Kidney Of Bivalves

Gryphaea i&SSS:'

Dactylioceras

Figure 13.4 Theoretical morphospace created by the computer simulation of shell growth (a) and some computer simulations matched with reality (b). (a, based on Raup 1966; b, from Swan 1990.)

burrowing and boring strategies together with migrations to freshwater habitats were secondary innovations. There are over 4500 genera of living bivalves, with fewer than half of that number described from the fossil record. In view of the wide range of life strategies and their relationships to particular sediments, the bivalves are good facies fossils. Although non-marine bivalves have been used extensively, in the absence of other groups, to zone parts of the Upper Carboniferous and by Charles Lyell in his classic work in the 1820s and 1830s to subdivide the Tertiary (the increasing proportion of living forms in fossil faunas through the Tertiary was used to subdivide the system; see p. 29), their biostratigraphic precision is limited.

Basic morphology_

Bivalves are twin-valved shellfish superficially resembling the brachiopods and common in modern seas (Fig. 13.5). In contrast to the Brachiopoda, bivalve shells are always composed of calcium carbonate, usually arago-nite, and many have a plane of symmetry parallel to the commissure separating the left and right valves from each other, i.e. the two valves are virtually mirror images of each other. Bivalves have sometimes been termed lamellibranchs or pelecypods, but they were first named Bivalvia by Linnaeus in 1758.

In the bivalves the molluskan head is lost, only the anterior mouth indicates its position. Sensory organs are concentrated instead on the mantle margins and include eye-spots, chemoreceptors and statocysts. The bivalve exoskeleton has two lateral valves, left and right, essentially mirror images of each other, united dorsally along the hinge line by an elastic ligament and usually interlocking teeth and sockets; the valves open ventrally. The valves are secreted by mantle lobes. The attachment of the mantle is marked by the pallial line, which may be indented posteriorly with the extension of the siphons. The earliest-formed parts of each shell, the beaks or umbones, may be separated by the cardinal area supporting the dorsal ligament. When the valves are closed, a pair of adductor muscles, situated anteriorly and posteriorly, is in contraction. While the shells are closed, the hinge ligament is constrained between the dorsal parts of the shells; when the adductors relax, the ligament expands and the shells spring open. The scars of these shell-closing muscles may be seen usually as clear roughened and depressed areas inside both valves.

Classification of the bivalves is based primarily on gill structure (Fig. 13.6a). Dentition is of secondary importance (Fig. 13.6b). Teeth may be all along the hinge line or separated into discrete cardinal (subumbonal) and lateral (both anterior and posterior of the hinge line) teeth. The three most important tooth arrangements are: (i) taxodont - numerous subequal teeth arranged in a subparallel pattern; (ii) actinodont - teeth radiating out from beneath the umbo; or (iii) heterodont - a mixture of cardinal (beneath umbo) and lateral teeth. Various other terms have been employed in the past when the teeth are thickened, modified or reduced, but are now less commonly used.

In most cases the umbones of the valves point or face obliquely anteriorly, the pallial sinus (if present) is situated posteriorly and the posterior adductor is usually the larger of the two scars. In some forms the anterior adductor is lost, together with the foot. When the valves are held with the commissure between the two valves vertically, the anterior end pointing away from the observer and the umbones at the top, then the right and left valves are in the correct orientation.

Main bivalve groups_

The Bivalvia are classified by zoologists mainly on the basis of soft-part morphology such as features of the digestive system and the gills; paleontologists have usually attempted to use details of the hinge structures. There are seven basic features that are of use for classification at various levels within the Bivalvia: gill structure (subclass and infrasubclass levels), dentition (all levels), ligament insertion (infrasubclass down to ordinal levels), adductor muscle scars (superfamily to orders), pallial line (family level and below), shell shape (all levels), and shell fabric (infrasubclass down to superfamily level). Two subclasses are recognized: (i) the Protobranchia with simple pro-

beak dorsal cardinal teeth socket ligament area hinge plate beak dorsal cardinal teeth socket ligament area hinge plate

