Adam Urbanek

Biotic crises in the history of Upper Silurian graptoloids: A Palaeobiological model

Four major biotic crises are recognized in the history of the Upper Silurian graptoloids. While the causes of extinction are largely unknown, the scenarios of the crises are fairly well ascertained. A model of recovery proposed here implies two alternatives: (1) some of the relic species exhibit great abundance frequently associated with a subnormal phenotype (the post‐event syndrome) and the ensuing rediversification involves indigenous speciation or (2) as a result of the lack of evolutionary response on the part of the local survivors, vacant habitats are filled by immigrants. Numerous cryptic and Lazarus taxa among the immigrants indicate the presence of pelagic refuges and centers of evolution. They may be identified as Central Water masses bounded by gyres.

Morphogenesis of uniaxiate graptoloid colonies; a mathematical model

-Morphological gradients in graptoloid colonies are explained by the production and diffusion of a morphogen from the sicula distalwards. Size of the thecae is inversely related to the amount of morphogen present. The graduate decrease of morphogen is given by a set of algebraic equations, and its effect on zooid growth is described by a modified Michaelis-Menten relationship. Changes in size of thecae computed on the basis of these equations fit the changes observed in graptolite colonies. Sets of differential equations are also given to describe the rate of morphogen diffusion and some other processes possibly involved in the development of uniaxiate graptoloid colonies. The suggested basic model of these colonies can be completed by a number of additional assumptions; however, computer experiments reveal that such assumptions do not affect the main properties of the model, namely the appearance of graded series of thecae. New evidence for regeneration of the sicular portion of the broken rhabdosome is presented, providing arguments that morphogen was produced by the siculozooid as a single bolus of secretion. Some local exclusions from the regular size gradient are discussed, and tentative explanations are suggested. Adam Urbanek and Janusz Uchmanski. Department of Paleobiology, Polish Academy of Sciences, Unit for Graptolite Research, ul. Newelska 6, Pl-01-447 Warszawa, Poland Accepted: September 15, 1989

Phyletic evolution and iterative speciation in the persistent Pristiograptus dubius lineage

The paper focuses on patterns of the evolution of the simplest and longest-ranging (approximately 18 Ma) Silurian graptolite Pristiograptus dubius. The Pristiograptus dubius species group consists of the P. dubius stem lineage represented by a sequence of a number of subspecies displaying only small morphological changes as well as derivative species produced from the stem lineage by means of iterative speciation. This long raging graptolite lineage is the only one, apart of one retiolitid, which survived the most severe environmental event for graptolites, the Cyrtograptus lundgreni Event. Based on three-dimensional, isolated material two P. dubius groups taxa are distinguished. One group has an obtuse angle between the thecal lip and the succeeding thecal wall, the second group has a right or acute angle. Other characters differentiating P. dubius forms are: the shape of the apertural lips, differences in rhabdosome shape and size, and a different number of sicular rings. Sixteen species and subspecies of Pristiograptus from the East European Platform, Poland and Lithuania are discussed. Five new subspecies P. dubius magnus, P. dubius paezerensis, P. dubius praelodenicensis, P. dubius postfrequens; and P. dubius postmagnus are proposed.

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.

Organization and Evolution of Animal Colonies

Based on the theory of organization and evolution of colonies in an extinct group of hemichordate graptolites (Urbanek, 1960, 1990) the relationship between the events in the late astogeny of bryozoan colonies and their somatic and reproductive cycle is proposed. The bryozoan colonies with simple organization are compared with graptoloid colonies and their structure is interpreted within the framework of the morphogen gradient theory. A morphogen produced by the founder-zooid (oozooid) diffuses along the long axis of the colony and controls the phenotypic expression of the size and structure of zooids. Evolutionary changes in the graptoloid colonies involve introduction of new characters and their spreading is also accompanied by gradient changes of their manifestation. Evolutionary mechanisms in bryozoan colonies are considered in terms of penetrance and expressivity of genes. In contrast to graptolites, many bryozoan colonies display multiple zones of astogenetic changes and repetitions.

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.