A classic, 1917 restoration of North America's Gastornis.
Read Caption
From Matthew, Granger, and Stein, 1917.
A classic, 1917 restoration of North America's Gastornis.

Giant Prehistoric Bird Crushed Seeds, Not Little Horses

If the huge bird were still alive, Gastornis would be an ornithophobe’s nightmare. Equipped with an extraordinarily deep beak, this six-foot-tall bird was among the largest creatures to roam the forests of prehistoric Europe and North America. And it’s exactly that imposing avian aspect which spawned a long-running fossil meme about the ancient bird.

While not so huge as the largest non-avian dinosaurs, Gastornis was nevertheless a giant in its Paleocene and Eocene heyday between 55 and 40 million years ago. In Europe the bird towered over the mammals who inhabited the same forests – the largest herbivores and carnivores of the day were about the size of a German shepherd, with many being considerably smaller. (In North America, where Gastornis fossils were previously labeled “Diatrymapreviously labeled “Diatrymapreviously labeled “Diatryma, some of the contemporary herbivorous mammals grew to bigger sizes, but there were still many smaller beasts running about.) So it seemed only natural that the monstrous bird would have preyed on the scurrying mammals, pouncing on “dawn horses” and cleaving lemur-like primates in two with it’s powerful beak. In museums and documentaries, Gastornis marked the last gasp of dinosaur dominance before mammals took over the world.

[In this clip from Walking With Beasts, Gastornis reprises the classic apex predator role.]

But recent research has found that Gastornis wasn’t so terrifying, after all. While a 1991 paper concluded that the bird’s beak could have made short work of many small mammals, other publications pointed out that such a beak would have been just as well-suited to cracking seeds and crunching tough fruit. More recently, tracks of Gastornistracks of Gastornis – née “Diatryma” – found in Washington show that the bird had blunted toes rather than vicious talons, and a preliminary study of dietary clues preserved in the bones of a German specimen of the bird suggested a menu of plants rather than flesh. And now paleontologist Delphine Angst and colleagues have added another line of evidence that Gastornis probably wasn’t a rapacious mammal-muncher.

Chemical signatures in the extinct bird’s bones are at the center of the Naturwissenschaften study. Angst and coauthors studied carbon isotope (δ13C) traces in the bones of Gastornis, small herbivorous mammals the bird lived alongside, and modern hawks and ostriches. This isotope acts as a proxy for diet. Generated inside plants, the carbon isotope becomes preserved in the tissues of herbivores that eat those greens and, further down the line, in the tissues of the carnivores that consume those herbivores. Locked in bones and teeth, this carbon isotope allows paleontologists to outline what individual animals were consuming and how they may have split up resources in the same habitat.

By themselves, these chemical traces don’t automatically label an individual as an herbivore, carnivore, or omnivore. What the carbon isotope values mean rely on comparison and analysis. In the case of Gastornis, Angst and colleagues used the data to investigate the opposing views of the bird as carnivore or herbivore.

Part of what makes carbon isotopes useful in paleontology is that they can be tied back to different types of plants that photosynthesize in different ways. This detail is  what led  Angst and coauthors to throw out the idea that Gastornis sliced small mammals. If the big bird was a carnivore, the researchers found, then the carbon isotope signatures inside it’s bones would indicate that it ripped open prey that, in turn, relied upon C4 plants – grasses and other plants that rely on a distinct form of carbon fixation. The snag is that plants didn’t evolve that C4 method of photosynthesis until about 14 million years after Gastornis lived. The chemical trace didn’t match up with the ecology of the time.

View Images
The skeleton of Gastornis on display at the National Museum of Natural History. Photo by Brian Switek.

When Angst and coauthors looked at the carbon isotope through the lens of Gastornis being an herbivore, however, the signature was a better match and was comparable to those of herbivorous mammals living at the same time. The bird’s carbon isotope profile was that of an avian that crushed seeds and crunched thick-skinned fruit.

And the researchers went a step further. Through dissections of modern birds ranging from Darwin’s finches to Eurasian sparrowhawks, Angst and colleagues studied the anatomy and connection points of the external adductor muscle in modern herbivorous and carnivorous birds. This is a major muscle that powers bird bites, and the herbivorous, seed-cracking birds typically had wider muscles with increased space for attachment on the lower jaw. That fits they way they feed. Much more power is needed to bust open hard fruits than to tear soft flesh.

The actual muscles of Gastornis rotted away over 40 million years ago, but the bird’s lower jaw shows a wide space for the external adductor muscle to attach. Taken together, the bird’s beak, feet, reconstructed musculature, and chemical signature best fit a large herbivore that snacked on plants rather than the mammals that lived underfoot.

Does this mean that Gastornis never took a swipe at little Eohippus? Of course not. Animals are not so strict about a herbivorous or carnivorous diet as our species can be. Gastornis may very well have snagged the occasional unwary mammal or tried carrion. But the bird’s anatomy did not evolve for a life of predation like that of the famous “terror birds”, and the traces in bone suggest a life primarily fueled by vegetation.

Visions of Gastornis as an herbivore towering over the heads of Paleogene mammals is stronger than ever before. But there is still one line of evidence that could finally cement this shift in paleo imagery. If you want to know about a prehistoric animal’s diet, there’s no better source of information than the food that actually passed through their mouth – gut contents, mashed-up food preserved in the intestines (cololites), and fossil feces (coprolite). As far as I’m aware, no one has yet described such trace fossils with Gastornis, but such finds are not outside the realm of possibility. With stomach scraps or a prehistoric pellet in hand, paleontologists could finally outline Gastornis grub.