Brachiopods have a variety of shapes, and the classification of brachiopods is based partly on shape. Two basic aspects of shape are the appearance of the brachiopod from (1) the side or lateral view, which is called its profile, and (2) from the top or bottom view, called its outline.
Brachiopod profiles are commonly described with a terminology based on the curvature of the valves. A compound-word term describes first the curvature of the brachial (dorsal) valve, followed by the curvature of the pedicle (ventral) valve.
Convex valves are outward-curving or bulging, like an arch, whereas concave valves curve inward, like a depression. A concavo-convex (sometimes spelled concavoconvex or concavi-convex) profile has a brachial valve that is concave (inward curving) relative to the exterior of the shell, and a pedicle valve that is convex (outward curving) relative to the exterior of the shell. The following images show terms used to describe brachiopod profiles and what the corresponding dorsal (brachial) and ventral (pedicle) valves look like both separated and hinged together.
Biconvex: Both the pedicle and brachial valves have convex profiles. Both valves are clearly visible when viewed from the side. In some cases, modifiers such as “equally biconvex” or “subequally biconvex” or “unequally biconvex” can be used to indicate if both valves have equal curvature, near-equal curvature, or unequal curvature. The terms dorsi-biconvex and ventri-biconvex are also sometimes used to describe the brachial (dorsal) or pedicle (ventral) valve as having greater convexity and generally being the larger of the two valves.
Concavo-convex: Brachial (dorsal) valve is concave (inwardly curving, depressed). Pedicle (ventral) valve is convex (outwardly curving, arching). Pedicle valve is generally larger and encompasses at least part of the brachial valve. Hence, the brachial valve may be hidden within the pedicle valve when viewed in profile.
Convexo-concave: Brachial (dorsal) valve is convex. Pedicle (ventral) valve is concave. Brachial valve generally is larger and encompasses at least part of the pedicle valve. Hence, the pedicle valve is commonly hidden within the brachial valve when viewed in profile.
Convexo-plane: Brachial (dorsal) valve is convex. Pedicle (ventral) valve is flat or near flat. Brachial valve is usually larger and may encompass, at least in part, the pedicle valve. Hence, the pedicle valve may be hidden within or at the base of the brachial valve when viewed in profile.
Dorsi-biconvex: Both valves are convex, but the brachial (dorsal) valve is more convex and generally larger than the pedicle (ventral) valve. The modifiers “subequally” or “unequally” biconvex apply to both dorsi- and ventri-convex profiles. Both valves are visible when viewed from the side.
Geniculate: Smaller valve bent anteriorly where it butts up against the interior curvature of the larger valve.
Plano-convex: Brachial (dorsal) valve is flat or near flat. Pedicle (ventral) valve is convex. Pedicle valve is usually larger and may encompass, at least in part, the brachial valve. Hence, the brachial valve may be partly hidden within or at the base of the pedicle valve when viewed in profile.
Resupinate: Both valves are broadly wave-form (falling and rising in cross section), and an area of the valve toward the hinge has a shallow dip or concave curvature, and a broader area toward the anterior of the valve is convex. In this profile, pedicle (ventral) valves are thin and at least partly recessed within the brachial (dorsal) valve. Hence, the pedicle valve is partly hidden within the brachial valve when viewed in profile. Both valves are usually relatively thin.
Ventri-biconvex: Both valves are convex, but the pedicle valve is more convex and generally larger than the brachial valve. The modifiers “subequally” or “unequally” biconvex apply to both dorsi- and ventri-convex profiles. Both valves are visible when viewed from the side.
Brachiopod outline is another feature used to describe the shape of a brachiopod. A wide variety of geometric terms (circular, oval, etc.) are used to describe the outline of the larger valve or individual valves of a brachiopod when viewed from the outside of the valve.
Use of the geometric terms in the diagram are somewhat subjective because many shapes are transitional between each other, and because actual brachiopod shapes may occur between the example shapes shown here.
Some brachiopod genera have a single, distinctive outline shape, but others have different shapes for different species of the genus, or for different life stages (juvenile versus adult) of the same brachiopod species (called ontogeny or ontogenetic stages).
Outline shapes are often described relative to width and length of the valves. Width is usually measured as the side-to-side measurement, subparallel to the hingeline. Length is the direction across the valve from the posterior (front) to the anterior (rear). Shapes described as “transversely” are wider than long. Shapes described as “elongate” are longer than wide.
A wide range of other shape adjectives (e.g., teardrop, spatulate) may also be used to describe brachiopod outlines. In some cases, where orders or families of brachiopods (in classification schemes) have a distinctive shape, the order or family name may also be used to describe the outline shape. For example, a shape or outline described as “pentamerid” would indicate a brachiopod with the general appearance of the genus Pentamerus; “productid” would have the appearance of the genus Productus; “spiriferid” would be a shell with the general appearance of the genus Spirifer.
It is important to recognize that terms for outline shape are based on loose, whole specimens. How a fossil is preserved can influence its relative appearance. Shells preserved in a 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 shape.