A long-held principle of evolution states that animal lineages tend to increase in body size over geological time. New research on a colonial marine invertebrate, however, has revealed a striking exception to this rule, documenting a consistent and significant decrease in size over the course of 200 million years. The study provides one of the most detailed counter-examples to the established theory, suggesting that for some organisms, shrinking can be a successful long-term evolutionary strategy.
The findings focus on a group of cyclostome bryozoans known as Berenicea, which are small, aquatic animals that form interconnected colonies of individual units called zooids. An international team of scientists analyzed fossil evidence from the Late Triassic period to the present day and discovered a persistent trend of zooid size reduction. This contraction challenges “Cope’s Rule,” the widely observed pattern of size increase in lineages, and opens new questions about the complex pressures and advantages that shape the evolution of life.
A Microscopic Trend Across Eons
Bryozoans are filter-feeding invertebrates that construct colonies, often appearing as crusts on rocks and shells in marine environments. While the colony grows, the size of the individual zooids that comprise it is a key functional trait. A collaborative study between researchers at the Nanjing Institute of Geology and Paleontology (NIGPAS), the University of Oslo, and the Natural History Museum, London, set out to track this trait over a vast evolutionary timescale. The research, published in the journal Palaeontology, reveals a clear and unexpected trajectory for Berenicea.
The scientific team meticulously measured the maximum zooid width from 200 different samples of Berenicea-like fossils. The specimens spanned a timeframe from the Late Triassic, over 200 million years ago, through subsequent geological periods to modern examples. The data showed that, contrary to the expectations of Cope’s Rule, the zooids did not get larger or stay the same size; they steadily became smaller. This finding is particularly notable when compared to another major group of bryozoans, the cheilostomes, which have maintained a relatively constant average zooid size over their evolutionary history. The discovery refutes an earlier hypothesis that both groups likely shared similar patterns of body-size evolution.
Searching for an External Cause
To understand the potential drivers behind this unusual trend, the researchers investigated several external environmental factors that could have theoretically favored smaller zooid sizes. They used multiple time-series models to test the correlation and causality of these potential pressures against the observed size reduction. The team’s analysis systematically ruled out the most prominent environmental hypotheses.
The Oxygen and Temperature Hypotheses
One leading theory was that changing atmospheric and oceanic oxygen levels could be the culprit. A smaller body size results in a greater surface-area-to-volume ratio, which can be advantageous for metabolic processes in lower-oxygen conditions. The researchers did find a statistical correlation between ancient oxygen levels and the average zooid size in Berenicea. However, when subjected to more rigorous causal analysis using formal time-series tools, the relationship did not hold up. The models showed no direct causal link, forcing the scientists to look elsewhere for an explanation. Similarly, the team considered the potential influence of temperature, examining whether the paleo-latitude of the fossil samples accounted for the size changes. This factor was also ruled out as a primary driver.
Competition with a Rival Lineage
Another compelling hypothesis centered on interspecies competition. During the time Berenicea was shrinking, another group of bryozoans, the cheilostomes, was rising in dominance. Cheilostomes are known to be superior spatial competitors, often overgrowing other encrusting organisms. The researchers tested whether the increasing origination rate and community presence of cheilostomes exerted evolutionary pressure on Berenicea, perhaps forcing it into a niche that favored smaller size. Yet again, the analysis found that the rise of this competitor was not directly associated with the size reduction in Berenicea zooids.
An Internally Driven Process
Having systematically eliminated the leading external pressures, the study concludes that the shrinking trend in Berenicea is most likely the result of intrinsic factors. Rather than being a passive response to environmental change or competition, the size reduction appears to be an inherent evolutionary trajectory. The research team proposed two primary explanations for why shrinking could be advantageous.
The first possibility is an optimization of metabolic efficiency. Over millions of years, evolving to have a smaller, more streamlined body plan could have conserved energy and resources, allowing the colonies to thrive under a wide range of conditions. A second, related possibility is a shift in feeding ecology. A change in zooid size may have enabled Berenicea to exploit different food sources or capture prey more efficiently, carving out a durable ecological niche that favored a smaller architecture. These factors suggest that for these bryozoans, “less is more” was a winning formula for long-term survival.
Evolution in Bursts, Not a Smooth Decline
The study’s detailed time-series analysis also revealed that the 200-million-year size reduction was not a smooth, constant process. Instead, the data identified two distinct periods where the rate of change significantly accelerated. The first of these evolutionary bursts occurred between 165 and 160 million years ago, during the Middle Jurassic period. The second took place much later, around 78 million years ago in the Late Cretaceous.
These phased changes suggest that while the overall trend may be intrinsic, it could have been punctuated by specific events or triggers that are not yet fully understood. Identifying these “switches” in the tempo of evolution adds another layer of complexity to the story of Berenicea. The finding highlights that macroevolutionary patterns can remain consistent over vast timescales while also featuring dramatic shifts in their rate of change.
A New Perspective on Evolutionary Rules
This comprehensive study on Berenicea stands as a significant contribution to evolutionary biology. It provides a robust, long-term case study of a lineage defying Cope’s Rule, demonstrating that the drive toward larger body size is not a universal law. The continuous decrease in zooid size over 200 million years suggests that, for some organisms, downsizing is an effective and enduring evolutionary path.
By ruling out major external drivers like oxygen, temperature, and competition, the research points toward more subtle, internal drivers related to metabolism and feeding strategy. The findings encourage a more nuanced view of evolutionary trends, showing that the pressures shaping life are complex and can lead to surprising outcomes. The story of Berenicea illustrates that on the vast timeline of evolution, the path to success is not always about getting bigger, but about adapting in whatever way ensures survival.