The study of Silurian sedimentary formations in the central part of the Altai Mountains (Gorny Altai) revealed two groups of spherical objects during dissolution of limestones: large microspheres 90-120 microns and small nanofossils (nanospheres) 5-18 microns in diameter, respectively. Their double-layer walls are composed of standard-sized siderite microcrystals replaced by goethite. A low content of calcium (<0.5%) does not allow to interpret the Altai microspheres as calcispheres. The Altai Silurian microspheres and nanofossils (nanospheres) are presumably attributed to the biomineralized remains of shell houses (loricae) of various generations of euglenic algae.
