SHAPE AND FUNCTION OF THE SHELL: A COMPARISON OF SOME LIVING AND FOSSIL BIVALVE MOLLUSCS
Authors: EAGAR R. M. C. 
Published: January 2008

The review is mainly concerned with Carboniferous non‐marine Anthracosiidae and Myalinidae, of which only the shells are known, and with certain unspecialized non‐byssate suspension‐feeding bivalves which had smooth shells and burrowed shallowly.

Limited experimental evidence and observation of living bivalves suggest that in certain Recent siphonate species and in some members of the non‐siphonate Anthracosiidae the shape of the shell was functionally related to movement through the sediment in the same way. Predicted optimum shapes of shells for downward burrowing and for upward near‐vertical movement in sands and silts were apparently realized in the Anthracosiidae, which constituted a series of highly variable opportunistic assemblages. It is stressed, however, that the shape of the shell always appears to be a compromise between several functional requirements.

In both the early Anthracosiidae and in several analogous Recent marine genera, orientation of the long axis of the shell was the same for downward burrowing and for upward pushing, that is near the vertical, with posterior end upward.

Invasion of the Pennine late‐Namurian delta took place when marine bivalves pushed upward, thereby avoiding sedimentation from delta lobes moving seaward relatively swiftly. The evolution ofCarbonicolaoccurred at about this time (the Marsdenian Age) when the bivalves acquired a smooth elongate shell of ‘streamlined’ form, having a hinge plate with swellings and depressions on it (later to evolve into teeth). All these features tend to characterize the active shallow burrowers of today.

Entry into soft‐bottom eutrophic conditions of fresh water is characterized in several unionids by increase in height/length (H/Lorw/m) ratio of shell, in anterior end/length (A/L) and in obesity (T/L) (see Fig. 2, centre). These changes also took place in established faunas ofCarbonicolacharacteristic of richly carbonaceous shales, in faunas of supposedAnthraconaiain more carbonaceous sediments of mid‐and late‐Carboniferous times in the U.S.A. and inAnthraconautaof the British late Carboniferous (Westphalian C and D). The genusAnthracosphaeriumepitomizes the culmination of these trends in the Anthracosiidae, and species of the genus were probably epifaunal or shallowly infaunal active burrowers on soft bottoms in Westphalian upper A and B time.

Two contrasting patterns of growth characterize the shells of the widely variable unionidMargaritifera margaritifera.In the first, dorsal arching of the shell, with straightening and reflexion of the ventral margin, provides increased weight but decreased ligamental strength. In the second, in which the ‘hinge line’ tends to remain straight while the dorsal margin becomes more rounded and obesity increases, there is increased metabolic efficiency for active surface movement. The maintenance of these trends within the species, which may be regarded as secondarily opportunistic, affords a means of insurance for survival within the highly variable environments of fresh water. The same trends are recognizable in established faunas ofCarbonicola, where it is likely that they performed the same function, as well as in Mesozoic and Cenozoic Unionidae.

The functional explanation outlined in paragraph (6) may be extended to provide an ecological meaning for Ortmann's ‘Law of Stream Distribution’, which states that obesity of unionid shells increases downstream. This applies broadly, within a fairly wide range of variation, a fact which again suggests ‘insurance’ of faunas against the variable hazards of fresh‐water habitats.

In bivalves having considerable thickness of shell in relation to their size, and having strong umbonal development, specific gravity of the living bivalve is correlated withH/LandT/Lratios of the shell, as in the veneridVenerupis rhomboides.In this species, differences in the specific gravities of the bivalves, as well as their shape, appear to be functionally related to shallowly infaunal burrowing in different substrates.

The conclusions of paragraphs 6 and 8 provide a functional explanation, in terms of selection, for the palaeoecological ‘law’ of Eagar (1973), which is applicable to established faunas ofCarbonicolain mid‐Carboniferous time, and relates variational trends in two main groups of shells primarily to increases in the relative water velocities of the palaeoenvironments.

Where the growth of relatively unspecialized bivalve shells has been measured, allometric relationships have usually been found inH‐LandT‐Lscatters. Logarithmic lines have two inflexions and linear scatters a sigmoidal form. A similar pattern of allometric growth has been found in bothCarbonicola (H‐L) andAnthraconauta (m‐w).These patterns appear to be related to the optimum requirements of water‐borne larvae, the initial byssal phase of settlement, when ability to burrow quickly is essential, and the main period of growth and activity. It is herein suggested that the final second inflexion, which indicates a falling off of gain inH/LandT/Lratios, may be a genetically controlled modification of the growth pattern which counteracts the operation of the ‘cube‐square rule’ (of Thompson, 1942) and prolongs productive life.

Patterns of relative growth of the shell may be significantly modified by conditions of the habitat; bothT/LandH/Lratios may be increased, with general reduction in size, in the less ‘favourable’ habitats. Both these ratios have been similarly modified, the one in the ‘natural laboratory’ of a lake formed by the damming up of streams, and the other in transplant experiments with livingVenerupis.In both these latter cases, phenotypic changes took place in the same direction as those expected on the basis of natural selection. Direct response to environmental factors cannot therefore be ruled out as an agent in similar changes noted inCarbonicolaand supposedAnthraconaiain paragraphs (5) and (9) and may have been operative in those of paragraphs (7) and (8).