Conclusions

Based on our analyses of Mellopegma and other stenothecids, we conclude that: (1) calcitic semi-nacre probably occurred in the early Cambrian Mellopegma uslonicum, extending back the first appearance of this unusual type of shell microstructure and thereby providing additional support for a fundamental homology between the shell of brachiopods and early molluscs; (2) the periostracum was present in middle Cambrian Mellopegma and other molluscs; (3) the protoconchs of Mellopegma reveal that these shells were likely adults and that this genus was lecithotrophic; and (4) stenothecids like Mellopegma appear to have been increasingly under predation pressure from the early to middle Cambrian and in response became better adapted to avoid predation, suggesting an early arms race between predators and their molluscan prey.