External morphology of brachiopods

Brachiopod shells grow by accretion. Mineralized material is secreted from the mantle lining the shell. Linguliforms are phosphatic, and craniiforms and rhynchonelliforms are calcareous. In the latter group the shell is multilayered and may have thin, tubular structures perpendicular to the layering. These are called punctae and are a useful diagnostic feature of brachiopods in thin section. Key aspects of the brachiopod external morphology are shell shape, shell sculpture, and the form of hinge area (Fig. 6.2).

Brachiopod shape is determined by the curvature of the valves. In order to accommodate the soft parts, at least one valve is always convex (has a rounded shape in cross-section). Both valves may be convex (biconvex) and the degree of roundness may be the same or unequal. Alternatively, one valve may be flat or concave (curves inwards), producing variations in the shell profile. Brachiopods commonly have an exterior surface texture. This may be in the form of ribs radiating from the beak, growth lines, or wrinkles.

The line of closure of the valves (commissure) may be straight or corrugated. It may also have a deep medial depression (sulcus) and a corresponding elevation (fold). The hinge area is very important in brachiopod classification. The hinge line may be straight (strophic) or curved (astrophic). The pointed extremity marking the start of valve growth is known as the beak and each valve has one. The area between the beak and the hinge line is known as the interarea. This may be flat or curved. In some brachiopods the beak is more prominent and curves over. In this case the posterior extremity is called the umbo.

The hole for the pedicle is called the pedicle foramen. It is sometimes closed by a single plate (deltidium) or plates (deltidial plates). Rather than a rounded hole, in some brachiopods the opening for the pedicle is more of a notch (delthyrium). This gap may be extended to the dorsal valve (notothyrium) to enlarge the opening.

In epifaunal brachiopods, as well as protection, the main functions of the shell are to guide the food-bearing water into the mantle cavity, to limit the contamination of these nutrient-rich currents by expelled waste-bearing water, and to prevent sediment from entering the shell through the open valves. In most brachiopods the incoming feeding currents and discharged waste water are drawn in and expelled through separate parts of the shell. Specific flow patterns occur in brachiopods depending on the shell shape and orientation of the lophophore. The development of a central fold and sulcus in the brachiopod shell may have helped separate the incoming currents from the discharged waters, reducing the risk of refiltration of the exhalent waters.

Another modification seen in brachiopod shells is the development of a crenulated, or zig-zag, commissure. With the valves open this has the effect of increasing the length of the gape, without over-opening the shell and allowing larger particles of unwanted suspended sediment to enter the mantle cavity.

(a) Pedicle foramen:

opening for pedicle

Delthyrium closed by deltidal plates

Commissure: line (b) Interarea: Delthyrium where valves meet ventral valve

(a) Pedicle foramen:

opening for pedicle

Delthyrium closed by deltidal plates

Growth line

Fig. 6.2 Brachiopod external morphology: (a) astrophic, and (b) strophic.

Beak: rounded area which marks point of valve growth

Notothyrium

Notothyrium

Growth line

Fig. 6.2 Brachiopod external morphology: (a) astrophic, and (b) strophic.

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