Juvenile stage of the species|
Pay attention to the central sphere (cubical?).
Kjell R. Bjørklund (2000/11/20)
could it be that this species is a sort of norwegian fjord variant of one of the taxa mentioned in the title, that was drifted into your area and got trapped in the fjords during some warm time period ?
It would be a sort of process opposite to the one you and Neil propose for the variants of Amphimelissa setosa.
Once trapped it could have developed a slightly different morphology, and even got as far as "speciating".
After all, Lozano (1974) mentions some possibilities of drifting for A. antarcticum even as far north as the Argentine Basin (not quite the typical habitat for this species...), and drifted faunas are not new, as you know, in the Nordic Seas either...
In some samples from ca. 30-40 S I have two or even all three taxa occurring together, plus some juvenile morphotypes which resemble somehow your figured specimen in many ways.
Unfortunately I cannot provide any electronic pictures (but I will send you a couple of videoprints by regular mail), but I think that the "little arches" that is possible to note in your picture both inside and outside the outer cortical sphere, where the sphere intersects the spines (see the top half of your first picture) are also present in my juvenile specimens, along with a quite similar general appearance and main spines' arrangement.
Giuseppe Cortese (2000/11/21)
I think there are a few differences between the one species I have illustrated and the one "group" you are suggesting Actinomma medianum/antarcticum/arcadophorum.
I agree that you have them all three in the south, but if you look carefully on the morphological differences, you will see that:
1) Actinomma medianum/antarcticum/arcadophorum "group" all have a spongy, rather complicated outer medullar sphere.
2) arcadophorum normally have an irregular outer sphere, bumpy in a way.
3) antarcticum is having an outer spongy sphere in addition to the spongy outer medullar sphere.
4) Finally, and most of all, the proportions of the three spheres are not corresponding, a criteria that I feel is the best of those mentioned.
I think that there is no doubt that it is an Actinomma species that we have in question, but I got uncertain when I looked on the inner medullar sphere. Its shape is not typical for an Actinomma! For the time being I prefer to keep it as an Actinomma and I still ask for other observations and thoughts about this species.
Kjell R. Bjørklund (2000/11/21)
The taxa in sediment cores from the Norwegian Sea|
When I was studying radiolarians in two sediment cores, HM 79-6.2 (62°58'N, 02°42'E) and HM 79-4 (63°06'N, 02°33'E) from the SE Norwegian Sea, I found a few specimens, which I believe, belong to the same taxa as shown above. The youngest occurrence was dated to 3.800 yrs. B.P. and the oldest to 13.100 yrs. B.P.
Jane K. Dolven (2000/11/26)
Peppe suggested that Actinomma sp. also could have been a result of "drifting" from the south and into the
Norwegian Sea, with the group Actinomma medianum/antarcticum/arcadophorum as a possible source for its
development. In the north Atlantic only A. medianum is the candidate. You have your first Actinomma sp. at
13.100 yrs. B.P. According to Jansen and Bjørklund (1985), the first siliceous sediments in the Norwegian Sea
was dated to ca 13.000 yrs. B.P. (an age that is not too accurate, but probably within the nearest few hundred
years). Therefore, there is no doubt that the post-glacial recruitment of radiolarians in the Norwegian Sea is from
the north Atlantic, an event that probably took place somewhere between 13.500 and 13.000 yrs B.P. The species
you have posted is identical with the one I have in the recent material in the Norwegian Sea. Peppe suggested that a
form from the A. mediaum group should have drifted from the north Atlantic and "Once trapped it could have
developed a slightly different morphology, and even got as far as "speciating"" Your finding suggest to me that if
Peppe's model should hold, this speciation must have taken place in the north Atlantic prior to the introduction to
the Norwegian Sea. This because your specimen is morphological identical to the modern forms. I have not come
across any specimens of A. medianum in the Norwegian Sea surface sediments, and if my memory is right, you
Jane have not recorded any A. medianum down the cores you have studied. During the glacial periods the Artic
and Norwegian Sea fauna was probably located in a more southern position, i.e. the 18K Polar Front at 45oN.
During this time the warm and cold water faunas were squeezed together in a much smaller area than today and had
a better chance to mix and develop new morphotypes. If any this is the case, may be the stress developed under
glacial periods can explain the origin of Actinomma sp. However, the origin is still a mystery!
Kjell R. Bjørklund (2000/12/04)
Kozo illustrated (tho only with one SEM image) something that looks rather
similar. There is no description as such provided but in a short comment he
indirectly indicates the presence of 2 medullary shells.
Dave Lazarus (2000/12/06)
I have looked up both Takahashi (1981 thesis) and Haeckel (1887) and do feel that we are closing in now, but still not quite. I regard these three criteria, in order of priority, as important in searching for similarities: 1) The relative size difference between the two innermost spheres. 2) The pore pattern and their shape on the outer sphere, and 3) the number of connecting radial beams. On this basis I find "my species in question" more similar to A. acradophorum than to A. capillaceum, but it is still not the same. A. acradophorum do have numerous (40 till 80 or more) thin radian beams connecting the outer medullar and the cortical spheres. We are not even close to this number in the Norwegian Sea specimens. The cortical sphere in A. acradophorum has a pore pattern and shape that resemble the Norwegian Sea specimens. I do conclude that we are here dealing with a new species, and sometime in the future it will be described. I am still searching, so any new information is still appreciated.
In the same go, I have started to use a different term on the spumellarian structures. I will try to be consequent and use the term sphere instead of shell. It is obvious from a linguistic point of view, that radiolarians do have no shell. A shell is an outer hard part, surrounding either a seed, and egg or a mollusk. Haeckel (1887) and most of us have not been too concerned about this aspect, but from now on I will try to be strict on this matter.
Kjell R. Bjørklund (2000/12/12)