With their immaculate dinner-suit plumage, smiley beaks and cute waddling gait, penguins are some of the most unusual and endearing members of the bird kingdom. A new fossil find, however, has revealed that one of their ancestors was a far more fearsome beast.

The fossil, which was discovered in Peru and is described today in detail by scientists reveals a creature that was over 1.5 metres tall and weighed as much as a person. The 36 million year old tropical bird's intimidating appearance was topped off with powerful forearms, a chunky neck and a potentially vicious 18 centimetre long spear-like beak. The discovery of the giant penguin is also shaking up scientists' understanding of penguin evolution. The finds indicate that penguins made the journey to equatorial regions much earlier in their evolutionary history than researchers had realised.

And because the penguins lived during a period when the Earth was experiencing a 'greenhouse' climate the pair of species are challenging what researchers thought they knew about how species adapt to hotter temperatures.

"It's a monster," said Julia Clarke at North Carolina State University who described the fossils with her team in Proceedings of the National Academy of Sciences today. The two main finds are remarkably complete and well preserved. "The bone preservation is extremely good," she said, "We have so few relatively complete penguins from that period of penguin evolution." The detail is so good that the researchers were able to see fine patterning on the beak of the giant penguin left by a sheet of keratin - the material that makes up feathers.

The giant species has been christened Icadyptes salasi, while Perudyptes devriesi, was around half as big. If it were alive today, Icadyptes would tower over the largest penguins in the planet - the emperors. Familiar to fans of natural history documentaries, their epic migration across the Antarctic wilderness to bring food to their chicks was celebrated in the film March of the Penguins. But even these modern day giants were only around half as big as their prehistoric ancestor.

The team do not have any direct evidence for the penguins' diet, but the wings of both birds are adapted for swimming and both were found in sediments laid down just off shore. Icadyptes' elongated beak would have been capable of snaring large fish, but its shape is so unusual, the team believe it used a previously unknown technique for catching prey. "It is distinct from anything we have in living penguins," said Prof Clarke. Attachment points for neck muscles are also very large suggesting that it had a powerful neck, perhaps for spearing prey.

The discovery goes against the general rule that as climatic conditions get warmer, species tend to evolve a smaller body size. The theory is that large size is useful in the cold because it reduces the ratio of surface area to volume, making it easier to conserve heat. But Icadyptes was found in a region that resembled the modern day Atacam desert at a time when the Earth was experiencing an extremely warm period in its history. The researchers speculate that there may have been an increase in ocean upwelling at the time around what is now the Peruvian coast. This would have fertilised the food chain leading to an abundance of fish and so easy pickings for the mammoth birds. The find also contradicts the idea that penguins did not reach equatorial regions until between 4 and 8 million years ago, well after a cooling period had set in that began to swell the ice caps at the poles. Today, only one species - the Humboldt penguin - is found on the coast of Peru.

The team are keen to point out that although these species were adapted to a tropical lifestyle, it does not mean that current penguin species will be able to adapt quickly to rising temperatures associated with man-made climate change. "Current global warming is occurring on a significantly shorter timescale. The data from these new fossil species cannot be used to argue that warming wouldn't negatively impact living penguins," said Prof Clarke.

The Icadyptes fossil is the most complete of any giant penguin yet discovered. But it may be smaller than the largest giant known. Anthropornis nordenskjoeldi or Nordenskjoeld's Giant Penguin, which is thought to have lived up to six million years ago, could be up to 2 metres high and weighed in at nearly 100 kilogrammes. It is known from fossils on Seymour Island off Antarctica and New Zealand. The Peruvian monster penguin and its older cousin were discovered in the Ullujaya Valley and Quebrada Perdida respectively. Both locations are in the Ica Department of Peru.