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The morphology and fine structure of the Ordovician Cephalodiscus-like genus Melanostrophus

100%

Mierzejewski P., Urbanek A.

Acta Palaeontologica Polonica

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2004

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tom 49

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nr 4

The holotype and a new specimen from the type locality, as well as a few new specimens of Melanostrophus fokini Öpik, 1930, an enigmatic invertebrate from the Ordovician of the Baltic region, have been examined using combined LM, SEM and TEM techniques. This form is reinterpreted as a ?cephalodiscid hemichordate. Its skeleton or coenecium is an encrusting assemblage of uniform zooidal tubes, forming a circular or subcircular palisade−like structure.The zooidal tubes are long (up to 50 mm) and slender, similar to zooidal tubes of the extant pterobranch hemichordate Cephalodiscus (Orthoecus). The fine structure of the skeleton wall is similar to that in graptolites and four components have been recognized within periderm: (i) thick, outer cortical layer, (ii) very thin fusellar layer, constructed of annular growth bands, with their oblique sutures arranged randomly, resembling the fusellar layer of some pterobranchs and primitive graptolites, (iii) inner cortical layer, and (iv) thin, enamel−like inner lining. The periderm is abundantly perforated by pits and holes of different diameters; some of them were probably caused by saprophytic or parasitic borers, but the largest ones (up to 100 µm) are probably primary and mark a tube bifurcation. It is concluded that cortex formation is not a synapomorphy for graptolites.

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Cortical fibrils and secondary deposits in periderm of the hemichordate Rhabdopleura [Graptolithoidea]

100%

Mierzejewski P., Kulicki C.

Acta Palaeontologica Polonica

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2003

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tom 48

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nr 1

Coenecia of extant hemichordates Rhabdopleura compacta and Rh. normani were investigated using SEM techniques. Cortical fibrils were detected in their fusellar tissue for the first time. The densely packed cortical fibrils form a characteristic band−like construction in fusellar collars, similar to some Ordovician rhabdopleurids. No traces of external secondary deposits are found in coenecia. Two types of internal secondary deposits in tubes are recognized: (1) membranous deposits, composed of numerous, tightly packed sheets, similar to the crustoid paracortex and pseudocortex; and (2) fibrillar deposits, devoid(?) of sheets and made of cortical fibrils, arranged in parallel and interpreted as equivalent to graptolite endocortex. There is no significant difference in either the shape or the dimensions of cortical fibrils found in Rhabdopleura and graptolites. The cortical fabric of both rhabdopleuran species studied is composed of long, straight and more or less wavy, unbranched fibrils arranged in parallel; their diameters vary from 220 to 570 µm. The study shows that there is no significant difference between extinct and extant Graptolithoidea (= Pterobranchia) in the histological and ultrastructural pattern of their primary and secondary deposits of the periderm. The nonfusellar periderm of the prosicula is pitted by many depressions similar to pits in the cortical tissue of graptolites.

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The ultrastructure and building of graptolite dissepiments

80%

Urbanek A., Mierzejewski P.

Acta Palaeontologica Polonica

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2009

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tom 54

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nr 2

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.

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Wczesna filogeneza zwierzat tkankowych w zapisie kopalnym

71%

Dzik J.

Kosmos

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1996

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tom 45

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nr 4

657-686

Ograniczanie wyników

3 Acta Palaeontologica Polonica

1 Kosmos

3 Mierzejewski P.

2 Urbanek A.

1 Dzik J.

1 Kulicki C.

1 2009

1 2004

1 2003

1 1996

The morphology and fine structure of the Ordovician Cephalodiscus-like genus Melanostrophus

Cortical fibrils and secondary deposits in periderm of the hemichordate Rhabdopleura [Graptolithoidea]

The ultrastructure and building of graptolite dissepiments

Wczesna filogeneza zwierzat tkankowych w zapisie kopalnym