Lamellibranch shells vary greatly in size. Some are only few centimetres while other, like Tridacna are more than a metre wide.
Brachiopods first appeared in the early Cambrian era, and became extremely abundant, during the Palaeozoic era. They multiplied into a number of different morphologies and even participated in the build-up of ancient reefs. At the end of the Palaeozoic era, about 250 million years ago, they were annihilated in the worst mass extinction of all time. This was the Permo-Triassic event, which leaves only about 30 species today, most of which are found in polar or very deep waters.
Many different brachiopods show similar adaptations to epifaunal bivalves (for example; pedicle attachment or thick and ribbed shells). They also show a number of unique adaptations, designed to protect the sensitive respiratory lophophore apparatus from sediment clogging and also self-contamination by faeces.
Trilobate shapes
The shell shows a 3-lobed form that is designed to separate the paired inhalant and single exhalant water currents set up by the lophophore, in order to prevent recirculation and self-contamination. Zigzag commissures occur in many shallow water brachiopods living in areas of varying sediment sizes; the commissure is not straight but shows zigzags whose amplitude increases towards the anterior margin. For the same angle of opening at the hinge, this adaptation allows greater control of the particle sizes drawn into the shell. This is often associated with strongly ribbed shells.
"Snow-shoe" shapes
These occur in brachiopods that lived in soft sediments, are thin-shelled and have a flat and wide contact surface to spread their weight and prevent them sinking. Some of these forms had spines, which also helped spread their load.
A few forms of brachiopods were able to live partially buried just below the sediment surface (out of view from predators), by having the anterior margin of the shell extended into an upward curving "flange" or "trail". By having no siphon in their soft parts, the shell became adapted to serve a snorkel-like function. These forms are called quasi-infaunal. The lower valve was often very large with the upper valve being thin and lid-like. The resulting centre of gravity also tended to restore them to the correct position if disturbed. Such quasi-infaunal adaptations appeared relatively late in brachiopod evolution, not until the Devonian, before becoming the most common brachiopod in the Carboniferous and Permian eras.
Virtually all articulate brachiopods are epifaunal. (The lack of a foot or siphon and the sensitivity of the lophophore to sediment clogging prevented them from becoming infaunal). The only true infaunal example is the dorso-ventrally flattened and parallel-sided inarticulate brachiopod genus Lingula, which has existed virtually unchanged since the Cambrian era (and must therefore be extremely well adapted to its shallow marine and estuarine habitat). It has no foot, but possesses a contractile pedicle, which allows it to withdraw into its "burrow".
In Lamellibranchs the shell consists of two hinged and usually identical (equivalve) halves, the left and right valve. The dorsal margin near the hinge line is pointed (umbo); the opposite ventral margin is more rounded or straight. A line drawn approximately perpendicular to the hinge and passing through the umbo divides the shell into an anterior and posterior end. The posterior end is usually larger and in life position is located uppermost. Lamellibranch shells are hinged by means of teeth and sockets, which together make up the dentition. Dentition in Lamellibranch shells is very important in their classification.
Figure 3
Taxodont dentition: A series of small parallel to sub parallel teeth, which are perpendicular to hinge line.
Heterodont dentition: having cardinal teeth and lateral teeth either in front and/or behind beak.
Desmodont dentition: having an internal ligament and a chondrophore, but usually lacking well defined teeth.
Schizodont dentition: having prominent bifurcating or diverging teeth.
Dysodont Absent or insignificant teeth.
Cardinals are teeth at the side, whereas laterals are teeth at the front. The Resilifer is a small depression along the hinge plate, which holds an internal ligament; may be a single pit or consist of multiple pits. The Chondrophore is the pit the ligament is in. Pallial sinus is an indentation in the posterior part of the pallial line, a line of mantle attachment, where siphons can be retracted. There are two different distinctions about the pallial sinus, integrepalliate, when the pallial sinus is circular, or sinulapalliate, when the pallial sinus has a kink in it.
Lamellibranchs adductor scars can also reveal a lot about the Lamellibranch, and they are divided in four ways;
Bimyarian- A valve having two adductor muscle scars; one anterior and one posterior.
Isomyarian- A bimayrian shell where two adductor scars generally equal in size.
Anisomyarian- A bimyarian shell where the two adductors are of unequal size; usually the posterior scar is the larger of the two.
Monomyarian - A shell having only one adductor scar; which is usually a centrally positioned posterior adductor scar.
Figure 4
The ribbing of a shell also reveals a lot about where the Lamellibranch lived, if it has large ribs, and a thick shell, then it can be supposed that it lived in rough seas, or where it was likely to attack from predators, if it has a thin shell and little ribbing it is likely that it is a burrowing Lamellibranch, not needing protection from the sea or predators.
The soft parts of some bivalves have a muscular foot at the anterior end for burrowing and a muscular siphon at the posterior end (a snorkel tube which may be twice as long as the shell when extended). These structures appeared in the Devonian, allowing some bivalve groups to exploit infaunal as well as epifaunal habitats, which led to greater overall morphological variety. Brachiopods lack equivalent soft parts and are thus all epifaunal. The soft parts of bivalves (the mantle) are also better at separating out and expelling any inorganic material drawn into the shell.
In some cases infaunal shells have a permanent opening at the anterior end (pedal gape) or posterior end (siphonal gape) even when the valves are closed; this is mostly seen in boring taxa (pedal) or deep burrowers (siphonal) which live out of the reach of most predators (which would otherwise exploit the gape to get at the flesh).
In Infaunal Lamellibranchs the following characteristics are common depending how it lived;
Burrowing Shells are usually equivalved and isomyrian (or anisomyrian) with a distinct pallial line. They include: the nuculoid burrowing deposit feeders, the shallow burrowing non-siphonate forms lacking a pallial sinus, and deep burrowing siphonate forms identified by a distinct pallial sinus.
Boring Shells are usually thick, equivalved, and cylindrical in cross section. Some forms are moderately ornamented with ridges and stout spines whereas others such as the "ship worms" are tubular in form.
In Epifaunal Lamellibranchs the following characteristics are common depending how it lived;
Reclining Shells are commonly inequivalved with a larger lower (usually the left) valve, which is more inflated or convex while the upper valve may be planar. Some also exhibit spines, especially on the lower valve, to aid in stabilisation in soft substrates in a manner similar to some brachiopods. Many have a small attachment area at beak where earliest growth stages were cemented. The giant clam Tridacna, who has photosymbionts similar to hermatypic scleractinian corals, is a recliner even though it had a functional byssus during its earliest juvenile stages.
Swimming Shells are usually equilateral but not equivalved. The lower (usually the left) valve is usually slightly larger. Swimming forms are typified by having a greater umbonal angle (greater than 105°). Furthermore, swimming forms typically have a single (monomyrian), large, centrally located adductor muscle.
Cementing Shells are commonly inequivalved with the lower (usually left) valve assuming the form of the object to which it is cementing, a condition called xenomorphism. In such cases, both valves are usually highly variable in shape, as in the common oysters and other forms as well. Some groups such as the Cretaceous rudists could reach very large sizes and were able to form reefs mimicking corals in both morphology and ecology.
Both Brachiopod and Lamellibranch morphology has occurred over millions of years, and both were very successful species, evolving to better suit their environment, giving us the variation we see today in their fossils.
