NEW CHANCELLORIIDS FROM THE EARLY CAMBRIAN SEKWI FORMATION WITH A COMMENT ON CHANCELLORIID AFFINITIES

Journal of Paleontology, Sep 2005 by Randell, Robert D, Lieberman, Bruce S, Hasiotis, Stephen T, Pope, Michael C

Occurrence.-Sekwi Formation, locality 1; ?Qiongzhusi Formation, Early Cambrian (Meishucunian), Meishucun, Jinning County, Yunnan, China (Qian and Bengtson, 1989); ?Wirrealpa Limestone, Early Cambrian (Toyonian), South Australia (Brock and Cooper, 1993); and ?Early Cambrian (Botomian), Tuora-Sis, Lena River region, northern Siberia (Missarzhevsky, 1989).

Discussion.-Archiasterella fletchergryllus resembles A. pentactina, but in the latter the recurved basal ray is shorter than the laterals and correspondingly is less robust at its proximal end. Though morphological variation is evident in A. fletchergryllus the extreme development of the basal ray is consistent and warrants its distinction from A. pentactina. Although typical A. pentactina differs from A. fletchergryllus, some sclerites figured as A. pentactina by Missarzhevsky, 1989 (pi. XXII, fig. 6), A. cf. pentactina by Qian and Bengtson, 1989 (fig. 6a-c), and also Chancelloria sp. Brock and Cooper, 1993 (fig. 6.7), appear to resemble A. fletchergryllus and they are all questionably treated as conspecific. The potentially broad geographic distribution of material referable to Archiasterella fletchergryllus would be compatible with that of other chancelloriid taxa. Morphological extremes seen within Archiasterella fletchergryllus might by themselves merit taxonomic distinction, but a range of intermediate forms is seen. At this point we view the various morphotypes as integrated within organisms belonging to one species.

The basal ray of A. palmiformis Vasil'yeva in Vasil'yeva and Sayutina, 1988 is highly reduced, and this species may well be synonymous with A. pentactina. Archiasterella palmiformis was described using isolated sclerites but may represent the more basal sclerites of the A. pentactina scleritome. Archiasterella sp. Mehl, 1998 is perhaps synonymous with A. palmiformis (and hence A. pentactina), though it does have a slightly different suture arrangement in the central disc. Taxa clearly distinct from Archiasterella fletchergryllus include: A. robusta VasiPyeva, 1985, which appears to possess a central foramen; A. antiqua Sdzuy, 1969, which does not display the marked size differentiation between ascending laterals and horizontal rays seen in A. fletchergryllus; and A. hirundo Bengtson in Bengtson et al., 1990 and A. cf. hirundo Bengtson in Bengtson et al., 1990, which are exclusively 4 0. Archiasterella tetractina Duan, 1984 lacks a recurved basal ray and probably should not be assigned to Archiasterella. The articulated exoskeleton of Archiasterella fletchergryllus is unique among described chancelloriids because sclerites can tessellate (Fig. 8.4). A similar arrangement could also be envisaged, though it has not been reported, for Archiasterella pentactina, A. robusta, A. palmiformis, and possibly A. antiqua', it is unlikely for A. hirundo, whose typically 4 0 sclerites probably could not tessellate in such a manner. The scleritome of A. cf. hirundo (Walcott, 1920, pi. 88, fig. Ic) has sclerites a rays' length apart and the orientation of rays precludes close articulation and thus tessellation. Thickened basal bulbs were not observed in Archiasterella fletchergryllus.