Fossil Bivalve Shells and the Length of Month and Year in the Cretaceous

Abstract

Berry and Barker¹ have suggested that laminae, ridges and troughs, and bands on present day and Cretaceous bivalve shells, are growth increments of the day, month and year, respectively. By counting the number of laminae in a number of ridges and troughs they therefore find that the number of days in the month in the Late Cretaceous was 29.65±0.18 (S.D.). They also count in specimens of Idonearcus vulgaris of Late Cretaceous age 24.98±0.04 (S.D.) half-monthly clusters in annual bands or 12.49±0.02 months in the year. Multiplying these two results, they find 370.3 days in the year in the Late Cretaceous. Wells² had proposed that such observations on palaeontological clocks should be compared with a plot of the length of the day through geological time obtained by the extrapolation of the astronomically determined secular lengthening of the day since the seventeenth century. Because this value (2 ms/century) leads to a value of 371 days in the year in the late Cretaceous, Berry and Barker find their results highly satisfactory.