palaeoclimatic conditions.