8.1 Introduction

While recent brachiopods are a rather rare and insignificant group, their long fossil history shows that they were at times the most prominent animals in the seas. Consequently, brachiopods receive only passing interest from zoologists, but a great deal of attention from paleontologists. The phylum is quite important for biostratigraphy, paleoecology, and evolutionary studies because it shows a great variety of changes in form and function through time.

Brachiopods appear near the beginning of the Cambrian, but did not become abundant until the Early Ordovician. The remainder of the Paleozoic could be termed the Age of Brachiopods-several orders dominated the shallow shelf environments throughout the era, giving way only reluctantly to the rapidly diversifying bivalved molluscs and gastropods of the Mesozoic. Only a few groups survive today.

8.1.1 Functional Morphology

General Form

Brachiopods are solitary, entirely marine animals, each with a shell consisting of two opposing parts (valves) that enclose most of the soft body. The animal and its shell are bilaterally symmetrical about a plane drawn perpendicular to the line of contact of the closed valves (the commissure). In most brachiopods, the shell is made of calcite, but a few groups have shells made of calcium phosphate with varying amounts of organic material.

Feeding

Inside the shell is the feeding structure characteristic of the brachiopods-the lophophore. This consists of a pair of ciliated, twisted projections that create water currents and then filter out microscopic food particles. Often the lophophore has a calcareous supporting brachidium. Fossil and Recent brachiopods have a variety of accessory supports for the feeding apparatus.

In order to increase the amount of water filtered and still protect the delicate lophophore from overly large paticles, some brachiopod lineages (notably the Rhynchonellida) developed a zig-zag commissure. The advantages of this system are apparent in the figure below. The zig-zags also bring the sensitive mantle edges closer together, giving the animal more control over the quality of incoming material.

Articulation and valve movement

The most common class of brachiopods, the Articulata, is characterized by the presence of two opposing calcareous valves hinged along the posterior edge. They usually have a series of sockets and teeth which allow valves to open anteriorly for feeding; they can also keep the valves firmly closed when necessary. In some brachiopods the articulating structures have been reduced or lost during evolution.

Two major muscle sets open and close the valves. Diductor muscles attach at one end to the floor of the ventral valve, and at the other end to a projection (cardinal process) in the dorsal valve. When these muscles contract, the hinge acts as a fulcrum, opening the valves anteriorly. Adductor muscles, which are attached between the floors of both valves, contract to close the valves and hold them shut.

Relation to substrate

Most brachiopods have a fleshy stalk, termed the pedicle, that protrudes posteriorly through one valve or between the valves and attaches permanently to the substrate. When the pedicle exits through a valve (by definition the ventral valve), it leaves an opening that varies greatly in form among brachiopod groups. In many the pedicle was lost during either ontogeny or the evolution of the lineage, leaving as evidence a hole partially or completely closed off by accessory plates or growth of the ventral valve.

Some brachiopods had no pedicle and either lived freely on the substrate or attached their ventral valve directly to some firm object. The free-living types developed a wide variety of devices to protect themselves from burial in the sediment or disruption by currents (except for opening and closing the valves and some limited movement on the pedicle, brachiopods are strictly sessile)A few added heavy stabilizing calcite to the posteior and ventral portions of the shell; others had spines that could attach to the substrate or function as a "snowshoe" in muddy areas. Other brachiopods without pedicles were able to grow at a rate that kept the commissure above the sediment surface.

Sensory structures

Recent brachiopods have series of small bristles (setae) extending from grooves at the valve and mantle edges that serve as tactile sensory devices. Many fossil brachiopods have similar grooves, indicating they probably had the same type of system.

Strophomenid brachiopods sometimes have hollow spines which may have carried continuous strips of living mantle tissue from the shell interior to their tips. If so, then the spines would have extended the sensory field of the animal.

8.2 Classification

The brachiopods are divided into two classes, based primarily on shell morphology. The inarticulates have unhinged valves generally of a chitinophosphatic composition, while the articulates are brachiopods with hinged calcareous valves.

