Leaf anatomy of Ningxiaites specialis from the Lopingian of Northwest China

Abstract

Leaves are important vegetative organs of plants. However, our knowledge of fossil conifer leaves has been mostly obtained from compression or impression specimens. Compared with the abundant impression and compression leaf fossils, permineralized conifer leaf fossils with preserved cellular details are extremely rare. Therefore, leaf anatomies of fossil conifers remain poorly understood. Here, we describe permineralized leaves of a fossil conifer, Ningxiaites specialisFeng, 2012, from the Lopingian Sunjiagou Formation of Northwest China. The leaves are linear and helically arranged on shoot axis. They are characterized by a single vascular bundle surrounded by transfusion tissue, a single-layered endodermis, a thick mesophyll, and a single-layered epidermis. Tracheids in the vascular bundle exhibit helical and scalariform thickenings. The transfusion tissue is of the Pinus-type and composed of transfusion tracheids and transfusion parenchyma. The mesophyll comprises palisade and spongy tissues. The palisade mesophyll consists of thin-walled elongate cells, and is present on the adaxial side of the leaf. The spongy mesophyll comprises subcircular to subrectangular shaped cells and is present on the abaxial side. The hypodermis is discontinuously present beneath the adaxial epidermis and possesses two to three layers of sclerenchyma cells. The epidermis consists of a single layer of rectangular parenchyma covered by a thick cuticle with abundant cylindrical papillae. Stomata are predominantly present on the adaxial leaf surface and are arranged in parallel rows. This is the first report on the leaf anatomy of a Paleozoic conifer from the North China Block and sheds new light on the evolutionary history of conifers.