Spirally-Coiled Radiolarians in the Latest Jurassic–Earliest Cretaceous

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 (FigSpirally-coiled radiolarians in the latest Jurassic–earliest Cretaceous