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Bivalves/ Lamellibranchia /Acephala) 2

Bivalves/ Lamellibranchia /Acephala) 2

Palaeontology 1

Unit II by. Prof. S.K. Kulshrestha

Phylum-

Class -

1. Pelecypoda (Bivalves/ Lamellibranchia /Acephala) 2. Gastropoda 3. Cephalopoda

Class-1 Pelecypoda (Bivalves)-

Name Pelecypoda is coined from latin word Pelecys meaning sickle shaped or the blade of axe. They are also called Lamellibrachia sheet like or lamellar form of gills (respiratory organs). The soft body organs are well protected within two valves, which are joined along their dorsal margin and can articulate. The two valves are designated as right and left valves. Since these two valves are equal in shape and size but in equilateral. The name Clam may be applied to all pelcypods. Many fresh water and a few marine (e.g. Mytilus) Clams are called Mussels.

Majority of Bivalves are aquatic (both freshwater and marine) a few can live on land (terrestrial) as well. Some are fast swimmer e.g. Lima and Pecten. Others are sluggish movers and bottom dwellers. A few forms can live firmly attached with bottom sediments. They are sedentary in nature.

Attached by Byssal threads e.g. Mytilus.

Attached by cementation e.g. ,Gryphea

Some are burrowers or borers- a) Sand burrowers b) Mud burrowers c) Rock borers

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d) Wood borers

Shell Morphology- the is more or less triangular in shape which is broadly rounded towards anterior side and tapering posterior. The earliest portion of the shell is known as , which normally points toward anterior, followed by a raised portion of the shell which is smooth is called . Rest of the external surface of the shell is marked by the concentric growth lines starting from the beak, and parallel to the free margin of the shell.

Beside this, the external surface is variously ornamented by radial, ribs, ridges or reticulate pattern. The side on which the two valves are joined together and articulate is dorsal side and the opposite side/margin is ventral margin.

On the internal surface of the shell, area is occupied by long platform known as Hinge plate. The area between the hinge plate and the dorsal margin is known as Cardinal area. In living forms this cardinal area (inter area) is occupied by linear depression in which the is attached. The function of ligament is to open the two valves. The closure of the valves is performed by adductor muscles, which are represented as the inner surface by scar marks. The Ventral most limit of cavity is marked .

Figure 1 Internal, external and dorsal view of shell of the

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Types of Ligaments-

1) Alivincular-e.g. Lima

2) Duplivincular- e.g. Arca

3) Multivincular- e.g. Gervilia

4) Perivincular- e.g. Venus

Muscle Scars-

1) Monomyarian- when only one scar mark is present, it is always posterior in position. E.g. Pecten.

2) Dimyarian- when two scar marks present.

a) Isomyarian-Two scar marks are similar in shape and size. E.g. Arca.

b) Anisomyarian- Scar marks of dissimilar in shape and size E.g. Mytilus.

DENTITION- The hinge plate is marked by various patterns of (projections) and sockets (depressions).

1) EDANTULOUS- Hinge plate lacking teeth and sockets. E.g.- Edmonia (sea mussel) 2) TAXODONT- Characterized by numerous teeth on the hinge plate. E.g. Arca, Glycemeris. 3) HETERODONT- Characterized by hinge teeth of distinct type- Cardinals beneath the beak and lateral teeth. E.g. Lucina, Cyrima 4) ISODONT-Characterized by two subequal prominent hinge teeth on one and corresponding sockets in the opposite valve. E.g. Pecten 5) SCHIZODANT – having prominent bifurcating teeth and corresponding sockets. E.g. Trigonia 6) DYSODONT- Characterized by very rudimentary teeth or by absence of hinge teeth. E.g. Mytilus. 7) DESMODONT- Hinge teeth are absent and the hinge area is occupied by the enlargement of ligament. E.g. Mya. 8) PACHYDONT- Thickened specialized teeth developed as coral like valves in which right valve is in the form of deep hollow conical shape and left valve in the form of a flat lid, bearing 5 large teeth on its underside which hangs in conical right valve. E.g.- Hippurites.

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Classification-

Phylum-Mollusca

Class- Bivalvia

Sub-Class- I. Paleotarodonta-( to Recent)

Order-1 Nuculoida E.g. Nuculena

Sub- class-II. Cryptodonta-( to Recent)

Order-1. Solemyoida-E.g.

2.Pre-cardioida-E.g. Cardiola

Sub- class-III. Pteriomorpha-(Ordovician to Recent)

Order 1. Arcoida E.g. Arca

2. Mytiloida E.g. Mytilus

3. Pteroida E.g. Pteria,Pecten

Sub-class- IV Palaeoheterodonta (Palaeozoic to Recent)

Order 1. Unionoida E.g. Unio

2. Trigonoida E.g. Trigonia

Sub-class-V ( to Recent)

Order 1.Veneroida E.g Venus,Cardita,Lucina

2. Myoida E.g. Mya

3. Hippuritoida E.g. Hippurites

Sub-class VI- -(Ordovician to Recent)

Order-1. Pholadomyoida E.g. Pholadomya

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GEOLOGICAL HISTORY-

The earliest known record of Pelecypods is from lower Ordovician rocks. They are simple shell, examples are- Ctenodonta, Glysoterea, Modiolapsis. Before the close of Ordovician the bivalves diversified both in number and variety in the later part of Paleozoic.

At the close of Paleozoic and beginning of many ancient genera became extinct and new genera appeared which have persisted to the present. During Mesozoic they reached their acme.

Pelecypods have been used for deforming large units of Geologic time. Due to their slow rate of evolution they are of little use for correlation of further sub conclusion.

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References

1. Palaeontology Palaeobiology By P C Jain Amp M S Anantharaman 2. Shrock, R. R. (1953). Principles of invertebrate paleontology. INTERNATIONAL SERIES IN THE EARTH SCIENCES. 3. Clarkson, E. N. K. (2009). Invertebrate palaeontology and evolution. John Wiley & Sons. 4. PALAEONTOLOGY INVERTEBRATE by Woods H. 5. Moore, F. R., Lalicker, C. G., & Fischer, A. G. (1952). Invertebrate . McGrawHill Book Co. Inc. Copyright.

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