Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization

Isaac D Rudnitzki,Anatoliy B Kudryavtsev,Amanda K Garcia,Daniel J.G Lahr,J William Schopf,Luana Morais,Thomas Rich Fairchild,Guilherme R Romero

doi:10.1017/jpa.2017.16

Abstract

AbstractVase-shaped microfossils (VSMs) occur in dolomitic extraclasts of indeterminate provenance within the basal diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group) of west-central Brazil, having an age constrained between 889±44 Ma (K-Ar; basement rocks) and 587±7 Ma (40Ar/39Ar age of early metamorphic cryptomelane in overlying manganese ore). Early isopachous carbonate cement entombed these VSMs, preserving rare direct evidence of original wall composition that is carbonaceous (now kerogenous) in practically all specimens. Some tests are siliceous or composed of a quartz-kerogen mixture; secondary replacement explains some features of these tests, but original biomineralization seems more likely for others. This interpretation, coupled with test morphology, suggests affinity to arcellinid testate amoebae. Five VSM taxa are recognized in the deposit:Cycliocyrillium simplexPorter, Meisterfeld, and Knoll, 2003, andC.torquataPorter, Meisterfeld, and Knoll, 2003, originally described in the Chuar Group (USA), and three new monospecific genera—Palaeoamphora urucumensen. gen. n. sp.,Limeta lageniformisn. gen. n. sp., andTaruma ratan. gen. n. sp. Most of the taxonomically important characteristics of these VSMs occur also in extant testate amoebae, but the combinations of some characters, such as organic-walled tests having exceptionally long necks that exhibit terminal apertures (L.lageniformisn. gen. n. sp.), are evidently novel additions to the known diversity of Neoproterozoic VSMs. Evidence of glacially influenced deposition in the conformably overlying Santa Cruz Formation may indicate that the Urucum Formation slightly preceded or was penecontemporaneous with a major Neoproterozoic glaciation, although the VSM-hosting extraclasts must be older, possibly rivaling the age of the testate amoebae of the Chichkan Formation (766±7 Ma) that are currently regarded as the oldest record of protists in the geological record.

Highlights

Molecular phylogenies and the fossil record indicate that eukaryotic diversification appreciably preceded the early Cambrian appearance of metazoans with hard parts (Knoll, 2014; Butterfield, 2015)
We show that the Urucum Vase-shaped microfossils (VSMs) include two species previously described from the Chuar Group (Porter et al, 2003) as well as others that are sufficiently distinct to warrant erection of three new monospecific genera, one of which is identifiable in figures of previously unnamed specimens from the Chuar Group
The VSM-bearing dolostone extraclasts were collected at three localities, including that of Zaine (1991), within a single extensive outcrop of diamictite near the base of the Urucum Formation at the northern end of Morraria do Rabicho, in the channel leading from the Paraguai River to the Lagoa Negra

Summary

Introduction

Molecular phylogenies and the fossil record indicate that eukaryotic diversification appreciably preceded the early Cambrian appearance of metazoans with hard parts (Knoll, 2014; Butterfield, 2015). If it could be proved that glacial activity during deposition of the Santa Cruz Formation overlying the Urucum Formation was penecontemporaneous with sedimentation of the Puga Formation at Morro do Puga and in the Serra da Bodoquena, 200 km to the southeast from the site here studied (Walde and Oliveira, 1980; Boggiani, 2010), the age of ~706 ± 9 Ma (U-Pb, SHRIMP) obtained by Babinski et al (2013) for the youngest detrital zircon grain in the Puga Formation would be relevant to that of the Jacadigo Group Corroboration of this suggestion would require additional data supporting both the correlation between the Puga and Santa Cruz formations and the interpretation that the glaciation affecting the two formations was related to a global rather than a local event. They are possibly older than the 740–750 Ma-old VSMs of the Chuar and Yukon groups of North America (Porter et al, 2003; Strauss et al, 2014) and the ~770 Ma-old VSMs of the Chichkan Formation of Kazakhstan (Sergeev and Schopf, 2010)

Materials and methods

Findings

Conclusions