Lamellibranchia Fossils

posterior adductor posterior pallial line ventral pallial sinus posterior adductor posterior

pallial line beak ventral pallial sinus dorsal beak dorsal anterior

Palaeontology Arca Pallial Line

posterior ventral posterior

ventral shell teeth gonad ligament style sac digestive diverticulum mouth palp shell anterior adductor muscle foot

kidney heart

shell teeth gonad ligament style sac kidney heart anterior adductor muscle

posterior adductor muscle exhalent siphon inhalent siphon position of the gill (shown in part)

anus

Figure 13.5 Bivalve morphology based on a living bivalve: (a) internal features of the right valve, (b) external features of the left valve, and (c) reconstruction of the internal structures attached to the right valve. (Based on Treatise on Invertebrate Paleontology, Part N. Geol. Soc. Am. and Univ. Kansas.

posterior adductor muscle exhalent siphon inhalent siphon position of the gill (shown in part)

Figure 13.5 Bivalve morphology based on a living bivalve: (a) internal features of the right valve, (b) external features of the left valve, and (c) reconstruction of the internal structures attached to the right valve. (Based on Treatise on Invertebrate Paleontology, Part N. Geol. Soc. Am. and Univ. Kansas.

anterior anus tobranch gills very like those of the archetype mollusk, that are deposit feeders; and (ii) the Autolamellibranchiata that mostly have large leaf-like gills modified for food gathering as well as for respiration (filibranch and eula-mellibranch types), but some have lost their gills altogether and use the mantle cavity for respiration (septibranch) (Fig. 13.6a).

A number of taxa from the two subclasses, the protobranchs and autolamellibranchs, are illustrated in Fig. 13.7.

Protobranchs

The Nuculoida is the oldest and most primitive infrasubclass, characterized by

Filibranch

protobranch filibranch eulamellibranch septibranch

protobranch filibranch eulamellibranch septibranch

taxodont taxodont

schizodont dysodont dysodont heterodont

isodont

schizodont heterodont

Figure 13.6 (a) Main gill types in the bivalves. (b) Main types of bivalve dentition.

Figure 13.6 (a) Main gill types in the bivalves. (b) Main types of bivalve dentition.

Lamellibranchia Types Dentition

Figure 13.7

Figure 13.7

Some bivalve genera: (a) Glycimeras (Miocene), (b) Trigonia (Jurassic), (c) Gryphaea (Jurassic), (d) Chlamys (Jurassic), (e) Mya (Recent), (f) Pholas (Recent), and (g) Spondylus (Cretaceous). Magnification x0.75 for all.

prismato-nacreous shells, taxodont dentition, equivalved shells and protobranch gills. Most are detritus-feeding infaunal marine animals, such as Nucula, and most abundant today in deeper-water environments. Ctenodonta has a typical taxodont dentition, an elliptical shell and an external ligament; it is principally Ordovician in age.

Members of the infrasubclass Solemyoida are specialized, infaunal burrowers with an anteriorly elongate shell. Most have symbiotic autochemotrophic bacteria allowing them to live in fetid muds, ranging in age from Early Ordovician to Recent.

Autolamellibranchs

Autolamellibranchs were derived from the protobranchs by the earliest Ordovician, possibly via a group of nuculoids that developed hinge-teeth allowing greater opening of the valves. This is necessary to avoid sediment inadvertently trapped by the gills during the food-gathering process.

The pteriomorphs are mainly marine, fixed benthos, attached by a byssus, or pad of sticky threads, modified from the foot, or they may be cemented. They are an important part of bivalve faunas from the earliest Ordovician and most had an outer mineralized shell layer of calcite; the gills are of filibranch grade. The group includes the mussels Modiolus and Mytilus and the ark shells Arca and Anadara, the scallops Chlamys and Pecten, and the oysters Crassostrea and Ostrea.

