THE world was an unfamiliar place 62 million years ago when Waimanu waddled along the New Zealand shore and slipped smoothly into the ocean. The dinosaurs had been dead for just a few million years and you wouldn’t have recognised most of the animals. The climate was much warmer than today. Yet anyone who spotted Waimanu would have known instantly that it was a penguin. A rather strange penguin, to be sure, but unmistakably a penguin.
Recently described in detail for the first time, Waimanu is part of a revolution in penguin palaeontology that, along with two other new fossils from Peru, is transforming our understanding of the evolution of these enigmatic birds. Perhaps more importantly, Waimanu could also help settle a question that has had palaeontologists and geneticists at loggerheads for years: when did modern birds first appear?
The evolutionary history of birds has always been a vexing question. We know that the first birds evolved around 150 million years ago, near the end of the Jurassic period, and that they then diversified rapidly. Most palaeontologists accept that birds evolved from dinosaurs. What is still not clear, however, is when the lineage leading to modern birds – the groups that we see around us today, including the penguins – first appeared.
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Cretaceous birds
If you look at bird fossils from the Creataceous period (145 to 65 million years ago), most of them belong to a group called the “opposite birds”, or enantiornithes, which disappeared with the dinosaurs at the end of the period. The rest are hard to place on the bird family tree, but none belongs unambiguously to any modern group.
In the absence of such fossils, palaeontologists proposed the so-called “big bang” theory of bird evolution. This holds that the mass extinction that wiped out the dinosaurs also killed off most of the birds. The few groups that survived evolved rapidly and gave rise to all modern birds.
So far so good, but when molecular biologists tackled the same question they came up with a very different answer. Using genetic differences between living birds to construct a genetic family tree, they put the origin of modern birds at about 100 million years ago. If correct, many groups of modern birds must have somehow weathered the asteroid impact that killed off the dinosaurs, while the opposite birds were wiped out.
Palaeontologists, unsurprisingly, were sceptical, suspecting that these “molecular clocks” failed to account for the rapid diversification after mass extinctions.
New fossil evidence
Two years ago, the a story took an important turn when Julia Clarke of North Carolina State University in Raleigh came up with new fossil evidence: the partial skeleton of a previously unknown bird. Found on Vega island off the coast of Antarctica in 1992, the skeleton initially proved inconclusive, but Clarke was able to glean enough detail to conclude that it belonged to the waterfowl, an unequivocally modern group that includes ducks, geese and swans. The fossil was 66 to 68 million years old. Clarke named it Vegavis ().
Vegavis provided strong evidence in support of the molecular clocks. Waterfowl are several steps up the evolutionary tree of modern birds, which meant not only that modern birds were around in the Cretaceous but also that the lineage must have been diversifying long before Vegavis lived.
Vegavis, however, was only a single fossil, which is why the ancient penguin Waimanu is so vital to the debate. Penguins are modern birds, and so provide a crucial second data point. “Waimanu is pivotal in showing modern birds must have had a Cretaceous origin,” says Ewan Fordyce of the University of Otago in New Zealand, who was part of the team that . Waimanu lived after the dinosaurs, but penguins are even higher up the avian family tree than ducks and geese. Using the two groups to calibrate a molecular clock pushed the origin of modern birds back to 90 to 100 million years ago.
Solid bones
Not everyone agrees with this. Alan Feduccia, an ornithologist at the University of North Carolina who proposed the “big bang” theory, still has doubts about the age and identification of Vegavis. For now, however, the molecular biologists have the upper hand.
Modern birds aside, Waimanu also has implications for another evolutionary mystery – the origin of penguins themselves. Until very recently, the penguin fossil record was patchy and their evolutionary origins obscure. Although penguin bones are solid and fossilise more readily than the hollow bones of flying birds, their skeletons are usually scattered before fossilisation, so bones tend to be found in isolation.
The first penguin fossil was just such an isolated scrap – a single ankle bone unearthed in New Zealand in the 1840s and described by Thomas Huxley in 1859. Huxley named it Palaeeudyptes antarcticus (ancient winged diver of the south). Typical for that time, the discovery was not well documented, so its age is unclear: it may be anything from 34 to 23 million years old.
Rich fossil beds
Until very recently, much of what was known about penguin evolution came from the rich fossil beds of Seymour Island off the Antarctic Peninsula. These have yielded numerous species of penguin dating from about 45 million to about 34 million years ago, which have revealed a great deal about extinct penguins. It is obvious that many were giants, towering over today’s largest species, the Emperor. But the identity of the earliest penguins remained murky.
Enter Waimanu. At 62 million years old, it is 15 million years older than anything from Seymour Island, opening a new window on penguin evolution.
