The Fightin’ Ibis: Xenicibis and Evolution’s Arrow

What comes next for evolution? This seems like a simple question. Every day we are learning more about the history of life on earth, and we would expect that, over 150 years since Charles Darwin published On the Origin of Species, the life of the past could be used to extrapolate the trajectory of evolution’s arrow. Regarding our own species, especially, there is a pervasive desire to know what our distant descendants might be like.

But this question is flawed. Even though there are evolutionary constraints – not everything we can imagine is possible – life is not following a tightly-restricted road of progress towards a pre-determined goal. Natural selection, the primary engine of evolutionary change, generates both diversity and disparity – Charles Darwin’s “endless forms most beautiful and most wonderful” – as organisms are constantly subjected to changing conditions. There is no way to create an evolutionary law or take everything into account so that we can predict what will exist one thousand, one million, one billion years in the future. That is part of the wonder of evolutionary science. We can’t know how the vast array of lineages we see around us today are going to change in years to come, but we still feel compelled to ask what the future holds.

This isn’t to say that we can’t have a little fun with speculation. In 1981 the Scottish geologist Dougal Dixon published one of the lasting cult favorites of paleontologists – After Man: A Zoology of the Future. Imagining the world 50 million years after our extinction – which, one takes it, is probably not all that far off in geological terms – Dixon takes the reader on a tour of the world’s ecosystems. There is an odd familiarity to all of it, seeing the prospective descendants of modern creatures twisted into new and bizarre forms. In the seas, fully aquatic descendants of penguins (the Vortex and the Porpin) have replaced the long-extinct whales, the long-legged rabbucks stride across the grasslands, and my personal favorite, a terrestrial bat Dixon calls the Night Stalker, “roams screeching and screaming” in search of prey under the cover of darkness.

(Dixon also wrote two similar books: The New Dinosaurs [1988], chronicling the evolutionary history of still-living non-avian dinosaurs, and Man After Man [1991], a grotesque gallery of future humans modified through bioengineering. After Man was arguably his most successful outing, however; it was turned into a documentary and cartoon in Japan.)

Dixon’s restorations are so delightful because they take creatures native to our own sliver of time and transform them into alien shapes. Probable or not, they are wonderful because they are both recognizable and alien. The same can be said of actual creatures which existed just yesterday in geological terms. We live in the wake of the earth’s last great extinction. (In fact, we are arguably still in the midst of it.) The giant sloths, saber-toothed cats, glyptodonts, mammoths, giant hyenas, and other impressive Pleistocene beasts only became extinct within the past 15,000 years or so. Nor were all the strange creatures charismatic, mammalian megafauna. One odd ibis, first described over thirty years ago, was unlike any other bird to have ever evolved.

It took nearly a century to recognize the strange character of this bird. Sometime between 1910 and 1920 the zoologist Harold Elmer Anthony collected a few bird bones from Long Mile Cave  in Trelawny Parish, Jamaica. They sat, virtually unnoticed, in the American Museum of Natural History collections for decades, but in 1977 the ornithologists Storrs Olson and D.W. Steadman used Anthony’s fossils to describe the bird as a previously unknown fossil ibis. They named it Xenicibis xympithecus. About the size of a chicken, this bird only recently became extinct (around 2,200 years ago according to a table included in the new book Holocene Extinctions.)

The initial description of Xenicibis was followed two years later by the discovery of a humerus (upper arm bone) found in Jamacia’s Swansea Cave. The unusual, robust nature of this bone led Olson and Steadman to believe that Xenicibis was flightless that had evolved on Jamacia. Along with another fossil flightless ibis – Apteribis glenos – described by Olson and colleague A. Wetmore in 1976, Xenicibis was one of the first flightless ibis to be recognized by science.

As more complete remains of Xenicibis were discovered, it became even stranger. One partial skeleton included the lower arm bones (radius and ulna) and the fused wrist and knuckles (carpometacarpus), the latter of which was a thick, banana-shaped bone unlike anything seen in other birds. Bits and pieces of other individuals have completed the skeletal anatomy, and Olson recently teamed up with paleontologist Nicholas Longrich to reexamine the weird anatomy of this flightless ibis. Their analysis has just been published in the Proceedings of the Royal Society B.

The main object of interest is the bird’s bizarre hand. The carpometacarpus – part of the bird’s “hand” you tear the meat off of while having chicken wings – is wider in diameter than the bird’s thigh bone (femur) and has extraordinarily-thickened walls. Behind this bone, the radius – a lower arm bone in line with the carpometacarpus – was unusually thick compared to the relatively slender companion bone, the ulna. Additionally, there was a modified groove – properly called the carpal trochlea – on the end of the carpometacarpus closest to the rest of the body which allowed the wrist to be quickly swung forward in a way different from other avians. This was not the arm of flying bird, but it also differed from the wings of many ground-dwelling birds. Rather than becoming reduced or vestigial, the wings of Xenicibis actually became heavier and more robust. (See the comparison of ibis wing skeletons below.)

