A new study is contradicting the idea that marine life has increasingly diversified over the last 200 million years. The long-term analysis of marine evolution shows that ocean biodiversity levels have remained largely consistent for at least 540 million years.
The magnitude of global ocean biodiversity has become a subject of debate in recent years. A research team led by experts in the School of Geography, Earth, and Environmental Sciences at the University of Birmingham set out to investigate using fossil evidence collected over the past two centuries available in the Paleobiology Database.
Based on this data, the research team was able to examine regional-scale patterns of ocean diversity dating back to the Cambrian Explosion, when major groups of animals started to appear in the fossil record.
“Studies of marine animal diversity over the last five-hundred-odd million years have historically focused on estimating how ‘global’ diversity changed through time. The problem is that the fossil record is not really global, because both the amount and the parts of the world that are actually preserved in the fossil record changes so much through geological time. This means that so-called global diversity curves are misleading,” explained study lead author Dr. Roger Close.
“To get around this problem, we studied diversity at regional spatial scales. This meant that we could focus on places and times that are well-known in the fossil record. By comparing geographic regions that were similar in size, we could show how marine animal diversity varied across both time and space.”
The team also estimated the influence of environmental factors, such as coral reef systems. By looking at diversity patterns on a localized level, the researchers were able to recognize significant variations across the globe within specific time intervals.
“We think of reefs today as being hotspots of diversity, responsible for housing a disproportionate amount of animal species. So in areas where there are a higher proportion of reefs, diversity will inevitably be higher,” said Dr. Close.
“Importantly, though we don’t find any evidence that diversity increased in a continuous, sustained way through long intervals of geological time. This is a major departure from previous studies of global diversity. These studies concluded that marine animal biodiversity had increased steadily over the last 200 million years, culminating in modern levels that were greater than any point in Earth’s history. In contrast, our work suggests that modern levels of biodiversity – at least at the regional scales we studied – are not exceptional.”
On the other hand, the researchers did pinpoint evidence of one substantial increase in marine diversity at the end of the Cretaceous period, when the dinosaurs became extinct.
“Not long after this devastating mass extinction, we see a distinct shift towards greater regional diversity. This probably had something to do with ecological reorganization after many species were wiped out. In particular, we see a rebound to much higher diversity among gastropods – a huge group of invertebrates that we would recognise as snails and slugs. This suggests that such a widespread species loss cleared space for other groups to explode – and gastropods were able to take advantage of this,” said Dr. Close.
“When you look at these individual animal groups, you can see fluctuations in diversity that are often substantial. But taken together, these patterns sum to one of constrained diversity. Some groups might benefit from the misfortune of others, but the overall levels of diversity that we see have remained fairly stable for hundreds of millions of years.”
The study is published in the journal Science.
By Chrissy Sexton, Earth.com Staff Writer