As is commonly the case with unicellular organisms, radiolarian taxonomy has been influenced by disagreements about which features should be regarded as more significant. Some would arrange taxa based on the overal formation of the skeleton. Others would focus on the development of the initial embryonic spicule around which the cephalis develops. A recent phylogenetic analysis of living nassellarians by Sandin et al. (2019), based on both morphological and molecular data, found that overall skeleton morphology was a much better indication of relationships than the internal structure. One well supported subgroup of the Nassellaria is the superfamily Eucyrtidioidea.
Eucyrtidioids have a fossil record going back to the Triassic (Afanasieva et al. 2005). The cephalis is spherical and clearly distinguished from the following segments by a constricted basal aperture. The test is usually multi-segmented; members of the subfamily Theocotylinae may have just two segments but other members of Eucyrtidiidae have up to ten segments. Fossil families assigned to Eucyrtidioidea by Afanasieva et al. (2005) may have up to twenty (but as Afanasieva et al.'s concept of Eucyrtidioidea was not found to be monophyletic by Sandin et al., the affinities of these fossil families perhaps warrant re-investigation). Segments are commonly divided by distinct inner rings. The skeleton lacks feet, the term used for protruding spines around the basal aperture of the skeleton found in many other nassellarians.
The phylogeny of nassellarians indicated by Sandin et al. (2019) places the Eucyrtidiidae as the sister taxon to other living nassellarians. Other living families included in the Eucyrtidioidea by Afanasieva et al. (2005) were placed in more nested positions. The implication is that the multi-segmented condition may be ancestral for crown Nassellaria. Segments are added progressively during the life of the radiolarian, leading the organism to look quite different at different ages. Indeed, this metamorphosis is pronounced enough that one of the earliest influential researchers on radiolarians, Ernst Haeckel (he of Kunstformen der Natur fame), made the mistake of classifying different ages as different species, genera and even families. Our understanding may be better than in Haeckel's time but there may still be a lot to learn about these intricate organisms.
REFERENCES
Afanasieva, M. S., E. O. Amon, Y. V. Agarkov & D. S. Boltovskoy. 2005. Radiolarians in the geological record. Paleontological Journal 39 (Suppl. 3): S135–S392.
Sandin, M. M., L. Pillet, T. Biard, C. Poirier, E. Bigeard, S. Romac, N. Suzuki & F. Not. 2019. Time calibrated morpho-molecular classification of Nassellaria (Radiolaria). Protist 170: 187–208.
Suzuki, N., K. Ogane, Y. Aita, M. Kato, S. Sakai, T. Kurihara, A. Matsuoka, S. Ohtsuka, A. Go, K. Nakaguchi, S. Yamaguchi, T. Takahashi & A. Tuji. 2009. Distribution patterns of the radiolarian nuclei and symbionts using DAPI-fluorescence. Bulletin of the National Museum of Nature and Science, Series B 35 (4): 169–182.
The nomenclature with abdomen, feet, etc. feels weirdly arthropodal.
ReplyDeleteThe exact same thought passed through my head when I was preparing this post. I don't know who's responsible for the nomenclature but I wonder if it may have been Haeckel. There's an intriguing history behind early concepts of whether and how features of 'protozoa' corresponded to those of macroscopic animals but it's not a subject I know a great deal about. At the present time, I don't think there's a great deal of functional significance to the radiolarian 'thorax' and 'abdomen'.
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