Jurassic Bivalves and Ammonites

Hallam (1975) believed this trend to be due to selective advantages for increased size. These included: an increased ability to capture prey, an increased reproductive rate, an increased intelligence level due to a larger brain capacity, and increased ability to regulate heat loss. Another aspect of Cope’s rule that Hallam (1975) points out is that if animals groups tend to evolve from small forms towards a larger optimal size that this optimally sized animal would be more specialized and therefore more vulnerable to extinction (Hallam, 1975). These larger animals would also require more resources and would therefore need to decrease their population size to stay in their niche, again increasing their risk of extinction. This was observed for the bivalves in Hallam’s (1975) paper. As the maximum size increased, fewer specimens were found. Hallam (1975) also noted several occurrences of smaller organisms evolving from larger ones, but there was no

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)
evidence of these being gradual changes. Hallam (1975) believes that these extinctions could account for the instances in which small organisms evolve from larger organisms and that this evolution is the result of Cope’s rule leading to an increase in body size and a sudden extinction. One criticism for this paper is that it only deals with two taxa, both of which illustrate Cope’s rule. Hallam should have covered a broader range of taxa to make a stronger argument.