The Influence of Oceanic Oxygen Levels on the Body Size Evolution of Trilobites

The evolutionary trends of organismal body size have long attracted the attention of biologists. Over the past century, several hypotheses have been proposed to explain why body size changes over time.

Beyond the well-known Bergmann's rule, which links body size to temperature and latitude, other principles have also been suggested. Cope's rule proposes that lineages tend to increase in size through evolutionary time, while Foster's rule suggests that habitat size and isolation can influence body size patterns. The hypothesis discussed here, however, remains controversial: the idea that atmospheric or oceanic oxygen levels influence the body size of animals, with higher oxygen concentrations allowing organisms to grow larger.

Historically, studies of body size evolution have focused primarily on vertebrates. Invertebrates have received comparatively less attention, with relatively thorough investigations limited to a few groups such as insects and brachiopods. Consequently, our understanding of body size evolution among early invertebrates remains incomplete and requires further research.

Trilobites（Image source：PaleoNeolitic， CC BY 4.0 ）

Trilobites provide an excellent opportunity to explore this question. As one of the most abundant and diverse groups of marine animals during the Paleozoic, trilobites possessed rapid evolutionary turnover and remarkable morphological diversity. These characteristics make them ideal organisms for studying body size evolution in early invertebrates. A joint research team from China and the United States therefore used trilobites as a model to investigate patterns of body size evolution among early Paleozoic marine invertebrates.

To conduct the analysis, the researchers assembled a dataset spanning the Cambrian and Ordovician periods. Their sample included 152 trilobite families, 1,091 genera, and 2,435 species, represented by measurements from 4,732 adult individuals. The length of the complete exoskeleton was used as a proxy for body size. Fossil occurrences were divided into 24 time intervals averaging roughly three million years each, allowing the researchers to examine changes in trilobite size through time with relatively high temporal resolution.

The four major paleogeographic regions sampled in this study and the body size changes of trilobite specimens from each region used in the analysis（Image source：Sun, Z et al. (2025)， CC BY 4.0 ）

The researchers also considered the possibility that different trilobite families might exhibit distinct evolutionary trajectories. To address this, they selected 24 representative families covering roughly 70 percent of the species in the dataset and examined whether body size evolution within these families showed consistent directional trends.

The results revealed that trilobite body size evolution can be divided into six distinct phases. Within each phase, body size remained relatively stable, but sharp changes occurred at the boundaries between phases. This pattern is characteristic of episodic evolution.

Episodic evolution refers to a mode of evolutionary change in which organisms remain relatively stable for long periods, followed by rapid shifts associated with environmental changes or the emergence of key biological traits. The term reflects the resemblance to dramatic shifts between acts in a play.

Specifically, trilobite body size underwent pronounced reductions during several intervals: Cambrian Stage 4 (approximately 514 million years ago), the Guzhangian stage of the Cambrian (around 500 million years ago), and the late Katian stage of the Ordovician (about 450 million years ago). Conversely, increases in body size occurred during the Drumian stage of the Cambrian (about 506.5 million years ago) and during the Tremadocian stage of the early Ordovician (around 480 million years ago).

Body size evolution of trilobites through time; the red line represents maximum body size, the blue line represents mean body size, and the shaded area indicates the 95% confidence interval（Image source：Sun, Z et al. (2025)， CC BY 4.0 ）

When examining individual families, most trilobite groups showed relatively stable body sizes through time and did not exhibit consistent directional trends toward larger size. This finding indicates that trilobite evolution does not conform to Cope's rule.

What could explain the abrupt shifts in body size between these evolutionary phases? When the researchers compared these timing patterns with known geological events, they discovered a striking correspondence between changes in trilobite size and fluctuations in marine oxygen levels.

For instance, the body size reduction during Cambrian Stage 4 coincides with the Sinsk biotic crisis and a widespread episode of oxygen depletion in the oceans. The size decrease observed during the Guzhangian stage corresponds with the SPICE event, a major carbon isotope excursion associated with expanded oceanic anoxia. Later, the termination of the BSAE anoxic interval aligns with a significant increase in trilobite size during the Tremadocian stage. Finally, the late Ordovician reduction in trilobite size coincides with the Hirnantian oceanic anoxic event.

The relationship between trilobite body size evolution and anoxic events during different geological intervals（Image source：Sun, Z et al. (2025)， CC BY 4.0 ）

The researchers also noted that the generally cooler environmental conditions of the Ordovician period did not appear to influence trilobite body size. This suggests that oxygen availability may have been the primary constraint. Only when oxygen levels exceeded a certain threshold might temperature or latitude have begun to influence body size patterns, which could explain why Bergmann's rule does not apply well to trilobites.

Another long-recognized idea in evolutionary biology is that larger organisms tend to face higher extinction risks during environmental crises because they require more energy to sustain their metabolism. The findings of this study are consistent with that expectation. Large trilobites required greater amounts of oxygen to produce energy, so when oxygen concentrations dropped, they were less able to survive and were more likely to disappear.

Interestingly, the researchers observed that the extinction of large-bodied trilobites often occurred before the main extinction events themselves. In other words, the average body size of trilobites had already begun to decline before the onset of the major biodiversity crises. A similar pattern has been documented in gastropods during the late Permian, where the disappearance of large species occurred three to eight million years before the end-Permian mass extinction.

This pattern suggests that size-selective extinctions may be partly decoupled from the main extinction pulses, which could explain why they have sometimes gone unnoticed in previous studies focused primarily on the timing of mass extinction horizons.

Overall, the results indicate that reductions in body size may respond more sensitively to environmental stress than declines in biodiversity. In other words, changes in body size may occur earlier than large-scale species extinctions.

This insight carries an important implication for modern ecosystems. In a world experiencing rapid climate change, shifts in organismal body size could serve as an early warning signal that ecological systems are under increasing environmental pressure.

Author: Bai Leng

Reference:

Sun, Z., Zhao, F., Zeng, H., Erwin, D. H., Zhu, M. (2025). Episodic body size variations of early Paleozoic trilobites associated with marine redox changes. Science Advances.

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