The biggest penguin that ever existed was a ‘monster bird’

New Zealand has been a haven for earth-bound birds for eons. The absence of terrestrial predators allowed flightless parrots, kiwis and moas to thrive. Now researchers are adding two prehistoric penguins to this grounded aviary.

One species is a beefy penguin behemoth that waddled along the New Zealand coastline nearly 60 million years ago. At almost 160kg, it weighed as much as an adult gorilla and is the heaviest penguin known to science.

Mr Alan Tennyson, a palaeontologist at Museum of New Zealand Te Papa Tongarewa, discovered the supersize seabird’s bones in 2017. They were deposited on a beach known for large, cannonball-shaped concretions called Moeraki Boulders. The churn of the tide cracked open several of these 57-million-year-old boulders, revealing bits of fossilised bones inside.

Mr Tennyson and his colleagues identified the fossilised remains of two large penguins. The humerus (upper arm bone) of one, at more than 24cm long, was nearly twice the size of those found in emperor penguins, the largest living penguin.

Other boulders yielded bones from a smaller, more complete penguin species that also appeared to be larger than a modern emperor penguin.

The researchers described the ancient birds in the Journal Of Palaeontology. They named the larger penguin Kumimanu (a mash-up of the Maori words for “monster” and “bird”) fordycei and named the smaller penguin Petradyptes (“rock diver”) stonehousei.

By creating 3D models of Kumimanu’s humongous humerus and comparing its size and shape with the flipper bones of prehistoric and modern penguins, the researchers estimate that the “monster bird” weighed a whopping 154kg.

According to Dr Daniel Ksepka, a palaeontologist at the Bruce Museum in Greenwich, Connecticut, and an author of the new study, the Kumimanu’s fragmented skeleton makes it difficult to pinpoint its height. He estimates that it stood at 1.6m, giving it a stocky build.

Petradyptes was not a lightweight, either. It weighed 49.9kg, making it heftier than modern emperor penguins, which top out at 39.9kg.

Both Kumimanu and Petradyptes plied the waters off New Zealand during a sweet spot in oceanic history, according to Dr Ksepka. The asteroid impact that ended the dinosaur era had wiped out most marine reptiles while the ancestors of seals and whales were still on land. This meant there were few things that would mess with a black-bear-sized penguin.

Dr Ksepka said a 136kg penguin is not going to “worry about a seagull landing near it because it would just crush it”.

Despite their prodigious size, Kumimanu and Petradyptes possessed primitive flippers reminiscent of modern seabirds like auks and puffins that fly and dive.

Professor Julia Clarke, a palaeontologist at the University of Texas at Austin who studies the evolution of diving in birds and was not involved in the new study, said it would make sense for early penguins like Kumimanu and Petradyptes to retain several features left over from their ancestors’ airborne lifestyle.

The new species add evidence that prehistoric penguins became huge before they fine-tuned their flippers into paddle-like appendages. Heavier seabirds are able to dive deeper and longer than their lighter counterparts, Dr Ksepka said.

The extra paunch would also have helped these penguins stay warm in the water.

While Kumimanu was mighty, it did not crowd out its smaller penguin cousins. “You have super large penguins eating the largest prey items and you also have mid-sized and smaller-bodied penguins, and they can all specialise within a crowded penguin environment,” Prof Clarke said.

Despite plenty of seafood and little competition, these penguins could probably grow only so big.

“I believe that Kumimanu is close to the upper limit of a flightless seabird and I do not expect substantially larger penguins to be found,” said Dr Gerald Mayr, a paleontologist at the Senckenberg Research Institute in Frankfurt, Germany. Dr Mayr, who was not involved in the new study, notes that heavier birds would most likely crush their eggs into yolky smithereens.

As some of the earliest fossil penguins, Kumimanu and Petradyptes reveal just how quickly penguins packed on kilos after they stopped taking to the skies.

“Once you know you’re not flying anymore,” Dr Ksepka said, “the sky’s the limit.” NYTIMES