Gastropod outline shapes
A wide range of adjectives is used to describe outline shapes of gastropods, sometimes using geometric shapes, and sometimes using the names of modern gastropods that have distinctive shell shapes. For most gastropods, outlines are described based on a view looking at the aperture. For low-spired and planispiral (spiraled in the same plane) gastropods, a view perpendicular to the plane of spiral may be used to describe outline shape and whorls.
A wide range of adjectives (e.g., biconic, turbinate, etc.) are used to describe the geometric shapes of gastropod shells. In some cases, where a fossil is in the same order or family of a living gastropod with a distinctive outline shape, the modern order or family name may also be used as an adjective to describe the outline shape. For example, a shape or outline described as “trochiform” would indicate a gastropod with the general appearance of the genus Trochus, a conical shell with a flat base.
Some gastropod genera have a single, distinctive outline shape, but others have different shapes for different species of the genus, or for different life stages (juvenile vs. adult) of the same gastropod species (called ontogeny or ontogenetic stages).
It is important to recognize that outline shape terms are based on loose, whole specimens. How a fossil is preserved can influence its relative appearance. Shells preserved in rock matrix may only be partly exposed on the outside of the rock, which may mask their true outline. In some shales, shells may be compacted and flattened, which can also change their original profile and outline shape.
Content and graphics by Stephen Greb, Kentucky Geological Survey