Mammal ancestors evolved a killer instinct

Before dinosaurs, ancestors of mammals were the top predators. Research suggests these first large land carnivores evolved new tools to hone their skills as hunters.

The battle between predator and prey was heating up on land during the 60 million years before the ‘Age of Dinosaurs’ began 252 million years ago. As herbivores grew larger, ancient carnivores needed to find ways to give themselves an edge.

Palaeontologists in Britain looked at jaw anatomy and body size of carnivorous ‘synapsids’ to track these evolutionary changes.

Infographic showcasing the differences in jaw functional anatomy and body size in ancient predatory synapsids
Infographic showcasing the differences in jaw functional anatomy and body size and the potential ecological inferences found in the study of more mammal-like behaviours among ancient predatory synapsids. Credit: Artwork by Suresh A. Singh. Photo Insert Credit: Kruger Sightings HD.

Synapsids are a group which today includes humans and all other mammals. Ancient synapsids, which dominated during the Permian period (299–252 million years ago) used to be known as “mammal-like reptiles.” They were not reptiles at all, but the evolutionary ancestors of mammals. Now, they are sometimes referred to as “stem mammals” or “proto mammals.”

In the recent study in the journal Communications Biology, the researchers explained a major shift in jaw function about 270 million years ago. They theorise that came about due to a significant change in predatory behaviour in our earliest mammalian ancestors.

Sail-backed dimetrodons walking across shallow lake salt plain
Sail-backed Dimetrodons. Credit: Mark Stevenson / Stocktrek Images / Getty.

“Earlier synapsid predators such as the famous sail backed Dimetrodon [pictured above], had fairly long jaws with lots of teeth to ensure that once they ensnared their prey, it wouldn’t escape,” says lead author Dr Suresh Singh from the University of Bristol. “However, we saw a shift in jaw function toward shorter jaws with greater muscle efficiency and fewer teeth that were concentrated at the front of the jaw – these were jaws adapted to deliver deep, powerful bites.

This change brought Earth face-to-face with the first sabre-toothed carnivores like the Gorgonopsids pictured below.

Predators were beginning to evolve the ability to deliver powerful blows to their prey, causing massive injuries and blood loss. Larger, faster herbivores required new ways of hunting.

“The risks to carnivores of getting injured or killed went up, so some synapsid carnivores became bigger, better killers to overcome these risks,” says co-author Dr Tom Stubbs from Open University.

“The late Palaeozoic was the time when animals first began to live, eat and reproduce entirely on land,” says study co-supervisor Mike Benton, a professor at Bristol. “They became fully terrestrial, colonising new habitats and exploiting new resources further inland from the aquatic environments they’d previously relied on.”

A leopard coated lycaenops hunts among a forest
Lycaenops. Credit: Daniel Eskridge / Stocktrek Images / Getty.

“Our findings show how the selective pressures on these early land animals changed as they became better adapted for life on land – catching another animal that can move fast and grow to larger sizes is much more difficult than catching a slippery little fish or amphibian.”

“Predator-prey interactions are an important driver of animal behaviour today so it’s quite something to see that influence through anatomical evolution over millions of years, and find that they are potentially responsible for driving some big leaps in our own evolutionary history,” says University of Bristol professor and co-supervisor on the study Emily Rayfield.

“It highlights how palaeontologists can use the relationship between form and function to explore how different prehistoric animals may have lived, which can tell us so much about the evolution of life on Earth.”

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