There must be a reason why Xenicibis had such strange wings. The trouble is that no other bird had wings quite like it. Spurs, spikes, and other wing weapons are seen among birds like steamer ducks – which use them in combat with members of their own species – but nothing as extreme as the banana wings of Xenicibis. (The closest thing, Storrs and Longrich suggest, might be the formidable claws of mantis shrimp, though this analogy breaks down in terms of anatomy and function – imagine Xenicibis using its wings to trap and consume prey!) Being that many birds do have a modified carpometacarpus used for combat, however, it is a reasonable hypothesis that the odd wings of Xenicibis were used as some kind of club or flail. The bird could have swung its arms so that the wide, thick parts of the carpometacarpus near the tip of the wing struck its opponent.

It is easy to concoct adaptive stories for a particular trait. When paleontologist Henry Fairfield Osborn studied the small, incompletely-known arms of Tyrannosaurus rex in 1906, for instance, he famously proposed that the dinosaur’s small arms were used for “grasping during copulation.” Such an odd feature had to have a reason to exist, and Tyrannosaurus groping each other with their small arms seemed as good an explanation as any. This was wrong. We now know that the forearms of Tyrannosaurus were quite powerful and probably used like meathooks to helping stabilize prey. (The “meat-eating bull” dinosaur Carnotaurus, on the other hand, had truly vestigial forelimbs.) Obviously the wings of Xenicibis are quite different from the forelimbs of its even more ancient coelurosaurian cousin, but the scientific lesson is the same – it is not enough to look at a structure, speculate about what it could be used for, and leave it at that. As Longrich told LiveScience about the process of investigating Xenicibis, “We threw around all kinds of ideas — that the bird used the wings for climbing, or for digging, or even that the bird moved on all fours and it used the wings to help it walk.” (This last idea has been floated before in abstracts and on mailing lists, but never studied in-depth.) Other lines of evidence must be pursued to test the idea and investigate possible alternatives.

Fortunately, there are a few indications that Xenicibis truly was a fightin’ ibis. Other than the fact that some modern birds fight with their wings, Longrich and Olson cite two broken arm bones which show signs of healing. One, a humerus, began to heal after being split in two, and the other, a carpometacarpus, has a large callus of bone which probably grew in response to a fracture. These findings are consistent with the idea that these birds were using their wings to strike each other.

What remains unknown is what individual Xenicibis were fighting and why. The most plausible answer is “Other Xenicibis.” Since there is no indication that the anatomy of the weapon differed between males and females, Longrich and Olson propose that Xenicibis fought each other over territory, particularly for good nesting spots. The wing clubs could have also been used to fight off predators, too, and this brings up an important point. We can generate ideas about the function of the bird’s wings by looking at their anatomy and come up with multiple possible uses, but identifying those uses does not tell us why those particular traits evolved in the first place. The clubs of Xenicibis may have evolved as a result of ibis-on-ibis combat and could also be used to fight predators once they had already evolved, or perhaps the clubs are some sort of exaptation – a structure used for one function which became co-opted for something else. Given the state of the evidence, it is likely that Xenicibis used its clubs in combat with each other, but we should not be so careless as to say that the clubs evolved for that function.

The actual manner in which Xenicibis fought is not discussed in the paper – I assume that will be the focus of further research – but Longrich and Olson propose that the unique weaponry of this bird underscores a grander theme of evolutionary change. Of all the birds which have spurs, spikes, and clubs on their wings, none was quite like Xenicibis. Despite similar evolutionary pressures – fighting one another for territory, mates, or some other resource – multiple species of birds from various lineages have been adapted to have very different weaponry, with Xenicibis having the oddest of all. Even when selective pressures are the same, evolutionary quirks and contingencies can generate quite different outcomes.

As strange as it may sound, prehistoric creatures like Xenicibis make Dixon’s speculative beasts seem almost conservative in their anatomy. Many of the creatures featured in After Man are products of convergent evolution, testaments to the power of natural selection to funnel organisms down similar channels from disparate starting points. Convergence is a real and significant part of life’s pattern, but chance and contingency can open up unique evolutionary possibilities that cannot be predicted or foreseen. The more we learn about the fossil record, the stranger life becomes, and though I am a little saddened by the fact that I will never know what life is going to be like 50 million years from now, I am left in awe by the fantastic creatures which inhabited this planet in ages past.

References:

Nicholas R. Longrich, and Storrs L. Olson (2010). The bizarre wing of the Jamaican flightless ibis Xenicibis xympithecus: a unique vertebrate adaptation Proceedings of the Royal Society B : 10.1098/rspb.2010.2117

Osborn, Henry Fairfield; Brown, Barnum (1906). Tyrannosaurus, Upper Cretaceous carnivorous dinosaur Bulletin of the American Museum of Natural History, 22 (16), 281-296

WILLIAM SUA´REZ (201). Deletion of the Flightless Ibis Xenicibis from the Fossil Record of Cuba Caribbean Journal of Science, 37 (1-2), 109-110