Jurassic Bivalves and Ammonites

In his studies of Cope’s rule, Hallam (1975) used Jurassic bivalves and ammonites as his test taxa. He looked at the increase in maximum size in the two taxa and also looked on the rate at which these taxa increased in size by finding the number of zones (approximately 1 x 106yr) through which specific species or genera existed (Hallam, 1975). In the interest of this web page, I will only be concerned with the overall size increase of the two taxa studied, not the rate and its effect on longevity. The trend observed by Hallam (1975) was that throughout Jurassic time, both the bivalves and the ammonites showed an increase in size (Figure 1). When collectively looking at each taxa, it was found that a high proportion (69% of bivalves and 68% of ammonites) of the groups increased in size by a factor of two or more (Hallam, 1975).

Figure 1. Size increase trends for select bivalves in the Jurassic period. Species include Plagiostoma, Cardina concinna, Liostrea irregularis, Liassic Gryphaea, Isognomon, Gervillella Middle-Upper Jurassic Gryphaea, Myophorella, Camptonectesand Protocardia (after Hallam, 1975)