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Compare and contrast the morphological features of Lamellibranches and Brachiopods

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Joe Dickinson L6MD Compare and contrast the morphological features of Lamellibranches and Brachiopods Brachiopods can be split into two classes: articulate and inarticulate. . Articulate brachiopods have calcerous shells hinged by teeth and sockets, whereas Inarticulate brachiopods have horn-like shells made from a protein called chitin, and their valves are not hinged by teeth and sockets. In Brachiopods the shell consists of two hinged halves, a dorsal pedicle valve and a ventral brachial valve. The line where the two halves of the shell join is called the Commissure. The two valves often differ in size, thickness, and form. In life position either of the valves may be uppermost (furthest from sediment) or largest, but the plane of the commissure is always sub parallel to the sediment surface. In articulate Brachiopods the pedicle valve has a small hole at its posterior end, through which the pedicle emerges. The inner surface of the brachial valve contains a raised solid, calcerous structure, the Brachidium. This supports a delicate respiratory and feeding apparatus called the lophophore. This is basically a pair of grooved tentacles or arms called Brachia. Each fringed with cilia. Sulcus, a dip in the line of the commisure, the fold is the uppermost part; the sulcus is the part below the commisure, aids ingress and egress of water from which microorganisms filtered out. ...read more.


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. ...read more.


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. ...read more.

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