Polycystine radiolarians have siliceous shells. The shells of Nassellaria, one of the major orders of radiolarians, are generally tower-like. Spiral structures are known in the shells (e.g. Sashida and Tonishi, 1991) and spines (e.g. De Wever et al., 2003) of other radiolarians; however, none with spiral shells had been found in Nassellaria. We found two different genera of Nassellaria with spiral shells from the latest Jurassic–earliest Cretaceous sample from the Mariana Trench. The findings are extremely rare and are important when considering the shell formation in radiolarians.
The sample (181-R003) is a laminated tuffaceous radiolarian claystone or clayey radiolarite collected from the Mariana Trench oceanward slope (15°28.7′N, 147°50.6′E: 6,316 m water depth) by “Shinkai 6500” of JAMSTEC (Japan Agency for Marine-Earth Science and Technology). Because of the occurrences of the Loopus primitivus (Matsuoka and Yao) and Pseudodictyomitra carpatica (Lozyniak) (Matsuoka, 1998), the sample was considered to correspond to the lower part of the Pseudodictyomitra carpatica Zone (KR1) (uppermost Tithonian to lower Berriasian, uppermost Jurassic–lowermost Cretaceous) by Matsuoka and Ito (2019). Further detailed information on the sample and cooccurring radiolarian species are mentioned in Matsuoka (1998).
The sample yielded two specimens having spiral shells. Such specimens are called “spiral-mutant” in this article. The first specimen of the spiral-mutant belongs to Svinitzium pseudopuga Dumitrica (Figure 1). The shell of the spiral-mutant specimen appears to be uncoiled up to the third septa from the top, but from the fourth septa onward, the shell tilts in opposite directions on the front side (Figure 1A1, A2) and the back side (Figure 1A3, A4), indicating a spiral coiling structure. In contrast, all the septa of the normal specimens (Figure 1B, C) are parallel and uncoiled. The second specimen belongs to Xitus Pessagno (Figure 2). Oblique septa can be observed in the lower part of the shell of the spiral-mutant specimen (Figure 2A); parallel septa are recognized in the normal specimens (Figure 2B, C).
Some researchers have observed the growth processes of radiolarian skeletons (e.g. Anderson, 1981; Matsuoka, 1992; Suzuki, 2006; Ogane et al., 2010; Lazarus et al., 2021). Lazarus et al. (2021, p. 58) illustrated the Nassellaria specimen and noted, “the degree to which cellular growth control mechanisms are increasingly limited in effectiveness as growth extends further away from the point of origin (the first cephalic segments).” The spiral-mutant specimens shown in this article seem to have an uncoiled structure in the upper part of the shell (Figures 1A, 2A), consistent with the description by Lazarus et al. (2021).
Shells of a unicellular organism have strictly formed parts according to its “body plan” and parts that are not. The former parts are essential for survival and are ecologically relevant. The accumulation of examples, such as the spiral-mutant specimens reported in this study, should lead to a better understanding of the shell formation with intracellular roles and growth processes of radiolarians. Further, this will be important knowledge for radiolarian paleoecology and taxonomy.
Figure 1.
A, “Spiral-mutant” specimen of Svinitzium pseudopuga Dumitrica. A1 to A4 shift the focus to the same specimen. B, C, Normal specimens of Svinitzium pseudopuga Dumitrica. Scale bars are 50 µm. Specimen number: B, 21489; C, 21490. All specimens were extracted from the sample (181-R003) collected from the Mariana Trench.
Acknowledgements
We thank T. Kurihara (Associate Editor) and two anonymous reviewers for constructive comments on the manuscript.
© by the Palaeontological Society of Japan
