The ultrastructure and building of graptolite dissepiments
Adam Urbanek and Piotr Mierzejewski
Acta Palaeontologica Polonica 54 (2), 2009: 243-252
Dissepiments or connecting bars between adjacent stipes in rhabdosomes of dendroid graptolites were studied by means
of electron microscopy (SEM and TEM). The material, chemically isolated from rock matrix, originating from the
Ordovician of Estonia and glacial boulders of Baltic origin found in Poland, is assigned provisionally to the genus
Dictyonema” sensu lato. Early growth stages of dissepiments are made only of the fusellar component. Older
dissepiments are composed of the central core and the outer envelope: the central core is made of rather irregularly
arranged growth units made of the fusellar tissue, whereas the outer envelope has a distinct cortical appearance. TEM
observations indicate that the fusellar component is made of both typical fusellar and microfusellar tissues (the latter
with complete and reduced microfuselli). The cortical component of dissepiments is made both of dependent and
independent cortex. The opinion is advanced that the dissepiments were constructed externally by the mortaring
activities of zooids, similar to that of Recent
Cephalodiscus. Our observations indicate that bizooids were most probable
dissepiment constructors. These results, in general, does not support earlier opinions that dissepiments are made of
cortical tissue acquiring a fusellar aspect in some cases, and that dissepiments were produced by the extrathecal
membrane surrounding the rhabdosome.

Key words: Hemichordata, Graptolithoidea, Dendroidea, Dictyonema, dissepiments, skeletal tissues, ultrastructure,

Adam Urbanek [], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL−00−818 Warszawa, Poland;
Piotr Mierzejewski [], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL−00−818 Warszawa, Poland.
Dr. Piotr Mierzejewski