The first Waimanu fossil was found in the 1980s near Waipara on New Zealand’s South Island, but the initial discovery – a single bone – didn’t reveal much beyond the fact that it was an “an ancient proto-penguin,” says Fordyce. However, bones from three more individuals have since been discovered, allowing Fordyce’s team to reconstruct a near-complete composite skeleton ().
From the skeleton it appears that the penguin’s immediate ancestors were probably diving birds, rather like modern diving petrels, which use their wings both to fly and to swim. That would make their closest living relatives tube-nosed birds such as petrels and albatrosses.
Ecological niche
Unfortunately we will probably never know for sure. Waimanu is already so far down the road to penguinhood that many clues to its ancestry have been erased. By 62 million years ago, Waimanu had evolved many features that make modern penguins so distinctive. It had short, sturdy wings to propel it through the water and solid bones to help it dive. It had grown to roughly the height of a present-day king penguin – around 90 centimetres tall – and had short stubby legs and an upright posture.
All of this suggests that penguins evolved rapidly to fill the ecological niche left by the extinction of the plesiosaurs and other marine reptiles at the end of the Cretaceous, says Dan Ksepka of the American Museum of Natural History in New York.
Just as Waimanu promised to answer many of the outstanding questions about penguin evolution, however, two new discoveries set palaeontologists a new conundrum. Another unknown about penguins was where they evolved, and why such highly-mobile birds have never colonised the northern hemisphere despite extending right up to the equator today.
Early in 2006, Allan Baker of the Royal Ontario Museum in Toronto proposed a solution. Based on genetic analysis of the 17 species of living penguin, he concluded that penguins evolved around 70 million years ago in the cool climes of Antarctica, recently separated from Gondwana. These cold-loving birds only spread northwards as the global climate cooled during the Eocene-Oligocene cooling event of 34 million years ago. (). Even today’s tropical penguins live in the cool waters of the Galapagos, and are unable to cross into the equatorial thermal barrier.
Waimanu fitted nicely with that idea: 62 million years ago New Zealand was close to Antarctica. However, in the latest twist, the two new penguin fossils found on the coast of Peru cast the idea into doubt.
Huge beak
One fossil, Perudyptes, dates from 42 million years ago. It lived just 14 degrees south of the equator and was about the same size as Waimanu. The other, Icadyptes, lived in the same location about 36 million years ago and stood at least 1.5 metres tall. Icadyptes is the most complete fossil penguin ever described (żěè¶ĚĘÓƵ, 30 June, p 19), with the only complete skull from an extinct penguin, including a stunning pointed beak. Clarke describes it as “absurdly elongate” and “very unpenguinlike”. Its shape suggests Icadyptes ate large fish, which it might have speared before swallowing.
The discovery of two large penguins in the tropics has got scientists scratching their heads. Today, the biggest penguins live in Earth’s coldest places, and their large size is thought to be an adaptation to conserve body heat. This is a fair assumption: larger bodies lose heat more slowly than smaller ones. What’s more, most of the extinct penguins from Seymour Island are very large, while today’s tropical species, the Galapagos and Humboldt penguins, are small. Yet here was Icadyptes, bigger than any modern penguin, and Perudyptes, close to the size of the largest living ones, living in the southern tropics at times when average global temperatures were several degrees higher than today.
“The discovery of giant penguins in the tropics has got scientists puzzled”
This is a serious challenge to Baker’s model, says Clarke, who described the fossils (). The waters off Peru were probably relatively cool at the time thanks to an upwelling of deep water, but were still much warmer than a modern penguin could tolerate. “There’s a lot we don’t understand about the physiologies of these species,” says Clarke.
As yet there is no clear picture of the evolutionary events linking Waimanu to the giant penguins of Peru. All that we know is that the tropical giants survived until about 24 million years ago, then went extinct. “We don’t know why,” says Ksepka. One possibility is that they were out-competed by whales and seals, which are thought to have been spreading around the globe at that time.
Facing extinction?
All species of modern penguins are part of a lineage that probably originated in the Antarctic some time around 20 million years ago. Their origins remain somewhat obscure because fossils are rare. The oldest modern penguin, Spheniscus muizoni, was recently found in Peru dating from 11 to 13 million years ago. Today, penguins are distributed across much of the southern hemisphere, from Antarctica through New Zealand, Australia, South Africa and South America, including many islands where they can nest with little fear of predators.
Although some early penguins thrived in warmer climates, all modern penguins are adapted to cool conditions. “Emperor penguins never set foot on land; they breed on Antarctic ice shelves,” says Ksepka. Even the Galapagos and Humboldt penguins of the tropics live in cool waters.
That does not bode well for penguins in a rapidly warming world. “If the ice shelves melt, Emperor penguins are gone,” warns Ksepka. The others may fare little better. We now know that the long march of the penguins began in a world that is much warmer than ours. It’s up to us to make sure it doesn’t end that way too.
