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.

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