Tuesday 24 September 2013

The a Late Miocene-Pliocene Diatom redescribed from Scanning Electron Microscope images.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells.

The Neogene Diatom Thalassiosira dolmatovae was originally described from specimens collected from Karaginsky Island in 1992 by Tatiana Oreshkina of the Geological Institute of the Russian Academy of Sciences. It is known from Latest Miocene to Late Pliocene sediments from the Kamachatka Peninsula in Russia and the Alaskan Peninsula in the US, and is part of a species complex ranging from Japan to California, making it a potentially useful stratigraphic marker for dating marine sediments around the north Pacific. However Oreshkina's description was based upon light microscope images only, and no holotype for the species was provided, making the original description both technically invalid and of limited usefulness (a holotype is an individual specimen designated to represent a species for taxonomic purposes; when deciding if a specimen belongs to a species it is compared to the holotype).

In a paper published in the journal Phytotaxa on 29 August 2013, Andrey Gladenov, also of the Geological Institute of Russian Academy of Sciences, provides a formal redescription of Thalassiosira dolmatovae based upon Scanning Electron Microscope (SEM) images, and designates a holotype for the species.

SEM image of the newly designated holotype of Thalassiosira dolmatovae, external view of valve. The arrow indicates a central process not previously noticed in the species. Specimen is from the Milky River Formation at Sandy Ridge on the Alaska Peninsula, and is between 5.5 and 4.8 million years old. Gladenov (2013).

Light microscope image of a specimen of Thalassiosira dolmatovae. Again the arrow indicated the central process, but this is far from clear. Gladenov (2013).

Thalassiosira dolmatovae is a 12–37 μm flat or slightly convex disk shaped Diatom, the valves of which are punctuated by a number of areolae (holes) through which the organism obtained nutrients; these decrease in size from the center of the valve to the perimeter. At the margin is a 0.7–1.5 μm wide zone lacking such areolae. A central fultoportula (process) not previously observed connects the two valves.

Thalassiosira dolmatovae, view of the internal surface of the valve. Gladenov (2013).

Locations of the Cenozoic stratigraphic sections (black circles) in the North Pacific region where Thalassiosira dolmatovae is documented. (1) Karaginsky Island section, northeastern Kamchatka. (2) Sandy Ridge section, the Alaska Peninsula, southwestern Alaska. Gladenov (2013).

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