A flightless meat-eating bird that stalked South America 6m years ago overcame its prey by pecking the creatures to death with its huge skull and hooked beak, researchers say.

The bird, which resembled an emu, landed precision jabs on its victims, before withdrawing to a safe distance then attacking once more. Once its prey was dead, Andalgalornis steulleti, a species in the phorusrhacidae family or "terror birds", moved in and swallowed its victim whole or used its beak to tear morsels of flesh from the carcass.

"They used their good vision to make surgical strikes," said Lawrence Witmer, a paleontologist and co-author on the study at Ohio University. "Like Muhammad Ali, they would attack and retreat."

Scientists pieced together the bird's fighting style after taking x-rays of a skull of a terror bird found in north-west Argentina. The creature was 1.4 metres tall and weighed about 40kg (88lb).

The researchers, led by Federico Degrange at La Plata Museum, in Argentina, used the x-rays to create a 3D image of the skull, which was then analysed to see how stresses would build up when the bird delivered bites and shook its head.

Writing in the journal Plos One, the scientists say the creature's large, rigid skull, and strong neck muscles, made it formidable in swift, front-on attacks. But the skull had serious weaknesses that left the bird vulnerable to catastrophic skull fractures if it tried to shake its prey from side to side. "They used ambush when they could, moving rapidly on finding their prey. They were specialists in hatchet-like attacks," said Witmer.

Terror birds evolved in the region of South America about 60m years ago and diversified into 18 known species, some of which grew to more than three metres tall. "They would have been terrifying to humans," said Witmer. They lived alongside ancestors of chinchillas, ground sloths and porcupines, and extinct horse- and deer-like animals.

The birds are thought to have died out about 2.5m years ago. A change in the environment could have pushed them to the brink of extinction, Witmer said. Emily Rayfield, a paleontologist at Bristol University, said the weakness of the birds' skulls – the animal was vulnerable to catastrophic skull fractures if it tried to shake its prey from side to side – overturned the view that they attacked large mammals. Instead, the birds probably sought out smaller creatures including the cat-sized, hoofed animals called hemihegetotherium.

Monica Desai is a British Science Association media fellow