Morphology, palaeoecology and evolution of the genus Gryphaea in the British Lias

Abstract

A comprehensive study has been undertaken of Liassic Gryphaea specimens collected from various localities in Great Britain at stratigraphical horizons ranging from Upper Hettangian to Upper Pliensbachian. Study of the area of attachment, which is generally small, but increases in mean size up the succession, shows no discernible natural selection effect. Measurements of the length, breadth and height of the left valve of over a thousand specimens from thirteen different samples were subjected to statistical analysis with the aid of a computer. The regression of breadth and height on length is shown to be expressible more accurately in terms of logarithmic than normal values. Statistical comparison of samples reveals th at geographical variations, though sometimes important within the limits of one or two adjacent ammonite zones, are insignificant compared with a major trend up the succession towards broader shells with a looser coiling. The spiral angle tends to decrease in stratigraphically younger forms and increase during individual ontogeny, which indicates that the coiling of the left valve cannot be accurately represented by a logarithmic spiral. The gryphaeas of the British Lias are divided into three species, in ascending stratigraphical order G. arcuata, G. mccullochii and G. gigantea , the first two each being divided into chronological subspecies. Field evidence indicates that gryphaeas tend to be commoner in stratigraphically thicker sequences and in ‘ferruginous’ rather than ‘calcareous’ facies. The fairly symmetrical size frequency distributions are best interpreted in terms of low juvenile mortality following the successful settling of spat. Most shells are disarticulated, small ones more so than large. Taken in conjunction with orientation data, this suggests that post-mortem disturbance has frequently been important. It is deduced on various grounds that the life orientation of all species was with the left valve convex downwards. Flow channel experiments on shell stability gave support to the common sense presumption that there is a trend of increasing stability from G. arcuata to G. gigantea , that G. arcuata became progressively less stable with growth and that the posterior sulcus and lobe of G. arcuata incurva could have had a stabilizing influence. Regional variations in shape of contemporaneous populations of G. arcuata may possibly relate to variations of turbidity and sedimentation rate, and it is argued that food supply was the major factor controlling distribution. Several evolutionary trends up the stratigraphical succession are suggested in the lineage G. arcuata— G. mccullochii— G. gigantea . These are increase of size, reduction in degree of incurvature of the left valve, broadening of the shell and thinning of both valves, together with less notable changes in area of attachment and the development of a posterior sulcus. The evolutionary change from the Hettangian Liostrea is thought to have been relatively sudden and genetically simple. It was clearly adaptive in raising the mantle margins above the sediment bottom, but led to a reduction of stability. The subsequent evolutionary history of Gryphaea largely can be interpreted as an attempt to rectify this and achieve, by the steady operation of selection pressure, a paradigmatic condition.