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Image gallery: Sterrasters - interesting sponge microfossils. by Hugh
Grenfell, New Zealand |
Editor's note: In November, Hugh Grenfell sent us an intriguing enquiry, hoping Micscape readers could help with the identification of a microfossil. The author has since been able to identify the organism with the help of a specialist listserver's readers. But with the author's agreement we include the original query below with an update on its ID as it's a fascinating organism and structure which we hope are of interest.
Hugh Grenfell's original query (compiled from the author's email): I am a micropalaeontologist specialising in foraminifera. We keep finding these curious small egg-shaped microfossils in our samples and I wondered if it would be possible to post them on Micscape to get some feedback.
They are not calcareous (maybe siliceous) and have a small depression. They have a very distinctive sculpture (which may be eroded) and interior structure. Figure 2 (right hand image) best shows the stellate micro-ornament. On the others this is eroded. Figure 4 shows the radiating internal structure, which is probably the most interesting in that it shows a wonderful hexagonal porate structure in the interior.
The specimens come from near Napier (Hawkes Bay, NZ). They were in a Holocene age sample taken from a vibracore at 72-74cm depth. We also have other examples from short cores around New Zealand in Holocene and Pleistocene sediments - usually estuarine or non-marine.Wim van Egmond kindly sent me a transmitted light microscope image of an organism which he has been unable to positively identify and which I consider to be the same thing (Figure 5). At the moment the consensus seems to point to a sponge origin but as yet no-one can tell me exactly what! I have posted my query to a sponge listserver in the hope that the experts can rule that one in or out. So far I know it is not a diatom, foraminifera, thecamoebian, radiolarian, dinoflagellate, prasinophyte, phytolith or indeed any palynomorph I have come across (my PhD was on Palaeozoic spore, pollen and acritarchs).
Update on the identification (Dec. 9th): I have just yesterday been able to confirm the identity of the strange microfossils courtesy of contacts via the Web (sponge experts). I enclose additional info and links showing readers that the evidence clearly shows them to be derived from a family of marine sponges - the Geodiidae. This family produces particular opaline silica spicules called sterrasters in the outer layer of their "body". It gives them a tough leathery feel when touched apparently. I also yesterday come across a paper published here in New Zealand in 1958 which describes the very same thing from Plio-Pleistocene deposits. I suspect because of their small size, shape and robust construction these microfossils are easily reworked into Holocene deposits. So they can be eroded from older, truly marine sediments into younger estuarine / brackish or freshwater deposits. Some papers and images on the web to support the identity of the microfossils can be found athttp://biblioweb.dgsca.unam.mx/cienciasdelmar/instituto/1986-3/articulo253.html and
http://webdoc.gwdg.de/diss/2003/hoffmann/hoffmann.pdf (especially Plate 5).
Dr Hugh Grenfell
Geomarine
Research
E-mail: Hugh
Grenfell
(The author would be interested hear from readers regarding
any aspects of this organism or related topics).
Author's footnote: If any readers would like to see some of the foram handiwork by Geomarine Research, please visit http://homepages.ihug.co.nz/~bw.hayward/ .
Acknowledgement: Thank you to Wim van Egmond for sharing Figure 5.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5 (40x objective, DIC) by Wim van Egmond from his
Micscape article 'Some
enigmatic radiolaria'
where he sought confirmation of the identity of
this and other organisms.
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