Review of the wood anatomy of extant Ulmaceae as context for new reports of late Eocene Ulmus woods

Abstract

Wood anatomy of extant Ulmaceae is briefly reviewed to provide context for descriptions of corresponding fossil woods, with attention to newly recognized woods from the late Eocene of Oregon, USA. The extant genera can be distinguished from one another using porosity type, presence or absence of vessel clusters, axial parenchyma type, ray width, and crystal occurrence. The late Eocene woods from Post, Oregon, conform anatomically with those of extant Ulmus. To assess whether fossil woods might be identified to an infrageneric category, selected wood anatomical features of 21 extant species of Ulmus are summarized to evaluate whether any sections within the genus have distinctive anatomy. Porosity type in extant Ulmus is related to leaf longevity; extant evergreen species are diffuse porous and have relatively narrow rays. Other extant Ulmus species are deciduous and predominantly ring porous, only rarely semi-ring porous, with clustered latewood vessels that are tangentially arranged. Sometimes earlywood characteristics have systematic value and allow recognition of groups. However, in some species earlywood characteristics are variable, apparently being much affected by growing conditions. Eastern U.S. species belonging to Sections Chaetoptelea and Trichoptelea of Subgenus Oreoptelea consistently have thick-walled fibers, single rows of small-medium diameter earlywood vessels, and little difference between the diameters of the solitary earlywood vessels and latewood vessels. The late Eocene Ulmus woodii sp. nov. shares more features with this group than any other. Another of these late Post Eocene woods is referable to Ulmus danielii, a species previously known only from the Middle Eocene Clamo Formation. A third fossil wood type resembles diffuse porous Ulmus, but its lower preservational quality obscures some diagnostic features and precludes secure assignment.