Vase-shaped Microfossils from the Neoproterozoic Chuar Group, Grand Canyon: A Classification Guided by Modern Testate Amoebae

Abstract

Vase-shaped microfossil (VSM) assemblages front early diagenetic carbonate nodules in >742 +/- 6 Ma black shales of the Chuar Group, Grand Canyon, provide evidence for affinities with testate amoebae. Not only are VSMs exceptionally preserved in Chuar rocks, they exhibit a much higher degree of morphological diversity than was previously known. Using the taxonomy of modern testate amoebae as a guide, nine new species and eight new genera of VSMs are described, augmenting the eight species and two genera already recognized. Taxa described here are <i>Melanocyrillium hexodiadema</i> Bloeser, 1985, <i>Trigonocyrillium horodyskii</i> (Bloeser, 1985) n. comb., <i>T. fimbriatian</i> (Bloeser, 1985) n. comb., <i>Cycliocyrillium simplex</i> n. sp., <i>C. torquata</i> n. sp., <i>Bonniea dacruchares</i> n. sp., <i>B. pytinaia</i> n. sp., <i>Trachycyrillium pudens</i> n. sp., <i>Palaeoarcella athanata</i> n. sp., <i>Hemisphaeriella ornata</i> n. sp., <i>Bombycion micron</i> n. sp., and <i>Melicerion poikilon</i> n. sp. All of the test characters observed in VSM taxa (e.g., collars; indentations; hexagonal symmetry; lobed, triangular or invaginated apertures, curved necks) occur in modern testate amoeban taxa, though not always in the same combinations. Some VSM species have characters found today in diverse extant taxa, making it difficult to assess their relationships. A few species, however, have character combinations that closely approximate those found in specific genera of both lobose and filose testate amoebae, suggesting that at least stem group, and possibly crown group, representatives of these taxa were present similar to ~742 Ma. These fossils indicate that ecosystems were diverse and complex, that eukaryotic biomineralization had already evolved, and that the last common ancestor of animals+fungi had already appeared by similar to ~750 Ma.