Two Brachyphyllum species from the Lower Cretaceous of Jiuquan Basin, Gansu Province, NW China and their affinities and palaeoenvironmental implications

Conifers are common in the Lower Cretaceous Zhonggou Formation of Jiuquan Basin, Gansu Province, NW China. The excellent preservation of plant fossils allows detailed investigations of the leaf epidermis by light microscope (LM) and scanning electron microscope (SEM). Based on the leaf morphology and epidermal features, two Brachyphyllum species, B. ningshiaense and B. obtusum are described. They closely resemble one another in foliar morphology and epidermal features, but have different leaf apices, cuticle features, stomatal shape and the number of subsidiary cells. The specimens consist of leafy axes with imbricate, spirally arranged, scale-like leaves. Epidermal analysis shows rectangular, pentagonal or rhomboidal unspecialized epidermal cells and abaxially longitudinally arranged stomatal rows except for the marginal areas, as well as well-developed papillae, sunken stomata and Florin rings of subsidiary cells with papillae on the inner surfaces. The imbricate and spirally arranged leaves with rather thick cuticle in both species indicate an Araucariaceae (Araucaria) or Cheirolepidiaceae affinity. However, epidermal features and the presence of Classopollis pollen in the sediments indicate a cheirolepidiaceous affinity. The morphological and anatomical characters of both taxa are interpreted as adaptations to a warm seasonally and temporarily dry palaeoclimate.