Frenopyxis stierlitzi Bobrov et Mazei, 2020
Diagnosis: Shell bilaterally symmetrical, broad elliptic, sometimes almost circular, dorsoventrally flattened, yellowish to colorless; aperture is placed slightly eccentrically on a ventral part of a shell at the bottom of a wide funnel (invagination); aperture usually drop-shaped with narrow side oriented toward closest side; sometimes aperture circular. There is an internal thick organic lip surrounding an aperture and continuing in a bridle, which connects an aperture with the internal side of a shell wall and broaden in the place of connection of a bridle and a shell wall. Shell surface smooth on a ventral side and covered densely by mineral particles on a dorsal side. Organic cement with simple circular pores regularly and densely distributed on the surface. This feature is likely to cause the shell wall to become fragile and thus need to be reinforced by an apertural lip and a bridle connected with a shell wall.
Dimensions: Shell length 37.80–45.90 μm; shell breadth 32.40–37.80 μm; shell depth 13.50–24.30 μm; distance between the aperture and the top edge of the shell 8.10–15.30 μm; aperture length 8.10–15.12 μm; aperture breadth 6.21–13.50 μm; bridle length 5.40–8.20 μm; bridle breadth 3.24–6.75 μm.
Ecology: Type locality: Urban park “Bitsevsky forest”, Moscow. Mixed spruce-birchlinden forest. Hollow on spruce tree at a height of 0.5 m. Semi-decomposed litter with a small number of fine mineral particles. Also found in Berlin.
Remarks: F. stierlitzi is characterized by high polymorphism of both shell size and shape. The general shell shape is mostly elliptic but, in some specimens, it is almost circular. The aperture is normally drop-shaped but sometimes circular. A circular aperture can be found in both circular and elliptic shells. The variability of the shell length and shell breadth in concrete populations is low with variation coefficient ranging between
4.64 % and 8.85 %. However, when comparing different populations, the total size range considerably increases with the population from the 1.5 m hollow in spruce tree trunks characterized by a significantly high shell size than others. The variability of the ture size is much high with larger shells having larger apertures (from Bobrov and Mazei, 2020).