The study shows that Berenicea’s decline in zooid size over 200 million years contradicts Kapp’s theory.

The evolution of body size has long been a fundamental topic in paleontological research. Cope’s Law describes the tendency of many lineages to increase in body size during evolution. However, a new study focusing on bryozoans, a group of modular organisms, has revealed a different evolutionary pattern.

Associate Professor Ma Junye, from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS), and colleagues found that Berenicea, a genus of cyclostome bryozoan, has experienced a steady decline in zoogeographic size over the past 200 million years. This trend runs directly counter to Cope’s principle. The findings were recently published in the journal paleontology.

By analyzing 200 Berenicea morphotype specimens from the Late Triassic to the present, the research team documented a significant decrease in maximum zooid width over time. This pattern is in stark contrast to the long-term stability of zoonotic size in chelostome bryozoans, challenging the earlier hypothesis that the two groups followed similar trajectories of body size evolution.

To identify the driving factors, the researchers used multiple time series models to examine potential impacts, including changes in oxygen levels, rates of origin of chelostome bryozoans, and their proportions in ecological communities. The results show that while oxygen levels and zoo size exhibit a statistical correlation, no causal relationship exists. Similarly, the rise of chelostomes as dominant space competitors did not directly trigger the decline in Berenicea’s zooid size.

Additionally, this study ruled out potential effects of changes in palaeolite advection (and associated temperature) on zooid size, further emphasizing the endogenous nature of this reduction trend. Model analysis revealed that the downsizing process was not constant: it underwent two major phase transitions, approximately 165–160 million years ago and 78 million years ago.

This study marks the first systematic documentation of zoonotic size reduction in a cyclostome bryozoan over macroelutionary timescales. The team suggests that this trend may be linked to improved metabolic efficiency or shifts in feeding niches, rather than being a direct response to external environmental pressures or asymmetric competition.