Bivalvia
From Palaeos
| MOLLUSCA | |
| Taxonomy | Phylogeny |
o Eutrochozoa `--o MOLLUSCA `-+--Solenogastres `--+--Caudofoveata `--o Testaria `--+--o Polyplacophora `--o Conchifera |--Tryblidiida |?-- Stenothecoida `--+?--Bivalvia (if molecular phylogeny) `==Helcionelloida (paraphyletic) | |--+-- †Yochelcionellidae | | `?-- Cephalopoda | |? -- †Criconarida | `-+?--Bivalvia (if transitional fossils) | `-- †Rostroconchia | `-- Scaphopoda `--+--†Paragastropoda `-- Gastropoda | |
Contents |
Introduction
The Bivalves - clams, oysters, scallops, mussels, and their kin - are among the most modified group of mollusks, although some fossil groups seem to have resembled them. As the name indicates, all bivalves are characterized by a two-valved shell, which protects the soft animal inside. The valves are joined by a hinge on the dorsal side of the animal along a line parallel to the length of the body. The axis of symmetry is perpendicular to that of the other great group of bivalved marine filter-feeders, the brachiopods. In bivalves, the two shells are identical in profile (although in some types this is not the case) but one valve is on the right side of the animal, and the other on the left.
All bivalves are aquatic, with the majority of types living in shallow marine waters. There is however a single group of freshwater forms, the Unionoidea. Most are quite sluggish and sit on the bottom, or burrow into the sand and mud. Some can bore through wood and even rock. A few types, such as the common scallops and file shells can swim through the water for short distances by clapping the valves together strongly.
| BIVALVIA | |
| Taxonomy | Phylogeny |
Phylum: Mollusca
|
o Mollusca `--+?-Tuarangioida `--+--Fordilla `--+--Pojetaia o Protobranchia |--Nuculoida |--Solemyoida |--Praecardoida `--o Autolamellibranchia |== Modiomorphoida (stem group) |--o Pteriomorphia | |--Isofilibranchia | |--Prionodonta | `--Eupteridomorpha `--+--o Palaeoheterodonta | |--Trigonioida | `--Unionoida `--o Heterodonta |--o unnamed clade | |--Carditacea | `--Crassatellacea `--o unnamed clade |--Hippuritoida |--Chamacea `--+--Veneroida `--Anomalodesmata |
Physiology
The paired gills consist of two plate-like flaps hanging into the mantle cavity on each side of the foot. These have tiny hairs called cilia that draw a current of water into the mantle cavity. In most species, as the water passes between the gill filaments, food particles are strained out and transferred by sticky mucus to the mouth.
In more specialized burrowing forms, the mantle margins are fused and the entrance and exit of the water current is governed by the inhalant and exhalant siphons. The line of attachment of the mantle to the shell shows as a scar line (called the pallial line, see illustration) on the shell, and the indentation of the line left by the free lobe of the siphonal area is called the pallial sinus. Macpherson & Gabriel (1962).
Bivalve morphology and lifestyle
Despite the limitation imposed on them by their morphology, bivalves are a very diverse group, that have adopted to a large number of lifestyles. Modern bivalves can be grouped into a number of morphoecological categories (Stanley 1970):
- infaunal shallow burrowing
- infaunal deep burrowing
- epifaunal, attached by byssus threads to the substratum
- epifaunal, cemented to the rock
- free-lying
- swimming
- boring and cavity-dwelling.
The shape and general morphology of bivalve shells directly reflects their mode of life. Often unrelated groups will adopt similar forms and lifestyles. From an understanding of the ways in which modern bivalves are adapted to particular modes of life one can make reasonable inferences on how extinct bivalves lived. Clarkson (1993).
Relationships and Evolutionary History
Despite their specialized nature, Bivalves are among the earliest mollusks to appear, with both Fordilla and Pojetaia dating from the Tommotian. Fordilla continues through to the end of the Botomian, and Pojetaia to the middle of the Middle Cambrian. Pojeta (2000), vide Schneider (2001). Both belong to, or are the sister taxa of, the most primitive bivalve lineage, the Palaeotaxodonta. Runnegar & Pojeta (1992) have shown that the Fordilla had the same shell microstructure as the palaeotaxodont Pojetaia and placed them in the same family. The former genus, previously considered an isofilibranch, is thus now placed in or near the Palaeotaxodonta. These very early forms seem to have disappeared by the end of the Middle Cambrian, so that true bivalves are unknown in the fossil record until the Early Ordovician, when a new diversity of Palaeotaxodonts appeared. The other great clade of bivalves, the Autobranchia or Autolamellibranchiata, do not appear until the Middle Ordovician, and are clearly derived from the Palaeotaxodonta, making the latter group paraphyletic. This is confirmed by molecular analysis. Giribet & Wheeler (2002).
During the early Paleozoic, epifaunal and primitive infaunal siphonate suspension feeding was the most common bivalve life mode. Infaunal siphon feeders appeared in the middle and late Paleozoic, but they gained prominence after the Permo-Triassic extinctions. The infaunal and epifaunal forms increased in diversity throughout the Mesozoic until the Cretaceous-Tertiary extinction event, when the epifaunal suspension feeders were decimated. During the Cenozoic, the infaunal siphonate clams continued to diversify. The labial palp deposit feeders and the mucus-tube builders evolved early in the Paleozoic and remained relatively unaffected by the extinctions and diversifications of the remainder of the class. Clarkson (1993).
References
- Adamkewicz, SL, MG Harasewych, J Blake, D Saudek & BCJ Bult (1997), A molecular phylogeny of the bivalve mollusks. Mol. Biol. Evol. 14: 619-629.
- Macpherson, JH & CJ Gabriel (1962), Marine Molluscs of Victoria, Melbourne University Press.
- Moore, RC (ed.) (1969), Treatise on Invertebrate Paleontology. Part N. Mollusca 6. Bivalvia. 2 Volumes.
- Moore, RC [ed.] (1971), Treatise on Invertebrate Paleontology. Part N. Mollusca 6. Bivalvia. Oysters (Volume 3).
- Moore, RC, CG Lalicker & AG Fischer (1952), Invertebrate Fossils, McGraw Hill Book Co., 766 pp.
- Pojeta, J (1987), Class Pelecypoda, in AH Cheetham & AJ Rowell (eds.) Fossil Invertebrates. Blackwell Scientific Publs.
- Pojeta, J., 2000 Cambran Pelecypoda (Mollusca), American Malacological Bulletin 15: 157-166
- Runnegar, B. & J Pojeta (1992), The earliest bivalves and their Ordovician descendants. Am. Malacol. Bul. 9: 117-22.
- Schneider, JA (2001), Bivalve systematics in the 20th century. J. Paleontol., 75: 1119-1127.
Links
- Man and Molluscs Bivalve page - a comprehensive introductory description and detailed taxnomic listing.
- Class Bivalvia - short intro with some images - Animal Diversity Web
- Bivalvia - good page, detailed intro
- Bivalves - another good intro
- Mollusca - Class Bivalvia - short intro
- Bivalvia
checked ATW050615
Credits
MAK98, MAK020926 Palaeos com
