Piotr Mierzejewski

A possible new pattern of cortical deposit in Tremadoc dendroid graptolites from chert nodules

Summary Rhabdosome fragments of dendroids from upper Tremadoc chert nodules, studied with TEM, show an unusual pattern of cortical tissue. In contrast to previously studied tissues it consists of an irregular meshwork of wavy, branching and interconnecting fibrils. Differentiated in size and thickness, the fibrous material includes thick, strongly branched fibres. Their presence is a characteristic feature of the cortex in the dendroids examined. The observed pattern of ultrastructure is tentatively described as ‘reticulate fabric’. Its nature remains obscure. It may represent a primary feature, thus being an unusual state of aggregation of fibrous material, but its secondary origin due to preservational degradation of normal dendroid cortex seems more plausible.

Ultrastructure of the tuboid graptolite tubotheca

KurzfassungDie Wand der Tubotheke bei einem Vertreter der Tuboidea,Epigraptus eisenacki Mierzejewski, wurde mit TEM untersucht. Es zeigt sich, daß die Wand der Tubotheke ausschließlich aus typisch cortikalem Gewebe besteht, das dem entsprechenden Gewebe der Dendroidea und vonMastigograptus sp. sehr ähnlich ist. Ein weiteres Objekt warDiscograptus schmidti Wiman mit einer ungewöhnlich großen Tubotheke, die mit REM untersucht wurde. Es scheint, daß die untersuchten Objekte die Ansicht bestätigen, daß jede Tubotheke von einem nicht-graptolithischen Organismus bewohnt war. Gleichzeitig muß man annehmen, daß die Wand der Tubotheke, die diese Organismen überwuchs, eine echte Graptolithen-Struktur ist.AbstractThe wall of the tubotheca of the tuboid graptoliteEpigraptus eisenacki Mierzejewski has been studied with TEM. It was found that the tubothecal wall is composed of the cortical tissue which differs only slightly from the same tissue described in Dendroidea andMastigograptus sp. In addition a giant tubotheca inDiscograptus schmidti Wiman has been examined with SEM. The tubothecae studied seem to confirm a non-graptolite nature of the organisms living in them, while providing evidence that the tubotheca wall was a genuine graptolite structure overgrowing these organisms.РезюмеС помощью трансмиссионного злектронного микроскспа быпа изучена стенка туботеки уEpigraptus eisenacki Mierzejewski, представителя граптолитов из отряда Tuboidea. Оказалось, что стенка туботеки сложена исключительно из типичной кортикальной ткани, имеющей большое сходство с кортикальной тканью у Dendroidea иMastigograptus sp. Затем с помощью растрового электронного микроскопа был исследован образецDiscograptus schmidti Wiman, отдичафщийся необычайно большими размерами туботеки. Оба изученных объекта, по-видимому, иодтверждафт известнуф точку зрения, что в каждой туботеке помешался организм, не принадлежавший к граптолитам. В то же время необходимо подчеркнуть, что стенка туботеки, которая обволакивала эти организмы, представляла собой подлинно граптолитовуф структуру.

The ultrastructure and building of graptolite dissepiments

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.