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On the morphology and evolution of the Cucullograptinae (Monograptidae, Graptolithina)

100%

Urbanek A.

Acta Palaeontologica Polonica

1966

tom 11

nr 3-4

A description is given of the structure, astogenetic development and evolution of the Upper Silurian Cucullograptinae. It is based on the material contained in the core samples from the Mielnik on the Bug (Eastern Poland) deep-boring and from the Baltic drift. The graptolites were prepared by dissolving the limestone and marly matrix in hydrochloric acid. For detailed study the specimens were bleached. The stratigraphic position and significance of the fauna investigated are discussed. The main trend in the evolution of sicula consists in its simplification and is interpreted as due to foetalization. In contrast with sicula, the evolution of thecae displays progressive changes which primarily consist in the development of apertural apparatus, connected, in most lines, with the elaboration of its secondary asymmetry. The biological significance of this pattern of evolution of thecal characters is discussed. The Cucullograptinae supply convincing evidence for the distal introduction of phylogenetic novelties which makes up a prevailing mode of evolutionary changes of astogeny in this group of moriograptids. Problems, related to the mechanisms of evolution and organization of graptolite colonies are discussed. Using the data, now available, tentative phylogenetic relationships within the group are established. An attempt is made to show some implications of these data in the problem of origin of the Cucullograptinae. Fourteen representatives (species and subspecies) of the Cucullograptinae are described in the systematic part. Four species and two subspecies are new.

Convergent evolution of two Silurian graptolites

80%

Lenz A. C., Melchin M. J.

Acta Palaeontologica Polonica

2008

tom 53

nr 3

The two graptolite speciesCochlograptus veles(Telychian, Upper Llandovery) and Testograptus testis(lower Homerian, upper Wenlock) are remarkably and uniquely similar in being strongly ventrally and planispirally coiled and in demonstrating an abrupt deflection in the immediate post−sicular regions of their rhabdosomes. The two species, however, are separated by a relatively large biostratigraphic gap and a global mass extinction, and they differ morphologically in the proportion of thecal overlap, different angles of inclination of the interthecal septa, relative proportions of the widths occupied by the free metathecae, the position of the sharp dorsal flexure relative to the tip of the sicula, and the presence of a distinctive, keel−like structure on T. testis. It is suggested therefore, that in spite of the strong proximal morphological parallelism between the two species, their origin is best explained as a remarkable example of convergent evolution. It is suggested that C. veles perhaps evolved from some modified monograptid such as Stimulograptus, whereas the small Testograptus group may have derived from some monograptid such as Monograptus flemingii. Cladistic analysis fully supports the independent derivation the two species. If correct, this hypothesis supports the validity of separate generic names for the two species, despite the close and unique rhabdosomal similarities, including proximal metathecal form.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

1 Lenz A. C.

1 Melchin M. J.

1 Urbanek A.

1 2008

1 1966

Wyniki wyszukiwania

On the morphology and evolution of the Cucullograptinae (Monograptidae, Graptolithina)

Convergent evolution of two Silurian graptolites