Although they are relatively rare today and unfamiliar to most of us, during the Paleozoic the brachiopods, or “lamp shells,” were by far the most common of all invertebrate fossils. In many places in the United States, especially in the East and the Midwest, the limestones are chock full of brachiopods. They literally pave the ground (fig. 13.1), and they are excellent index fossils. If you can recognize the basic brachiopod groups, you can easily tell what time it is in the Paleozoic from brachiopods alone. There are only 120 living genera, but there were more than 4,500 fossil genera (900 in the Devonian alone). Nearly all of the modern brachiopods live in hiding places, such as under rocks or burrowed into the mud, to avoid predators. Consequently, brachiopods have long been intensely studied by paleontologists. However, most marine biologists have never seen one, and brachiopods get only a page in a typical invertebrate zoology textbook.
Figure 13.1 ▲
A dense pavement of brachiopod shells of Cincinnetina meeki. Such concentrations of brachiopods are typical in many Paleozoic limestone localities. (Courtesy of Wikimedia Commons)
Brachiopods are not familiar to most of us because of their scarcity in modern oceans. The only common name they have is “lamp shells,” which was given to one group of brachiopods that resembles the biblical oil lamp. Superficially brachiopods look like clams because their bodies are encased in two hard shells, also called valves. However, inside the shell their anatomy is completely different (fig. 13.2A), and they are not closely related to clams or any other kind of mollusc. Even their bivalved shell is very different from the shell of a clam (fig. 13.2B). The best rule of thumb for telling them apart is that brachiopods are symmetrical through the valves, so the right half of each shell is the mirror image of the left half. In contrast, a clam is a mollusc (an entirely different phylum), and it is symmetrical between the valves, so one shell is the mirror image of the other. However, there are a number of oyster-like brachiopods, as well as true oysters and a number of other clams that give up on symmetry altogether, so these rules of thumb don’t always apply.
Figure 13.2 ▲
(A) Anatomical features of a typical brachiopod, showing the fleshy pedicle used for attachment, the concentration of organs in the back of the shell near the hinge, and most of the internal shell cavity, which is occupied by the feathery lophophore. (B) The basic symmetry of brachiopods compared to the shells of clams (bivalves). (Illustrations redrawn by Mary Persis Williams)
Once you have determined the fossil is a brachiopod shell, the rest of the anatomy is straightforward. One valve is usually larger than the other, and many brachiopods have a small opening on the hinge for the long fleshy stalk known as the pedicle, which is used to attach the shell to the substrate (see fig. 13.2A). This has long been known as the pedicle valve, although now it is called the ventral valve because it usually sits in the ventral (bottom) position. The pedicle gives the phylum its name, Brachiopoda, which means “arm foot” in Greek. Hinged to the ventral valve is the other valve, which originally was called the brachial valve but today is called the dorsal valve. It is slightly smaller than the ventral valve, and it tends to sit on the top, or dorsal position, in most brachiopods.
Inside their paired shells (bivalved shells), brachiopods have a very simple anatomy. Their main feature is a large, feathery, filter-feeding device known as a lophophore, which is used to trap microscopic food particles as they pass through the gap in the open shell (fig. 13.3). The food then passes down through the mouth and the digestive tract as it does in most other coelomates. They have all the other usual organs, such as gonads, excretory system, digestive glands, and a simple circulatory system. Brachiopods have a nervous system, but no eyes. Instead, bristles around the margin of the shell, called setae, are sensitive to changes outside the shell and warn the animal to close its shell when danger approaches.
Figure 13.3 ▲
The interior of the brachiopod shell has a lophophore, which is supported by a rigid structure. This example is a spiriferide brachiopod, whose lophophore was shaped like a spiral. (Courtesy of Wikimedia Commons)
Most important are the muscles that close and open the shell. One pair, known as the adductor muscles, pull the shell closed; a second pair pulls on the lever arm of the internal hinge to pull the shell open. Because they have paired muscles controlling the shell, brachiopods tend to stay closed when they die, and they tend to be buried and fossilized as complete shells. In contrast, clams and other bivalve molluscs have adductor muscles to close the shell, but a flexible ligament in the hinge spring-loads the shell so it opens automatically when the clam is not trying to close the shell. Thus, when clams or scallops die and the adductor muscles relax, they tend to open automatically. The shells often break apart, and it is rare to see both shells preserved together.
Many details of the shell, especially in the hinge area, the shape of the lophophore, and the detailed microscopic structure of the shell, are used in identifying the brachiopods and creating their classification. For a book of this level, however, we will look at only a few anatomical features that are helpful for identifying the external shape of the shell (fig. 13.4). The outside of the shell can have numerous growth lines, which form arcs radiating out from the hinge, and fine ribs known as costae, which radiate out from the hinge-like spokes. Some shells have large corrugations radiating from the hinge, known as plications. Certain groups of brachiopods have a large, trough-like depression on the midline called a sulcus, and a corresponding large ridge on the midline of the other shell called a fold. The edge of the shells where they join is called the commissure. In brachiopods with strongly plications, the commissure has a zigzag shape.
