Australasian Science Magazine

Its 5–6 metre wingspan, narrow wing shape and light skeleton would have made Pelagornis an efficient long-distance glider. Credit: Peter Trusler/Museum Victoria

By Erich Fitzgerald

For 55 million years, giant seabirds with serrated beaks successfully soared above the waves before vanishing 2.5 million years ago. Now fossils uncovered in Melbourne show for the first time that these bizarre birds called Australia home and reached every continent, deepening the mystery of their extinction.

“It’s not a penguin.” That thought occurred to me as I stood among Museum Victoria’s vast Geosciences Collection pondering the identity of a 9 cm-long honey-coloured fossil. It was 2004, and I was a PhD student immersed in the esoteric world of fossil whales.

So why was I studying a bird? How did I know that this one bone was not the relic of a primordial penguin? And if it wasn’t a penguin, what kind of bird was the former owner of this bone? Why does it matter anyway?

The solution to these riddles spans 60 million years of the Earth’s history, 7 years of human endeavour and the entire globe. But it begins with a chance discovery on a Melbourne beach. Let me explain.

By a wonderful coincidence of geology and human history, a window into the Southern Ocean’s past occurs within metropolitan Melbourne at Beaumaris, a suburb just 20 km south-east of the city centre. This gateway to deep time is manifested in sandstone that forms cliffs, erodes as beach boulders and pebbles, and lines the shallow floor of Beaumaris Bay. These sedimentary rocks were laid down in a shallow coastal sea during the late Miocene and early Pliocene epochs 5.0–6.0 million years ago. This comparatively recent twinkle in the eye of geological time bore witness to a world in which we would find much that is familiar, as well as the startlingly unexpected.

The incessant millennia of erosion at Beaumaris has yielded a bounty of fossils cataloguing an entire ecosystem of marine animals, and even the occasional landlubber mammal whose bodily remains must have floated seaward to join its salty relatives. The most abundant of all the marine mammals entombed within the sandstone of Beaumaris are the cetaceans – whales and dolphins – and these drew my attention at this remarkable site.

In 2004 a Museum Victoria field team was prospecting along the shores of Beaumaris Bay, with keen eyes peeled for the slightest hint of whale bones or a reflective flash from tooth enamel among the flaming orange-brown rocks. After much searching, one fossil in particular announced itself, eroding from the side of a boulder as if poised for collection and study.

Upon careful excavation from the surrounding sandstone, and return to the museum, I examined the bone and quickly concluded that this was no whale. Indeed, it was not even a mammal. It was the shinbone, or tibiotarsus, from the leg of a bird.

This realisation was not too surprising, as dozens of extinct penguin wing and leg bones, as well as the bill of an albatross, had been collected from this site over the past 100 years. Hence we already knew that the fossils of birds could be preserved in the sandstone of Beaumaris.

The limb bones of penguins have a tendency to join the annals of life’s history as fossils thanks to a peculiar property that distinguishes them from most other bird bones. Whereas the wing and leg bones of flying birds are virtually hollow and have extremely thin walls as a weight-reduction measure, those of the flightless penguins are extremely dense and have thick walls and almost solid interiors.

But as I closely inspected the shaft of the newly discovered shinbone from Beaumaris I observed that the bone was both extremely thin-walled and that the interior of the shaft was filled with sediment. This meant only one thing: this was not the fossil bone of a penguin. This was the shinbone of something else entirely. But what?

My knowledge of bird anatomy enabled me to deduce that this must be the leg bone of a flying bird. A big flying bird. It would be another 7 years before I learnt just how big this bird was and what kind of bird left its remains at the bottom of the sea 5 million years ago.

In the meantime, a PhD thesis had to be finished, followed by postdoctoral research at the Smithsonian Institution in the USA. In 2009 I returned to Melbourne and Museum Victoria to continue my research on the evolution of whales.

Nevertheless I continued to wonder about the bird fossil from Beaumaris. What I needed was the expertise of palaeontologists whose speciality lay in bird anatomy and evolution. Only with their assistance would it be possible to remove the shroud of questions surrounding this fossil.

Help arrived in 2011 in the form of a student, Travis Park, and an authority on fossil birds, Dr Trevor Worthy. Park, a student at Deakin University with an aptitude for palaeontology, had begun volunteering at Museum Victoria while completing the third year of a Bachelor’s degree. He was keen to pursue studies on fossil birds, so I suggested he have a look at the mystery bone from Beaumaris. Worthy, from the University of Adelaide, joined in the project to lend his wealth of knowledge on all that is ancient and avian.

Park and I began comparing the Beaumaris fossil with the shinbone of a representative species from each family of living birds. We narrowed the comparative set to include only the largest of living birds capable of flight, as well as those with aquatic lifestyles.

It soon became apparent that the fossil shinbone from Beaumaris was unlike that of any living bird family –and it was much larger! Even the shinbone of the wandering albatross, which has the longest wingspan of any living bird (3.6 metres), is dwarfed by the fossil from Beaumaris.

Having ruled out a close affinity with any living bird species, we sleuthed through the fossil record to determine whether any extinct groups of birds had a shinbone like our fossil.