The heteroconchs are a mixed bag of mainly suspension feeders, important in bivalve faunas from the earliest Ordovician and radiating during the Mesozoic when mantle fusion and the development of long siphons promoted a deep-infaunal life mode. They are and were very successful burrowers. Gill grades are mainly eulamellibranch and many have crossed-lamellar or complex crossed-lamellar shell microstructures. This group includes the typical clams such as the giant clam Tridacna, the horse-hoof clam Hippopus and the surf-clam Donax, together with the razor shells Ensis and Tagelus, the ship-worm Teredo and the cockle Cerastoderma.

The anomalodesmatans are predominantly suspension-feeding marine forms with pris-mato-nacreous shells and reduced dentitions, such as Pholadomya. They have eulamelli-

branch or septibranch gill grades. These too are found from the earliest Ordovician but only form a minor part of bivalve faunas.

Lifestyles and morphology_

There are seven main bivalve forms that relate to their modes of life (Stanley 1970): infaunal shallow burrowing, infaunal deep burrowing, epifaunal attached by a byssus, epifaunal with cementation, free lying, swimming, and borers and cavity dwellers. Specific assemblages of morphological features are associated with each life mode; these are summarized in Fig. 13.8. Steven Stanley's studies have been adapted by a number of authors for similar bivalve-dominated communities throughout the Phanerozoic (Fig. 13.9). Most bizarre were the rudists that built extensive reefs in the Cretaceous (Box 13.5).

Bivalve evolution_

The earliest known bivalves have been reported from the basal Cambrian. Two Early Cambrian genera are the praenuculid Pojetaia from Australia and China and Fordilla from Denmark, North America and Siberia. Both genera have two valves separated by a working hinge with a ligament, together with muscles and teeth. These probably came about 10 myr after the oldest rostroconch, Heraultipegma, and so the bivalves might just have evolved from rostroconchs (see p. 357) or something like them. The class evolved rapidly in the Early Ordovician to include basal forms of all bivalve infrasubclasses. Not only were tax-odont, actinodont and heterodont dentitions established, but a variety of feeding types had also developed following the Tremadocian and Floian radiation.

Following this major diversification, the group stabilized during the remaining part of the Paleozoic, although some groups evolved extensive siphons that aided deep-burrowing life modes. This adaptation, together with the mobility provided by the bivalve foot, were important advantages over most brachiopods, which simultaneously pursued a fixed epifau-nal existence. The earliest autolamellibran-chiate forms are known from the Early Tremadocian. The early Mesozoic radiation of the group featured siphonate forms with desmodont and heterodont dentitions,

1. Infaunal shallow burrowers Glycimeris

2. Infaunal deep burrowers Mya (^y

3. Epifaunal with byssus Mytilus

4. Epifaunal with >n Ostrea

5. Unattached reel its

Gryphaea

6. Swimmers

Pecten

7. Borers and cavity dwellers elongate valves with flat ventral surface and reduction of both the anterior part of the valve and the anterior muscle scar. Attached by thread-like byssus.

markedly differently shaped valves, sometimes with crenulated commissures; large single adductor muscle.

markedly differently shaped valves sometimes with spines for anchorage or to prevent submergence in soft sediment.

valves dissimilar in shape and size with very large, single adductor muscle and commonly with hinge line extended as

Teredo equivalved, adductor muscles of equal sizes and commonly with strong external ornament.

elongated valves, often lacking teeth and with permanent gape and a marked pallial elongate valves with flat ventral surface and reduction of both the anterior part of the valve and the anterior muscle scar. Attached by thread-like byssus.

markedly differently shaped valves, sometimes with crenulated commissures; large single adductor muscle.

markedly differently shaped valves sometimes with spines for anchorage or to prevent submergence in soft sediment.

valves dissimilar in shape and size with very large, single adductor muscle and commonly with hinge line extended as elongate, cylindrical shells with strong, sharp external ornament; cavity dwellers commonly grow in dimly lit conditions following the contours of the cavity.

Figure 13.8 Morphology and adaptations of the main ecological groups of bivalve mollusk.

equipped to handle life deep in the sediments of nearshore and intertidal zones where they diversified.

0 0

Post a comment