A well-preserved and moderately diverse acritarch assemblage was recovered from the upper Ordovician Maquoketa Shale of northeastern Missouri. Based on graptolite and conodont evidence, the Maquoketa in northeastern Missouri is considered Richmondian (=Ashgill) in age.
The acritarch assemblage comprises 28 species, distributed among 17 genera including two new genera, Caelatosphaera and Stictosoma. There are 11 new species, Baltisphaeridium adiastaltum, Caelatosphaera verminosa (type species), Dorsennidium undosum, Elektoriskos aktinotos, Lophosphaeridium acinatum, L. varum, Micrhystridium hirticulum, M. prolixum, Peteinosphaeridium accinctulum, P. septuosum, and Stictosoma gemmata (type species); together with two species similar to previously named species and three species left in open nomenclature. Cosmopolitan and stratigraphically important species identified include Baltisphaeridium perclarum Loeblich and Tappan, 1978; Cheleutochroa sp. cf. C. diaphorosa Turner, 1984; Dorsennidium hamii (Loeblich, 1970) Sarjeant and Stancliffe, 1994; Excultibrachium concinnum Loeblich and Tappan, 1978; Orthosphaeridium insculptum Loeblich, 1970; O. rectangulare (Eisenack, 1963) Eisenack, 1968, and Villosacapsula setosapellicula (Loeblich, 1970) Loeblich and Tappan, 1976. Polygonium gracile Vavrdová, 1966 emend. Sarjeant and Stancliffe, 1994 and Micrhystridium hirticulum new species dominate the assemblage, followed by Dorsennidium undosum new species, Baltisphaeridium oligopsakium Loeblich and Tappan, 1978, Veryhachium sp. cf. V. oklahomense Loeblich, 1970, Peteinosphaeridium accinctulum new species, and Lophosphaeridium acinatum new species. Additionally, the palynoflora contains chitinozoans, scolecodonts, cryptospores, and the enigmatic palynomorph Gloeocapsomorpha prisca Zalessky, 1917 emend. Foster, Reed, and Wicander, 1989.
Paleontologic-palynologic and sedimentologic evidence indicates that the Maquoketa Shale at the two studied localities accumulated under low energy, somewhat offshore, and unrestricted marine conditions. The present Maquoketa Shale acritarch palynoflora shows greatest similarity to those of the Sylvan Shale (Ashgill; Richmondian) of Oklahoma and the Maquoketa Shale (Caradoc and Ashgill) of Kansas. Whereas there are some cosmopolitan acritarch species present in the Maquoketa Shale, its palynoflora shows no pronounced similarity with age-equivalent acritarch suites from outside of North America.