8.2.1 *Class Inarticulata

Class Inarticulata contains five orders, only three of which are commonly encountered:

*Order Lingulida

• These are the most conservative of the brachiopods; one genus, Lingula, appeared in the Ordovician and is still alive today in essentially the same form. The shells of lingulids are made primarily of calcium phosphate, which appears dingy brown, and are always biconvex and oval to squarish in outline. The order first appeaxed in the Cambrian, and it has been found consistently throughout the record. They live burrowed in the soft sediment, anchored by their long pedicle.

• This group has also survived since the Cambrian with a fairly constant shell morphology. Acrotretid valves are generally subcircular to circular, unequally biconvex, and often have a pedicle opening. An important subgroup (the craniaceans) have no functioning pedicle; instead, they cement the ventral valve directly to the substrate.

• These brachiopods resemble the acrotretids and some articulates, and so are rather difficult to identify. Their valves are circular to oval, the ventral one with a pseudo-interarea and a pedicle opening of some sort. Obolellids are known only from Cambrian rocks.

The articulates are a diverse and comples class-they have proven to be the most useful brachiopods for a variety of studies. Seven orders are recognized.

*Order Orthida

• Apparently the most ancestral of the articualtes, these brachiopods are also the most difficult to work with. They are a generalized group lacking complex morphologic features, and they tend to resemble some of the other articulate orders. Orthids are almost always biconvex, with a fairly wide hinge line flanked by distinct interareas on each valve. The first known articulates to appear in the Cambrian are orthids. They change little in basic structure through the Paleozoic, and became extinct at the end of the Permian.

• This group usually has strongly biconvex valves that are smooth or finely costate. Their characteristic feature is a robust spondylium, which is a curved calcareous platform for muscle attachment in the beak region. The spondylium is seated in the ventral valve. Pentamerids probably arose from orthids in the later Cambrian; they went extinct in the Devonian. The pentamerids were most important in Silurian shelf seas.

• No other brachiopod order shows as much morphologic diversity as the strophemenids. Generally, they have plano-convex or concavo-convex shells with a costate surface and no pedicle opening. This simple plan though, has led to several distinct and sometimes exotic forms. They include:
  • Productids: These brachiopods have hollow spines and strongly concavo-convex shells. They include several large, very spiny brachiopods and specialized reef-forming groups. Productids are mostly upper Paleozoic.
  • "True Strophomenids": In this group the shell is usually wider than long and the body cavity is small. They are important in the lower Paleozoic, but range into the Jurassic.
  • Chonetids: These are small shells that resemble the "true strophomenids", but they have hollow spines along the postreior edge. Chonetids are especially prominent in upper Paleozoic rocks.
  • Oldhaminids: "A leaf in a gravy boat" is the best way to describe the most common oldhaminid genus, Leptodus. These are confined to the Upper Paleozoic and the Lower Mesozoic.

• The shells of this group are strongly biconvex, many to the point of being globose, heavily plicate, and have a very short hinge line. Rhynchonellids are simple internally and have no complicated supports for the brachidium. Most workers believe that they evolved from the pentamerids. The rhynchonellids appeared in the Middle Ordovician and are still extant. Most Mesozoic brachiopods are rhynchonellids.

• The most common of this group is the easiest to identify: these shells have a hinge line so wide that they look winged. The group is defined by the possession of a spirally-coiled brachidium that "points" towaxd the cardinal extremities. Most spiriferids are strongly plicate or costate, and they usually bear a fold and sulcus. The group is important in the middle and upper Paleozoic and parts of the lower Mesozoic.

• These are the most abundant brachiopods today; they are the typical "lamp shells" known to beachcombers. Terebratulid shells are strongly biconvex and bear a short hinge line. The beak is prominent and usually has a rounded pedicle opening. The surface of a terebratulid is smooth or finely costate. This group first appears in Lower Devonian rocks, but it only becomes abundant in Mesozoic and Cenozoic strata.