Ancient oceans show tropical diversity was not permanent

Today’s tropical peak has a deep but changeable history

Modern oceans are often described through a familiar geographic pattern: marine life is generally more diverse in tropical waters and becomes less diverse toward the poles. Scientists call this broad pattern the latitudinal diversity gradient. It is so widespread in contemporary ecology that the tropics can seem like the permanent, natural center of biodiversity.

A new fossil study suggests the pattern has been far less fixed over geological time. Researchers at Nanjing University and University College London analyzed marine invertebrate fossils spanning roughly 500 million years and found that the strength and shape of the latitudinal diversity gradient changed alongside long-term shifts in Earth’s climate. The work was published in Proceedings of the Royal Society B: Biological Sciences.

The finding does not challenge the richness of today’s tropical seas. Instead, it places that richness in a longer planetary context. The distribution of marine diversity appears to be responsive to climate state, rather than an unchanging feature of the ocean.

Using fossils to reconstruct ancient geography

The researchers drew on records in the Paleobiology Database, a public resource that compiles published evidence of past life. Their focus was marine invertebrates, whose fossil record can reveal how biodiversity was distributed across the globe at different points in the Phanerozoic Eon.

Fossils do not arrive with a ready-made map of their original locations. Continents move, coastlines shift, and the rock containing a fossil may be far from where the organism lived. To address that problem, the team reconstructed ancient geographic locations for the fossils. They then divided the world into equal-area grid cells about 250 kilometers apart and estimated diversity across latitude.

That approach also had to account for a persistent limitation of paleontology: uneven sampling. Some regions and geological intervals have been studied much more intensively than others. A place with more fossil collectors and more exposed rocks can look artificially diverse if raw fossil counts are compared directly.

The team therefore used coverage-based rarefaction, a statistical approach designed to make diversity estimates more comparable when sampling effort differs. In practical terms, it helps separate a genuine biological signal from the unequal chances that organisms were found, preserved, reported, and entered into the database.

Climate and diversity moved together

The resulting record indicates that marine latitudinal diversity gradients were not constant through time. Their intensity and overall form varied in association with long-term climate changes. That makes the present-day tropical maximum an outcome of a particular climate context, rather than a universal template for marine ecosystems.

Lead author Die Wen said the project grew from a question raised by earlier work: whether the global distribution of biodiversity differed between warmer and colder climate regimes in Earth’s history. The new analysis was designed to test whether the modern tropical peak is permanent or instead linked to the current climate state.

That distinction matters because climate is not merely a background condition for ocean life. It influences temperatures, circulation, habitat extent, sea levels, and the geographic conditions under which species persist and diversify. Over hundreds of millions of years, major changes in climate can therefore be expected to reshape where biological diversity is concentrated.

The study’s central contribution is the scale of its view. Rather than treating the modern ocean as the default and asking only why its tropics are diverse, it asks how the relationship between latitude and diversity has changed across a substantial portion of animal life’s history.

Why the result matters now

The authors’ reconstruction is not a direct forecast of how contemporary ecosystems will respond to current environmental change. Fossil data describe patterns over immense timescales, while present-day changes are unfolding on a much faster human timescale. Still, the work provides an important caution against treating today’s geographical pattern of biodiversity as immutable.

For researchers studying marine ecosystems, the result reinforces the value of combining modern observations with deep-time records. Current conservation and climate questions often focus on where biodiversity is concentrated now. The fossil record adds evidence that the locations and gradients of diversity can be reorganized as planetary conditions change.

It also highlights why methodological care is essential in historical biodiversity research. The fossil record is necessarily incomplete, and geographic coverage is uneven. By reconstructing paleogeography and standardizing estimates using coverage-based rarefaction, the researchers sought to make comparisons across regions and intervals more robust.

A dynamic map of life

The tropical oceans of the present remain extraordinarily important reservoirs of marine biodiversity. But the new research portrays that distribution as a dynamic map rather than a fixed rule. Over the Phanerozoic, the relationship between latitude and marine diversity changed as Earth moved through different long-term climate states.

That perspective turns an apparently simple ecological pattern into a historical one. The concentration of marine life near the equator is not just a property of latitude. It is also part of a changing interaction between climate, geography, and evolution over deep time.

This article is based on reporting by Phys.org. Read the original article.