Bark Anatomy of an Early Carboniferous Tree from Australia

Abstract

Our knowledge of the evolution of secondary phloem and periderm anatomy in early lignophytes (progymnosperms and seed plants) is limited by the scarcity of well-preserved fossil bark. Here, I describe the bark of a Mississippian (Early Carboniferous) tree from Australia based on macro- and microscopic observation of two permineralized specimens. The bark tissues are up to 1.5 cm in thickness. The secondary phloem is organized in repeated, multicellular tangential layers of fibers and of thin-walled cells that correspond to axial parenchyma and sieve cells. Fibers are abundant even in the youngest, presumably functional, secondary phloem. Older phloem shows a proliferation of axial parenchyma that further separates the fiber layers. Successive periderm layers originate deep within the phloem and lead to the formation of a rhytidome-type bark, one of the oldest documented in the fossil record. These fossils add to our knowledge of the bark anatomy of Early Carboniferous trees, previously based on a few specimens from slightly younger strata of Western Europe. The complexity of the secondary phloem tissue in Devonian-Carboniferous lignophytes and possible anatomical differences related to growth habit are discussed.