Global warming 40 million years ago transformed the size of Europe’s carnivorous mammals

Europe’s ancient predators did not vanish in a neat evolutionary handoff. For millions of years, early relatives of cats, dogs, and bears lived beside powerful competitors, even as climate change repeatedly reshaped their ecosystems.

An international team reconstructed body-mass changes among 155 fossil species spanning nearly the entire Paleogene, from about 57.2 million to 23.3 million years ago.

The results challenge a long-standing explanation for the rise of Carnivoramorpha, the group containing modern carnivorans and their close extinct relatives.

That theory held that carnivoramorphans gradually displaced rival meat-eating mammals because their teeth gave them greater ecological versatility. Much of the argument, however, came from North American fossils.

The European record tells a different story.

Early in the Eocene, carnivoramorphans and hyaenodonts were relatively small. Both groups remained below about 2 kilograms.

Larger predator roles instead belonged mainly to oxyaenodonts and mesonychians, which commonly reached between 10 and 100 kilograms.

“Body mass is a powerful clue of the ecological role of continental animals,” said Valentin Fischer, director of the Evolution & Diversity Dynamics Lab at the Université of Liège and co-lead author.

Because complete skeletons rarely survive, the team estimated body mass from teeth.

“Because of these strong bonds between ecology and body mass, it is actually possible to reliably infer the body mass of ancient predators by measuring key traits in their dentition,” Fischer said.

Different equations matched different groups. Carnivoramorph estimates used the first lower molar, while hyaenodont and oxyaenodont estimates relied on average molar length.

Researchers then followed minimum, maximum, average, median, and variation in body mass through time.

A major shift appeared around the Middle Eocene Climatic Optimum, or MECO, approximately 40.75 million to 40 million years ago.

After that warming event, both hyaenodonts and carnivoramorphans became larger.

Carnivoramorphans changed most dramatically.

Their body masses spread across a range from roughly 300 grams to 28 kilograms. Their variation in body size also became substantially greater than that of hyaenodonts.

Yet hyaenodonts did not immediately collapse.

Shortly before the Eocene-Oligocene transition, their average mass had climbed to about 21 kilograms. Carnivoramorphans averaged less than 6 kilograms.

That coexistence matters because it does not match a simple replacement scenario.

If carnivoramorphans had directly displaced hyaenodonts as they expanded into new body-size niches, the researchers expected a corresponding decline among their rivals.

Instead, the groups overlapped for millions of years.

Earlier changes had already altered Europe’s predator community.

Large oxyaenodonts and mesonychians disappeared after the Early Eocene Climatic Optimum. Their loss caused a sharp decrease in the maximum body size of European carnivorous mammals.

Equivalent large carnivoramorphans did not immediately take their place.

The gap between the disappearance of large earlier predators and the arrival of similarly sized carnivoramorphans lasted about 10.4 million years for animals around 20 kilograms.

For predators near 100 kilograms, the delay stretched to roughly 21.3 million years.

Those long intervals weaken the idea of an immediate competitive takeover.

Instead, carnivoramorphans gradually occupied a broader range of sizes as environmental conditions changed.

Statistical analyses identified the period following the MECO as an especially important breakpoint in European predator evolution.

A second transformation arrived near the Eocene-Oligocene boundary about 34 million to 33.5 million years ago.

This period includes the European faunal turnover known as the Grande Coupure.

By then, carnivoramorphans had already been expanding their body-size range for roughly 7 million years.

The later transition brought major losses among endemic hyaenodonts, early carnivoraforms, and hyainailourines. New carnivoran groups also dispersed and diversified.

Hyaenodont body masses then began declining during the early Oligocene, while carnivoramorphans continued increasing.

By the middle Rupelian, the largest European carnivorans had surpassed the hyaenodonts living alongside them.

Some approached 100 kilograms.

The timing again tied major biological changes to environmental upheaval rather than to a steady competitive replacement.

The North American fossil record produced a substantially different pattern.

There, predator body-mass variation was greatest during the first half of the Eocene and peaked around the middle Lutetian.

Carnivoramorphans were also more diverse than hyaenodonts through nearly all the Eocene and Oligocene intervals examined.

North America did not show the same dramatic carnivoramorphan body-size expansion during the MECO.

Its major statistical shifts appeared earlier, during the late Ypresian for hyaenodonts and the early Lutetian for carnivoramorphans and the broader predator community.

Europe’s largest transformation instead centered near 40 million years ago.

That continental contrast argues against one universal evolutionary contest producing the same outcome everywhere.

The analysis does not show that competition played no role.

Body mass captures only part of an animal’s ecology, and it cannot by itself explain every interaction among predators.

The researchers also tested several evolutionary models. One model containing three shifts in evolutionary trends received the strongest support in 84% of one set of evolutionary trees and 66% of another.

Still, the models generally did not identify enduring changes in preferred body size or long-term size trends for carnivoramorphans and hyaenodonts.

Their ranges could change greatly while averages remained comparatively stable.

That pattern fits a history shaped by multiple ecological pressures rather than one steadily advancing lineage.

The European fossil record changes how researchers can test broad explanations for mammalian evolution.

Patterns derived mainly from one continent may not describe what happened elsewhere.

Comparing regions can reveal whether an apparent competitive replacement reflects a general biological rule or a local response to climate, migration, and changing ecosystems.

The findings also give researchers a clearer timeline for investigating how warming and cooling episodes reorganized predator communities.

Rather than portraying modern carnivores as inevitable winners of a single evolutionary contest, the evidence points to lineages repeatedly responding to regional environmental change.

That distinction matters for reconstructing how the ancestors and relatives of today’s cats, dogs, bears, and other carnivorans acquired their ecological diversity.

These resources explore how climate, ecological competition, migration and major faunal turnovers shaped carnivorous mammals and other mammal communities during the Eocene and Oligocene.

Long-fuse evolution of carnivoran skeletal phenomes through the Cenozoic
Using more than 850 specimens from nearly 200 living and extinct carnivoran species, this research links major changes in carnivoran body form to climate transitions, including the Eocene-Oligocene cooling that opened new ecological opportunities. (Proceedings of the Royal Society B, 2025)

Two major ecological shifts shaped 60 million years of ungulate faunal evolution
This large-scale analysis traces how mammal communities responded differently across Europe, Asia and other regions, highlighting the Grande Coupure and the importance of climate, migration and regional biogeography in producing uneven evolutionary change. (Nature Communications, 2025)

Middle Eocene Climatic Optimum (MECO) and its imprint in the continental Escanilla Formation, Spain
Researchers reconstructed a terrestrial record of the roughly 40-million-year-old MECO in the Spanish Pyrenees, finding warm, generally arid conditions and demonstrating how this major warming episode affected continental environments. (Climate of the Past, 2024)

Drivers of the artiodactyl turnover in insular western Europe at the Eocene-Oligocene transition
This analysis examines the forces behind the Grande Coupure, when European mammal communities underwent extensive extinction and replacement, helping clarify how climate change, environmental disruption and immigration interacted during the Eocene-Oligocene transition. (Proceedings of the National Academy of Sciences, 2023)

Dietary niches of creodonts and carnivorans of the late Eocene Cypress Hills Formation
This North American study compares the ecological niches of hyaenodonts and carnivorans and argues that climate change, rather than straightforward competitive superiority, may better explain the later success of modern carnivorans. (Journal of Mammalogy, 2022)

Research findings are available online in the journal Biology Letters.

The original story "Global warming 40 million years ago transformed the size of Europe’s carnivorous mammals" is published in The Brighter Side of News.

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