450-million-year fossil record reveals warming seas are shrinking marine animals again, scientists sound alarm

A new study has found that marine animals have repeatedly shrunk during periods of intense global warming over the past 450 million years. The study suggests that the trend already being seen in today's oceans is part of a much older pattern rather than a modern anomaly.

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Researchers analysed nearly 9,000 recorded body size changes and more than 1.6 million individual measurements from fossils, historical records and living marine species. Their findings reveal that warming events consistently caused much greater reductions in body size than cooling events or periods of low oxygen alone.


The study, published in the journal Proceedings of the National Academy of Sciences (PNAS), offers one of the strongest pieces of evidence yet that rising ocean temperatures can reshape marine life on a global scale.

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Marine animals have been shrinking for hundreds of millions of years

The international team compiled one of the largest marine body-size datasets ever assembled, covering hundreds of millions of years of Earth's history.

Instead of focusing on a handful of species or one region, the researchers examined marine animals from oceans across the world and compared what happened during normal periods, environmental crises and recovery phases after those crises.

Their conclusion was clear: marine animals consistently became smaller during environmental upheavals. Whenever global warming triggered those crises, the shrinking was about twice as severe compared to environmental disasters caused by cooling or declining oxygen levels.

What is the Lilliput effect?

Scientists have long used the term Lilliput effect to describe the tendency of surviving species to become much smaller after major extinction events.

The name comes from the tiny people in Gulliver's Travels. Typically, after a mass extinction, surviving animals remain smaller for generations before gradually returning to larger sizes once environmental conditions improve.

This new research suggests that warming has been one of the strongest drivers behind that pattern throughout Earth's history.

Researchers say warming leaves the strongest fingerprint

The study was led by Paulina S Nätscher, a paleobiologist at Friedrich Alexander Universität Erlangen Nürnberg (FAU) in Germany.

Nätscher and her colleagues sorted thousands of marine records into three categories:

• Normal background periods
• Environmental crisis periods
• Recovery periods after crises

By comparing all three, they found warming consistently produced the largest declines in body size.

Until now, scientists had mostly relied on studies involving individual species or single fossil sites, making it difficult to know whether shrinking represented a global biological pattern or isolated events.

The new dataset helped answer that question.

Marine animals were actually growing smaller

One of the study's biggest findings is that this was not simply a case of larger species disappearing while smaller species survived.

Researchers found evidence of true dwarfing.

That means individual animals within the same species actually matured at smaller adult sizes.

Kenneth De Baets, a co-author of the study, noted that these effects tend to run about twice as strong during warming events compared to other forms of environmental stress.

The researchers also found that the largest ancient temperature increases generally matched the biggest drops in body size, although other factors—especially declining oxygen levels—also appeared to contribute.

Why warmer oceans make marine animals smaller

Scientists already understand much of the biology behind this process. Warm water naturally contains less dissolved oxygen than cold water.

At the same time, warmer temperatures increase an animal's metabolism, meaning it needs even more oxygen simply to survive. For cold-blooded marine animals such as fish, mussels and crustaceans, that creates a serious challenge.

As they grow larger, their bodies demand more oxygen than their gills can efficiently extract from increasingly oxygen-poor water.

Instead of continuing to grow, many species stop growing earlier and reach adulthood at a smaller size. Staying smaller becomes an advantage because it reduces the body's oxygen demand.

What it means for today's oceans

Researchers say the fossil record may offer an important warning for the future. Wolfgang Kießling, who leads a paleoenvironmental research group at FAU, believes ancient warming events provide a glimpse into how today's oceans may continue to respond as global temperatures rise.

The trend is already being observed in living marine ecosystems.

One previous study examining thousands of reef fish surveys across Australia found that average fish length declined by roughly 5% for every 2 degrees Fahrenheit increase in ocean temperature.

That matters because body size influences nearly every part of an animal's life, including:

• How much food it eats
• How many offspring it produces
• Its ability to avoid predators
• Its role within the marine food web

Smaller fish could eventually affect entire ecosystems, fisheries and food supplies that millions of people depend on.

A warning written in Earth's fossil record

The researchers say their study provides the clearest global evidence yet that ocean warming leaves a lasting biological signature.

Across hundreds of millions of years, warming repeatedly produced stronger and more unpredictable body-size reductions than other environmental stresses.

The shrinking already being documented in today's oceans appears to follow the same pattern recorded throughout geological history.

If ocean temperatures continue rising, researchers suggest many marine animals are likely to keep getting smaller—until warming eventually moves beyond limiting growth and begins pushing more species toward extinction.