Distinctive, well-preserved, and abundant microfossils of uncertain taxonomic and biologic affinity have been discovered in acid-resistant residues and thin sections from shales of the Walcott Member of the Kwagunt Formation (late Precambrian) of the eastern Grand Canyon, Arizona. These forms are referred to the new acritarch genus Melamocyrillium; however, these present forms are morphologically different from previously reported acritarch genera and do not comply with any existing, suprageneric, acritarch classification scheme. Melamocyrillids are organic-walled, opaque, unilocular and apparently unicellular, bulbous- to lachrymiform-shaped, vesicular, solitary microfossils. They range from 32 to 170 µm in length and have a length-to-width ratio ranging from 1.0:1 o 2.7:1 (N=600). Symmetry is bilateral or, more rarely, radial (longitudinal axis of symmetry). Furrows, basal scars, and evidence of surficial processes are lacking; chain and colonial forms are absent. The wall is dense, thick (5 to 7 µm), apparently unilayered, and has a psilate to faintly microgranular texture. Excystment is by a pylome. Three species are recognized: M. fimbriatum is characterized by a fringe which skirts the oral end; M. hexodiadema has an elevated

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hexagonal apertural crown; and M. horodyski has an inflated, turbanlike triangular margin at the oral end. All forms possess an equilateral triangular, or, more rarely, a circular operculum.

Associated microfossils include the acritarchs Chuaria circularis and Trachysphaeridium levis and a cyanophytic assemblage of solitary and colonial forms. The melamocyrillid acritarch community is significant for three reasons: (1) these forms apparently represent a morphologically and biologically advanced grade of organization heretofore unrecognized in Precambrian microfossils; (2) they could prove useful for regional and perhaps global biostratigraphic correlation of upper Precambrian strata; and (3) they provide another example of the usefulness in studying shale environments, in addition to silicified stromatolitic chert environments, to gain a better understanding of the biologic diversity of late Precambrian seas and of the dating of significant evolutionary events.

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