Only one family of birds proved to be a match: the Pelagornithidae. It dawned on us that not only had we discovered the first evidence of Pelagornithidae from Beaumaris, but the first proof of the existence of this group of birds in all of Australia. And thanks to recent research on fossils from Chile and Morocco, we were able to sharpen our identification of the Beaumaris fossil as a species of Pelagornis, which means “pelagic bird”. At last, the mysterious fossil bird bone had been identified.

But this was only the first step on the fossil trail to enlightenment. Exactly what were pelagornithids? As is often the case when uncovering the fossil record, fact can prove stranger than fiction.

In reality, pelagornithids were not really similar in appearance to any living bird. If you imagine looking at a living pelagornithid, you might see something that looks a bit like a pelican but has the wings of a super-sized albatross.

But even this comparison with living pelicans and albatrosses would not do justice to the most bizarre quality of these birds: their huge beak with long tooth-like fangs projecting from its edges. Not true teeth like yours and mine, these serrations and fangs are in fact bony outgrowths of the bones that form the upper and lower jaws. Hence the common name often given to pelagornithids is “pseudodontorns”, which means “false-toothed birds”.

What did Pelagornis and its kin do with its “false teeth”? Pending a detailed anatomical analysis of the “teeth” and beak of these birds, we can only speculate. However, it’s likely that the “false teeth” helped the pelagornithid beak to become an effective tool for grasping and trapping slippery squid and fish. These prey were perhaps caught by skimming the surface while gliding low over the sea. The long and narrow wing bones of Pelagornis indicate that pelagornithids were efficient long-distance soarers, not unlike the albatrosses of today – only bigger.

Just how big pelagornithids were has only recently been estimated with confidence after the discovery of a nearly complete Pelagornis skeleton in Chile. The wingspan of the skeleton alone is 4.5 metres, but if we factor in the minimum estimated length of primary flight feathers, the wingspan must have exceeded 5 metres and may have been greater than 6 metres! This is twice the wingspan of the living wandering albatross, and indeed the largest accurately estimated wingspan of any known bird. Because the Pelagornis shinbone from Beaumaris is almost equal in size to that of the skeleton from Chile, my colleagues and I estimate that the Australian Pelagornis was approximately the same size as the largest known pelagornithid.

Exactly where pelagornithids perch in the evolutionary tree of birds remains one of the great controversies of fossil bird research. Some scientists have reasonably suggested a close relationship with albatrosses, while others think that pelicans are the closest living relatives of these “false-toothed birds”.

More recently, it has been argued that these winged marvels are members of a large group of waterfowl and their cousins that includes ducks, geese and chickens. If this idea stands the test of future research, then your humble roast chook had some rather more grand extinct family relatives!

Yet the plot of pelagornithid pedigree thickens even more and becomes mysteriously murky. Intriguingly, there are details in the anatomy of their skull that may link pelagornithids with flightless birds such as the kiwi and emu. Watch this space.

While it is clear that pelagornithids have some relationship issues, their antiquity is better understood. Pelagornithid fossils have been found in rocks dating back to the Paleocene epoch (about 60 million years ago), which is only about 5 million years after the extinction of the non-bird dinosaurs and many other organisms.

The fact that pelagornithids were already airborne 60 million years ago hints at an even longer evolutionary history that we’ve yet to dig up. What we do know is that pelagornithids achieved a wide distribution early in their history, even reaching Antarctica about 35 million years ago. By the end of the Miocene epoch (about 6 million years ago), pelagornithids inhabited Europe, both the Pacific and Atlantic coasts of North America, New Zealand, Japan, Morocco, the west coast of South America and Australia.

The most recent pelagornithid fossils are about 2.5 million years old and have been found in Morocco and California, showing that these birds survived in both the Atlantic and Pacific Oceans until the end of the Pliocene epoch. Then, after successfully soaring above the waves across the globe for over 55 million years, the giant “false-toothed birds” vanish from the fossil record, presumably extinct. What happened?

This is perhaps the greatest mystery surrounding the pelagornithids. By 2.5 million years ago, the world was experiencing climatic upheaval as the cycle of ice ages that have prevailed right up to the present day set in, and the Earth cooled. The distribution of major ocean currents also shifted as tectonic forces closed the sea gap between North and South America.

Elsewhere on coasts and in the sea, the diversity of marine mammals was experiencing a revolution of sorts, with the diversification of modern species still alive at present. The modern world was dawning.

Perhaps, for the largest flying animals since the extinction of the pterosaurs, these relatively rapid environmental changes were too much. After more than 55 million years of evolution, the time of the pelagornithids had finally run out.

With the extinction of the pelagornithids, and that of the pterosaurs before them, the long history of truly giant aerial animals has reached its end – for now.

Today, you might be lucky enough to spot a magnificent wandering albatross and, squinting against the wind and sunlight, you might imagine a bird twice its size with fangs growing from the edges of its beak. But then step into the time machine of your mind, cast yourself back to 2.5 million years ago, and picture one of your early ancestors walking along the high water mark of an east African beach. Perhaps they too gazed out to sea and, squinting in the equatorial sun, spied a Pelagornis: the last of the flying giants.