planispirality

Planispirality is a form of spiral growth in which an organism’s shell or body coils in a single flat plane. In a planispirally coiled shell, the whorls lie in the same geometric plane and the axis of coiling is perpendicular to that plane, producing a disk-like or coin-shaped spiral. The opening of the shell (the aperture) is often oriented toward the edge of the coil, though exact alignment can vary among groups.

Planispiral coiling is common in certain mollusks as well as some other shelled organisms. It is characteristic of many planispiral gastropods and of numerous ammonoids and nautiloids in the fossil record. Among living groups, freshwater snails in the family Planorbidae exhibit planispiral shells with a flattened, circular outline. Planispirality also occurs in foraminifera, where tests form flattened spirals around a central axis.

In scientific descriptions, planispirality serves as an important diagnostic feature for identifying fossil shells and understanding evolutionary relationships, growth strategies, and ecological constraints. Planispiral forms can range from tight, compact coils to broader, more open discs, reflecting variations in growth and buoyancy control.

Etymology-wise, the term derives from Latin planus meaning flat and spirare meaning to coil, describing the flat coiled geometry that defines this mode of growth.