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Evolution of retiolitid graptolites - a synopsis

100%

Kozlowska-Dawidziuk A.

Acta Palaeontologica Polonica

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2004

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tom 49

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nr 4

Twenty million years of retiolitid evolution reflect environmental changes, the most severe being the Silurian Cyrtograptus lundgreni Event. Five biostratigraphically and morphologically constrained retiolitid faunas are distinguished and characterized according to their rhabdosomal modifications: (1) the oldest and long−ranging Llandovery group of mostly large and morphologically complex rhabdosomes, (2) the less diverse Telychian−Sheinwoodian group, (3) the Cyrtograptus lundgreni Biozone varied group of intermediate size, and two short−ranged (4) late Homerian, and (5) early Ludlow groups with small rhabdosomes. Although the evolutionary history of retiolitids was complex and not linear, a common tendency toward reduction of rhabdosome size in most lineages is observed. The greatest reduction in both number and volume of thecae, and in skeletal elements is demonstrated in the Gothograptus and Plectograptusfaunas. Contrary to the thecal decrease, a distinctive increase of sicula size is observed in retiolitids. Two types of colonies are distinguished: L−colonies with a small sicula and numerous large thecae of similar size, and S−colonies with a long sicula and a few, small thecae. These changes imply modification of the soft body: an increase in siculozooid length and a decrease in the size of the zooids. Thus, the siculozooid probably produced great amounts of morphogen inhibiting zooid growth. In consequence the phenomenon of colony reduction occurred. The most extreme stages of rhabdosome reduction in Ludlow retiolitids can be seen in Plectodinemagraptus gracilis of the Plectograptuslineage and in the new species Holoretiolites helenaewitoldi, possibly representing the last stage of skeletal reduction in the Gothograptus lineage; the next hypothetical stage would be its total loss. The sicula length of Holoretiolites, about 2 mm, is reported herein for the first time.

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The Silurian retiolitid graptolite Plectograptus: New observations and new species

80%

Bates D. E. B., Kozlowska A., Maletz J., Kirk N. H., Lenz A.

Acta Palaeontologica Polonica

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2006

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tom 51

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nr 3

The Ludlow genus Plectograptus, with the type species Retiolites macilentus Törnquist, 1887, collected from Thuringia (Germany), has been a widely−identified, monospecific, but poorly understood taxon for almost one hundred years. This was due to poor and incomplete preservation of the type material, and misidentification by subsequent authors up to 1995. The original, and only, type specimen of P. macilentus collected by Törnquist being lost, a neotype is herein selected from a small collection of Thuringian material. The genus has now been redefined and based on this, and SEM studies of isolated material, the defining characteristics of the genus are (i) the possession of a simple ancora umbrella with five radial lists with an incompletely developed rim; (ii) an ancora umbrella separated from lateral ancora sleeve walls by exceptionally large lateral orifices; (iii) the possession of mid−ventral lists; (iv) simple, orderly zigzag lateral wall ancora sleeve lists. Recently, two additional species, P. robustus and P. wimani, previously placed in different genera, were assigned to Plectograptus. This study recognizes three new species: P. mobergi, P. toernquisti, and P. trijunctus, bringing the total number of species to six. Species are distinguished by the presence or absence of genicular processes, inclination of the thecal ventral walls and mid−ventral lists, presence or absence of reticular lists, and three−way or four−way sleeve/lateral rod/apertural lip junctions.

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Evolution of the retiolitid Neogothograptus [Graptolithina] and its new species from the upper Wenlock of Poland, Baltica

70%

Kozlowska A., Lenz A., Melchin M.

Acta Palaeontologica Polonica

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2009

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tom 54

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nr 3

423-434

Neogothograptus reticulatus sp. nov. from the upper Homerian Colonograptus praedeubeli Biozone, and N. thorsteinssoni and N. alatiformis from the Lobograptus progenitor Biozone, are described for the first time from three localities: borehole, Baltic erratic boulder of East European Platform and Holy Cross Mountains of Poland. N. reticulatus, presently the oldest known species of Neogothograptus, is also tentatively identified from upper Homerian strata of southeastern Australia. The two other species are previously known only from Arctic Canada, and possibly China. The morphology of the Neogothograptus reticulatus rhabdosome, its appendix, thecal profile, densely reticulated rhabdosome and genicular hoods suggest a close relationship to N. eximinassa from Colonograptus ludensis Biozone. N. reticulatus and N. eximinassa are most similar to Gothograptus nassa, the earliest−known retiolitid to appear immediately following the Cyrtograptus lundgreni extinction event. The biostratigraphic position of N. reticulatus suggests it might be considered as a potential ancestor to all younger (Ludlow) species of Neogothograptus. Cladistic analysis, however, provides no direct support for such an interpretation and, instead, suggests that Baculograptus batesi may be the ancestor. The occurrences of Neogothograptus, as well as G. nassa, from a number of Silurian terranes mostly from low paleolatitude regions, but also from high paleolatitudes, demonstrate their tolerance to a broad range of paleoenvironments.

Ograniczanie wyników

3 Acta Palaeontologica Polonica

2 Kozlowska A.

2 Lenz A.

1 Bates D. E. B.

1 Kirk N. H.

1 Kozlowska-Dawidziuk A.

1 Maletz J.

1 Melchin M.

1 2009

1 2006

1 2004

Evolution of retiolitid graptolites - a synopsis

The Silurian retiolitid graptolite Plectograptus: New observations and new species

Evolution of the retiolitid Neogothograptus [Graptolithina] and its new species from the upper Wenlock of Poland, Baltica