Wyniki wyszukiwania

1

Artykuł dostępny w postaci pełnego tekstu - kliknij by otworzyć plik

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

100%

Mierzejewski P., Urbanek A.

Acta Palaeontologica Polonica

|

2004

|

tom 49

|

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.

2

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

100%

Mierzejewski P., Kulicki C.

Acta Palaeontologica Polonica

|

2003

|

tom 48

|

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.

3

Discovery of Pterobranchia [Graptolithoidea] in the Permian

100%

Mierzejewski P., Kulicki C.

Acta Palaeontologica Polonica

|

2002

|

tom 47

|

nr 1

The fossil remains of a hemichordate exoskeleton, recognized as fragments of the stolons and their cyst−like swellings connected with the fusellar zooidal tubes, were derived by chemical isolations from Late Permian (Kazanian) mudstones of the Svalis Dome (central Barents Sea, Norway). These fossils, referred to as Diplohydra szaniawskii sp. nov., are the first undoubted representatives of the class Graptolithoidea found in Permian deposits. The genus Diplohydra Kozłowski, 1959, known previously only from the Ordovician and originally established as a thecate hydroid taxon, is reinterpreted as an aberrant member of the order Rhabdopleuroidea. This strange hemichordate, characterized by fusellar tubes distinctly narrower than stolon−like tubes and their swellings, reveals a certain degree of dimorphism in the stolon system. D. szaniawskii sp. nov. also displays some peculiar morphological features common to the Ordovician rhabdopleuroid genus Rhabdopleurites Kozłowski and the stolonoid genus Stolonodendrum Kozłowski.

4

The stolon system in Rhabdopleura compacta [Hemichordata] and its phylogenetic implications

80%

Urbanek A., Dilly P. N.

Acta Palaeontologica Polonica

|

2000

|

tom 45

|

nr 3

201-226

Studies made with the light microscope on the stolon system of extant pterobranch hemichordate Rhabdopleura compacta Hincks, 1880 have revealed the presence of characteristic structures called herein diaphragm complexes. Each complex consists of the stolonal diaphragm proper and a thin-walled conical encasement, produced by a rapid inflation of the stolonal sheath around the diaphragm. Such structures have never been observed before either in the Recent or fossil Rhabdopleurida. However, both in their origin and in their relations to the stolon and to the zooidal tube, diaphragm complexes strongly resemble the internal portions of thecae as recognized in the sessile orders of the Graptolithina. The significance of the presence of these homologues of the enclosed initial portions of thecae in Rhabdopleura compacta for the understanding of the phylogenetic relationships between pterobranchs and graptolites is discussed.

5

The ultrastructure, development, and systematic position of the graptolite genus Mastigograptus

80%

Bates D. E. B., Urbanek A.

Acta Palaeontologica Polonica

|

2002

|

tom 47

|

nr 3

Fragments of rhabdosomes isolated by chemical treatment from an erratic boulder of Baltic origin and ?Middle Ordovician age, provisionally assigned to Mastigograptus aff. tenuiramosus (Wallcott, 1881) were studied with SEM. Although exceptionally well preserved, remains lack the thin−walled free portions of thecae. Rhabdosomes are provided with a strongly developed basal disc, short stem and many branched stipes. The latter consist of heavily corticalized chains of stolothecae with alternately disposed thecal bases. Stolothecae display a morphological gradient and increase in size and change in shape distalwards. The stolon system studied with SEM on naturally and artificially broken specimens, as well as traced through open thecal bases, reveals a regular triad budding but no stolon inside the stolothecal cavity. We tentatively suggest that crassal lining, recognized earlier by TEM studies, corresponds to an unusually inflated stolonal stolon, filling the entire thecal cavity and adhering tightly to stolothecal wall. The systematic position of Mastigograptus, a matter of long debate, seems to be defined by a number of structural features which imply a distinct difference between genus in question and all known orders of sessile graptolites. The order Mastigograptida nov. and the family Mastigograptidae nov. are proposed.

Ograniczanie wyników

5 Acta Palaeontologica Polonica

3 Mierzejewski P.

3 Urbanek A.

2 Kulicki C.

1 Bates D. E. B.

1 Dilly P. N.

1 2004

1 2003

2 2002

1 2000

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

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

Discovery of Pterobranchia [Graptolithoidea] in the Permian

The stolon system in Rhabdopleura compacta [Hemichordata] and its phylogenetic implications

The ultrastructure, development, and systematic position of the graptolite genus Mastigograptus