Figure 13.4 ▲
Terminology of the external features of the exterior of the brachiopod shell. (Redrawn by Mary Persis Williams)
Most brachiopod shells have two shells that bulge outward, so they are biconvex. However, some have a convex ventral valve, but a flat dorsal valve, and they are plano-convex. Some even have a dorsal valve that sinks down into the body cavity, and they are concavo-convex. A few have the same shape but the dorsal valve is convex and the ventral is concave, and they are convexi-concave.
BRACHIOPOD IDENTIFICATION
With over 4,500 fossil genera of brachiopods, it is not possible to describe how to identify every possible kind. Instead, we will look at the major orders of brachiopods, which are fairly easy to tell apart, even for the amateur. Knowing which major group of brachiopod you are looking at will enable you to identify the Paleozoic period from which your fossil comes.
INARTICULATE BRACHIOPODS
The most primitive of all the brachiopods are the inarticulates. The name doesn’t suggest they can’t speak fluently; instead, it refers to the fact that they have no mechanical hinge with teeth and sockets holding the shells together. The hinge is held together only by bands of muscles, with no mechanical articulation. One of the most common brachiopods is alive today and is found in mudflats all over the world. It is called Lingula (“little tongue” in Latin) because the shell has a tongue-like shape (fig. 13.5). Lingula live buried deep in mudflats, using a very long fleshy stalk-like pedicle to burrow down. This type of brachiopod has been around unchanged since the Cambrian when inarticulates became the first brachiopods on the planet, so they are a living fossil that has persisted for 550 million years. A number of other tongue-shaped, coin-shaped, oval, and disk-shaped inarticulates were found in the Cambrian, but only Lingula survives today. Inarticulates are also interesting in that they make their shells out of calcium phosphate (the mineral apatite), the same material used in our bones, rather than the calcite that nearly all other marine invertebrates use.
Figure 13.5 ▲
The living inarticulate brachiopod Lingula, shown on the sediment surface after having been extracted from its burrow (see also color figure 3). (Courtesy of Wikimedia Commons).
ARTICULATE BRACHIOPODS
All the rest of the brachiopods are articulate; that is, their shells have a mechanical hinge that holds together even when the animal dies. All of them make their shells out of calcite, the most common building material among marine invertebrates. They are also peculiar in that they have no anus. When the digested food accumulates to their limit, they expel all their waste products out of their digestive tract.
Order Orthida
The orthides are the most primitive of the articulate brachiopods, originating in the Late Cambrian during the heyday of the inarticulates, flourishing in the Ordovician before crashing during the Late Ordovician mass extinction, then straggling on to the end of the Permian extinction. Most are very similar, with a straight hinge and small pedicle opening, and with lots of fine ribs of costae radiating away from the hinge. During the Ordovician, genera like Orthis, Dinorthis, Hebertella, and Resserella (fig. 13.6A–B) were very common, and they are good index fossils of this period.
Figure 13.6 ▲
Typical orthide brachiopods: (A) a slab full of Orthis, a typical orthide; (B) the common Ordovician genus Hebertella. ([A] Courtesy of Wikimedia Commons; [B] photograph by the author)
Order Strophomenida
The strophomenides are the largest group of articulate brachiopods. They had two great periods of diversification. During the Ordovician, they were by far the most common type of brachiopod, and thus they are an instant indicator of Ordovician rocks. Ordovician strophomenides (fig. 13.7A) tended to look like Strophomena or Rafinesquina, with long straight hinges giving them a D-shape in top view. These strophomenides also tended to have concavo-convex shells, so when the valves were closed, they were like a pair of bowls nested inside one another and had only a tiny internal volume. They apparently lived with their convex side up, so they arched their shell above the substrate. The lack of a large pedicle opening tells us that they did not attach to anything, but instead they lived on the open seafloor.
Figure 13.7 ▲
Strophomenide brachiopods: (A) typical Ordovician strophomenides, like this Rafinesquina, had long straight hinges and looked like the letter “D” in dorsal view, and had concavo-convex shells; (B) Late Paleozoic productids were shaped like a cup or dish supported by spines, with a small lid-like dorsal valve to close them; (C) ventral view of a Permian productid with the spines still preserved. ([A] Illustration by Mary Persis Williams; [B–C] courtesy of Wikimedia Commons)
The D-shaped strophomenides were nearly wiped out during the Late Ordovician extinction, but the group had another great evolutionary radiation in the Late Paleozoic (Carboniferous-Permian). These brachiopods, known as productids, had a cup-shaped ventral valve and a tiny flat or concave dorsal valve that formed a lid on top of the shell (fig. 13.7B–C). They had no pedicle, and they lived on the open seafloor. Most had a dense cluster of spines on their ventral valve that acted as “stilts” or “snowshoes” to prevent their shell from sinking into the sediment. In fossil productids, the delicate spines are usually broken off, but the ventral valve is covered with a dense set of bumps where they once attached.
