(3111) Proposal to conserve the name <i>Xenikoon australis</i> (fossil <i>Dinophyceae</i>) with a conserved type

(3111) Xenikoon australis Cookson & Eisenack in Micropaleontology 6: 16. Jan 1960, nom. cons. prop. Typus: Brickhouse Bore, at 715 ft., Central Carnarvon Basin, 16 km SE of Carnarvon, Western Australia (Mus. Victoria No. P17850 [specimen circled & arrowed]); [illustrated in] Cookson & Eisenack in Micropaleontology 6: t. 3, fig. 16. Jan 1960, typ. cons. prop. Isabel Cookson and Alfred Eisenack (in Micropaleontology 6: 14, 16, pl. 3, figs. 16–17. 1960) introduced the monotypic dinoflagellate fossil-genus Xenikoon, with X. australis as type, describing it as follows: “Shell ovoidal, slightly pointed anteriorly, containing a spherical capsule, the wall of which is in direct contact with the wall of the shell except at the anterior end. A pylome is present shortly below the apex. In one example, which unfortunately is damaged, the shell seems to have been enclosed in a smooth outer membrane (plate 3, fig. 17).” Cookson & Eisenack (l.c.) illustrated two specimens, one well preserved (fig. 16) and another broken; the latter was designated as the holotype (fig. 17). Cookson & Eisenack (l.c.) classified Xenikoon under Incertae sedis but in the same publication used family names such as Gymnodinidae and Deflandreidae for other taxa, which reflects the “zoological” family names Gymnodinidae Kofoid and Deflandreidae Eisenack. This suggests that Cookson and Eisenack (l.c.) were employing zoological nomenclature. Stover & Evitt (in Stanford Univ. Publ., Geol. Sci. 15: 134. 1978) argued that “The holotype of X. australis (on Mus. Victoria slide P17850) is the specimen shown in Cookson & Eisenack, 1960a, pl. 3, fig. 16, not fig. 17 as stated in their text.” Although this is a logical assumption as Cookson and Eisenack (l.c.) acknowledged the limitations of their fig. 17, Stover & Evitt (l.c.) provided no supporting evidence for this claim. In the absence of unequivocal proof, the holotype must be accepted as the specimen depicted in pl. 3, fig. 17 of the protologue and designated as such, even though perhaps mistakenly. The two specimens illustrated in the protologue would now be considered to belong to two different fossil-genera. The specimen of pl. 3, fig. 16 in Cookson & Eisenack (l.c.) aligns with the “traditional” fossil-species concept of Xenikoon australis, featuring a 2a archaeopyle, as applied by later authors (see, e.g., Riding & al. in Newslett. Stratigraphy 26: 19–39, pl. 2/3 & 4. 1992). By contrast, the holotype of X. australis in Cookson & Eisenack (l.c.: pl. 3, fig. 17) shows an unclear archaeopyle that may be a combination involving intercalary and apical plates, possibly attributable to the dinoflagellate cysts fossil-genus Ovoidinium Davey (see Davey in Bull. Brit. Mus. (Nat. Hist.), Geol. 18: 351. 1970). Strictly adhering to the ICN (Turland & al. in Regnum Veg. 162. 2025) and current taxonomic concepts, X. australis would thus potentially be a heterotypic junior synonym of an Ovoidinium fossil-species, necessitating a new name for the concept currently applied to X. australis. Implementing this change would follow nomenclatural rules, including those on priority, but it would be highly disruptive. The current concept of the fossil-genus and the fossil-species Xenikoon australis, extensively used in the sense of Cookson & Eisenack (l.c.: pl. 3, fig. 16) would be altered, affecting its role as a biostratigraphical marker. Xenikoon australis is a reliable index fossil for the early and middle Campanian (Late Cretaceous) of eastern Gondwana. The type material of X. australis is from 217.93 m in the Brickhouse Bore, which was drilled 16 km SE of the coastal town of Carnarvon, Western Australia, in the central Carnarvon Basin. This horizon is of Campanian age according to Cookson & Eisenack (l.c.: 16). Marshall (Late Cretaceous dinoflagellates from the Perth Basin, Western Australia. Ph.D. thesis, Univ. Western Australia. 1984) determined that the range of this fossil-species is early and middle Campanian, based on material from the Perth Basin in Western Australia. The early Campanian “Xenikoon australis Zone” of Evans (in Reynolds, Rep. Dept. Natl. Developm., Bur. Mineral Resources Geol. Geophys., Commonw. Australia 134: 31–35. 1971) was modified by Helby & al. (in Mem. Assoc. Australas. Palaeontol. 4: 66, 68, figs. 40, 45. 1987). The latter authors stated that acmes of this fossil-species are confined to this zone, which is of early Campanian age (Helby & al., l.c.). Xenikoon australis was also reported from the uppermost Santa Marta Formation of San José Pass, NW James Ross Island in the Antarctic Peninsula by Riding & al. (l.c.: fig. 2, pl. 2/3, 4), and these occurrences were deemed to be early Santonian to early Campanian in age. Because of the superior independent dating of the type material, this Antarctic record is best interpreted as early Campanian. The late Campanian–earliest Maastrichtian age interpretation of X. australis from the Maud Rise and the Georgia Basin in the Southern Ocean quoted by Mohr & Mao (in Palynology 21: 41–65, fig. 9. 1997) is not considered to be justified. The latter authors appear to have interpreted dinoflagellate cyst ranges as anomalously young. To prevent significant disruption, particularly beyond systematics, and to avoid disadvantageous nomenclatural change (Art. 14.1), we propose conserving the fossil dinoflagellate name Xenikoon australis with a different type than that designated by Cookson & Eisenack (l.c.), i.e., with the specimen illustrated in pl. 3, fig. 16 of Cookson & Eisenack (l.c.). This will ensure stability in fossil dinoflagellate cyst taxonomy and its applied uses. James B. Riding publishes with the approval of the Director, British Geological Survey (NERC). We are grateful to Vânia Correia for providing a helpful review for GSC internal purposes, and to Tim Ziegler (Melbourne Museum, Victoria Museums) for verifying the type material.

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