The shallow-water cradle of vertebrate evolution

October 25, 2018

The first vertebrates on Earth originated and diversified in the shallow-water lagoons lining the mid-Paleozoic coastline, a new study finds. The results help to fill a gap in the understanding of early vertebrate diversification, a true grasp of which also requires understanding these organisms' earliest environments. While we know some things about early vertebrate evolution, the role of habitat on their early diversification has remained largely unknown and is widely debated. Opinions on the matter range from freshwater areas to the open ocean and are often based on evolutionarily distant species or small samples of select fossils. However, due to a lack of early vertebrate fossils and ancient habit data, the environmental context of early vertebrate evolution has been difficult to ascertain. To address this question, Lauren Sallan and colleagues analyzed fossil vertebrates spanning the mid-Paleozoic (aged between 480 and 360 million years ago), as well as the environmental markers that indicated associated ancient habitats. According to Sallan et al., the results suggest that all major groups of early vertebrates, including both jawed and jawless fish, originated and diversified within nearshore intertidal and subtidal environments, over a period of 100 million years. As these nearshore populations diversified, adaptations in body plans allowed them to spread out into other environments. The more strongly built creatures remained in the nearshore or freshwater habitats, while more gracile body forms colonized deeper water to avoid increasing competition as the shallow and confined nearshore habitats filled with life, suggest the authors. In a related Perspective, Catalina Pimiento underscores the role of nearshore environments as a persistent cradle for species diversification, even today. "The work of Sallan et al. shows us that without shallow-water habitats, vertebrates like us would probably not have evolved. Worryingly, it is precisely these ecosystems the ones that have been altered the most by human activities," writes Pimiento.
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American Association for the Advancement of Science

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