Productids were by far the most common brachiopods of the Carboniferous and Permian, and they often lived in dense clusters or colonies. In the Permian they evolved some truly strange forms, known as the leptodids. These weird creatures had a ventral valve that was shaped like a soap dish that was covered by a dorsal valve that looked like a comb or a grill. In contrast to the flattened leptodids, the other extreme were the richthofenids. These peculiar creatures had a ventral valve that was shaped like an ice-cream cone held up by stilt-like spines, with a tiny lid-like dorsal valve inside the opening of the cone. This was one of many examples of a group evolving into a conical and colonial coral-like shape or oyster-like habitat. A similar shape also happened independently in the rudistid oysters in the Cretaceous.
Order Pentamerida
During the Silurian, the most common group of brachiopods was the pentamerides. They had a robust biconvex shell with a narrow hinge, a large pedicle opening, and a relatively smooth shell covered with fine costae. Their name comes from the fact that their internal shell is subdivided into chambers by a series of small dividing walls. There are five chambers, hence the name “pentamerid” (penta is “five” and meros is “part” in Greek).
Pentamerides tended to form dense reef-like clusters during the Silurian, often in areas associated with the huge Silurian coral and sponge reefs (fig. 13.8A). In many places in the Midwest, the Silurian rocks are completely replaced by dolomite, including the filling of the shells. However, the shell itself remained preserved in calcite, so when these rocks weather, the shells dissolve away, leaving a steinkern or internal mold of the more resistant dolomite (fig. 13.8B). Their distinctive internal molds have a large cleft in the middle where one of the dividing walls used to be located.
Figure 13.8 ▲
(A) Pentamerides typically formed dense “reefs” of clustered brachiopods in the Silurian. (B) Many times, pentamerides are preserved as dolomite internal casts or steinkerns, which filled their original shell. The small clefts on the steinkerns are where the internal dividing walls of the brachiopod were located. ([A] Illustration by Mary Persis Williams; [B] courtesy of Wikimedia Commons)
Order Spiriferida
The spirifers are distinguished by their lophophores, which are arranged in a spiral or corkscrew pattern inside the shell (hence their name “spirifer,” which means “spire bearing” in Latin). The external shape of Spirifer, Mucrospirifer, Neospirifer, and their kin is also distinctive (fig. 13.9A–C), with a long straight hinge, strong plications, and a large fold and sulcus. Their shape resembles a pair of wings. Other spiriferides, like Atrypa, were plano-convex with many fine costae and a slightly shorter hinge. Spirifers first appeared in the Ordovician, but during the Devonian they had a huge radiation, and they are a classic index fossil of that period. They were decimated during the Late Devonian extinction, but genera like Spirifer flourished in the Mississippian, and the entire group straggled through the rest of the Paleozoic in small numbers. They were nearly wiped out by the great Permian extinction, but they recovered in the Triassic, only to vanish at the end of the Triassic.
Figure 13.9 ▲
Spirifers were the most common brachiopods in the Devonian and Mississippian. They all had long straight hinges, a deep fold and sulcus, and a spiral lophophore inside. (A) Mucrospirifer was a very long-hinged form whose shape resembled a pair of wings. It is an index fossil of the Devonian. A common Mississippian brachiopod is Spirifer: (B) dorsal view, with the prominent fold on the midline; (C) ventral view, with a deep sulcus on the midline. (Photographs by the author)
All the previous articulate orders are now extinct. Only two articulate groups survived the Mesozoic and are still around today.
Order Rhynchonellida
The rhynchonellides first appeared in the Ordovician, but they persisted through the entire Phanerozoic and are still found in great numbers in certain habitats. Nearly all rhychonellides have a short hinge with a pointed beak and strongly corrugated plications that give them a distinctive zigzag commissure (fig.13.10A). This body form was very conservative and typifies nearly all of their order, so they are easy to recognize.
Figure 13.10 ▲
(A) Rhychonellides typically have a sharp pointed hinge and short hinge line, and a corrugated shell with deep plications, producing a zigzag commissure. Typical of them is Rhynchotrema. (B) The common living brachiopods are terebratulides, with a smooth shell shaped like a biblical oil lamp. This is Terebratula maugeri. (Courtesy of Wikimedia Commons)
Order Terebratulida
The other surviving group of articulate brachiopods is the terebratulides. These are the most common form alive today, and they are shaped like the biblical oil lamp, hence the name “lamp shells” (fig. 13.10B). They are strongly biconvex, with a narrow hinge. They have a large pedicle, and the beak of the ventral valve curves around with a large pedicle opening. Living terebratulides attach to a hard surface and can orient their shells in any position to take advantage of currents.
