Systematics of organic-walled microfossils from the ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona

Abstract

The ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona, provides an exceptional record of life during the diversification of crown-group eukaryotes, just prior to the first Cryogenian glaciation. We document in detail the assemblage of organic-walled microfossils preserved in fine-grained siliciclastics throughout the unit. In contrast with earlier studies, we primarily used SEM to document fossil morphologies, augmented by transmitted light microscopy, FIB-SEM, and TEM. This resulted in the discovery of new species and the recognition of broad-ranging, intraspecific biological and taphonomic variation in other species. Twenty-two species and five unnamed morphotypes are described, including three new species:Kaibabia gemmulella,Microlepidopalla mira, andVolleyballia dehlerae; two new combinations:Galerosphaera walcottiiandLanulatisphaera laufeldii; and 17 previously described forms. The possible colonial green algaPalaeastrum dyptocranumButterfield in Butterfield, Knoll, and Swett, 1994 and the index fossilCerebrosphaera globosa(Ogurtsova and Sergeev, 1989) Sergeev and Schopf, 2010 (=C. buickiiButterfield, 1994) are described for the first time from Chuar rocks.Lanulatisphaera laufeldii, a locally abundant and globally widespread species characterized by submicrometer filamentous processes that form a reticulate network, may be a useful marker for the time interval just before the appearance of vase-shaped microfossils (VSMs) ca. 740 Ma.Organic-walled microfossil assemblages decline in diversity upsection, coincident with the appearance of VSMs and intermittent euxinia within the basin. Whether this pattern is due to preservational bias related to greater water depth or the higher TOC of upper Chuar rocks or instead reflects biotic turnover related to the spread of euxinic water masses in the basin is unknown.

Tiny vampires in ancient seas: evidence for predation via perforation in fossils from the 780–740 million-year-old Chuar Group, Grand Canyon, USA

“…Although originally spheroidal vesicles, the fossils now occur as 1-2 mm-thick discs, a few tens to hundreds of micrometres in diameter. Twenty-three species of organic-walled fossils have been described from the Chuar Group, including the 1-3 mm compression Chuaria circularis [19,20]. In addition, 17 species of vase-shaped microfossils (VSMs), interpreted as the remains of arcellinid and possibly euglyphid testate amoebae occur in the upper part of the unit [10,13].…”

“…Fossils were released from their host rocks via HCl and HF acid maceration and viewed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) systems housed at UCSB's Earth Science Department and Materials Research Laboratory (see [20] for more details).…”

“…By contrast, larger holes are found in fragments of Cerebrosphaera globosa (¼C. buickii; see [20]) (figure 2b; 1.2-2.1 mm in diameter; mean ¼ 1.3 mm; n ¼ 8) and C. circularis (figure 2a; 0.6-3.4 mm in diameter; mean ¼ 2.0 mm, n ¼ 42). Holes of different sizes may occur in fossils from the same rock sample, but always in different species (figure 3).…”

“…Similarly-sized holes are found in Leiosphaeridia sp. and Valeria lophostriata, as well as in two new species and an indeterminate form (described in [20]; figure 3). By contrast, larger holes are found in fragments of Cerebrosphaera globosa (¼C.…”

“…Five of these species and the unnamed form are interpreted to be the remains of vegetative cells or resting cysts (figures 1a,c,d,f,g and 2b; [20]); a sixth species is interpreted to be a possible scale-bearing protist (figure 1e; [20]) and a seventh species, represented by 30-100 mm sized smooth-walled fragments, is assumed to be C. circularis (figure 2a). The holes are circular in shape and occur in specimens that may otherwise show no signs of pitting or degradation (e.g.…”