Electron Microscope Study of a Diplograptus Species

Specimens of Diplograptus sp. from the late Middle Ordovician Lebanon Limestone in central Tennessee have been isolated from the matrix and examined with transmission electron microscopes at 60 and 650 kv. The fine structure of the fusellar layer in the metasicula and in the thecae is a mesh formed from fibers. The cortical layer has two sublayers, one of which is a mesh that is closely similar to the mesh of the fusellar layer and the other is formed from two sets of long fibers that are oriented at moderate to high angles to each other. The prosicula outer wall is a loosely-woven, open mesh formed of fibers that are 0.080–0.165 microns in diameter. The spiral thread appears to be a band that is also formed of interwoven fibers. The longitudinal rod is a bundle of long fibers. Prosicula microstructures of this Diplograptus are significantly different from those in an Orthograptus of the O. quadrimucronatus (Hall) group.