When mammals became predators: what fossils show

Predation is older than the modern mammalian orders. Mammaliaforms lived alongside dinosaurs for much of the Mesozoic, and their diets varied. Some were small and likely fed on insects; others were larger and could take vertebrate prey. The fossil record does not identify one moment when all mammals became predators.

The strongest evidence comes from preserved meals, bite traces and anatomy considered together. Teeth can indicate cutting or crushing, but a tooth shape alone does not prove what an animal ate. A fossil association can document an interaction more directly, although the circumstances of burial still matter.

Repenomamus and a fossil meal

A Lower Cretaceous skeleton of Repenomamus robustus from Liaoning, China, preserves juvenile Psittacosaurus bones in the mammal's stomach region. This is direct evidence that this individual had consumed a young dinosaur. It also demonstrates that at least some Mesozoic mammals were capable of eating vertebrates rather than being restricted to insects.

A later fossil from the same region preserves a mammal and a Psittacosaurus in close contact. The authors interpret the posture and completeness of both skeletons as a predation event interrupted by sudden burial. That interpretation is supported by the fossil's arrangement, but the specimen records one exceptional event. It does not show that mammals routinely hunted dinosaurs or that every association between two skeletons represents an attack.

Many lineages, different diets

Mammalian predators later included several groups with different evolutionary histories. Carnivorans, which include cats, dogs and bears, are one branch. Oxyaenodonts and hyaenodonts were separate extinct carnivorous groups; they were once often lumped with mesonychians under the informal historical label “creodonts.” That old grouping does not represent one natural predator lineage.

In Paleogene ecosystems, carnivorans shared habitats with extinct mammal groups whose teeth and jaws were adapted for meat eating. A predator's body size and tooth design can help reconstruct its likely prey range, but a specific kill is rarely preserved. Tooth marks on bone, stomach contents and unusually clear associations offer stronger behavioural evidence than a skeleton found near a possible prey animal.

What the record can and cannot date

The earliest known fossil in a lineage gives a minimum age for its existence, not necessarily the exact time when predation evolved. Small fossils and soft remains are unevenly preserved, and taxonomic relationships can change when new material is found. Researchers therefore distinguish the first direct trace of a feeding event from the longer evolutionary history inferred from anatomy and phylogeny.

Fossils show that predatory mammals occupied varied niches before and after the end-Cretaceous extinction. They do not support a simple story in which all small mammals waited until dinosaurs disappeared and then instantly became large hunters. For the broader Mesozoic context, read about research on mammals of the dinosaur age and compare mammalian feeding evidence with the much older record of land predators.