17-million-year-old fossil may solve an evolutionary mystery about giraffes
Giraffes are also into necking, but not the human variety.
It’s 1996 and paleontologist Jin Meng is exploring the Junggar Basin in Xinjiang, China. The valley is nestled between the mountainous borders of Kazakhstan and Mongolia in the northwest. The Basin consists of layered sedimentary rocks atop older igneous and metamorphic rocks that formed in the times of Pangea, more than 500 million years ago.
Suddenly, he notices an odd specimen sitting in the sand. This discovery becomes one of the highlights of his career: Discokeryx xieshi, a fossil from the early Miocene that’s nearly 17 million years old. Now, we know more about Discokeryx xieshi looked like and how it lived, which was quite, shall we say, heady.
New research shows Discokeryx xiezhi was a giraffe-like animal with a few stark differences from Earth’s tallest land mammal.
D. xiezhi, whose species name comes from the word for a mythical Chinese beast, lacked the giraffe’s iconic neck, for example. But the fossil tells paleontologists like Meng a story of what kind of life this animal led millions of years ago and how it might inform the evolution of the giraffe’s unique morphology. According to Meng, the story involves a lot of head-butting.
Head-butting, of course, requires proper headgear, and the animals that developed this headgear are called ruminants. Some ruminants have antlers and others have horns, while giraffes’ headgear is known as ossicones. Ossicones are bony protrusions from the head that are covered in skin, and also grace the heads of okapi, the giraffe’s only living relative.
If you’ve ever wondered why giraffes have long necks and why they have ossicones, a new study by Meng and his colleagues published today in the journal Science examines how the two may be related. D. xiezhi could be the lynchpin.
D. xiezhi had a thick, flat head plate that was optimal for headbutting.
What’s new — This paper proposes giraffes may have evolved long necks from sexual selection, and the origin comes from D. xiezhi.
Today, giraffes’ long necks serve an important purpose in feeding, fighting, and avoiding predators. In particular, the giraffe’s way of fighting, called necking, could stem from short-necked ruminants’ preferred method of fighting, headbutting. Ruminants needed headgear for fighting because they’re herbivores, so their teeth are more suited for grinding vegetation rather than cutting through flesh. They can’t deliver deadly bites like lions, so they smash their skulls.
Necking for giraffes is also a mating display, but not in the same way it is for humans. Male giraffes neck other male giraffes to vie for a female sexual partner. The male with superior headgear and articulated neck has the upper hand, so its traits get passed down after mating.
D. xiezhi’s horn was a large, flat disk about 5 centimeters thick and made of keratin, which is what human fingernails and rhino horns are made of. The word Discokeryx translates to “disk-horn” in Greek, Meng says. Curator-in-charge of fossil mammals at the American Museum of Natural History, Meng explains that the flat horn and complex neck skeleton made D. xiezhi likely the best head basher in the history of ruminants.
“I think its main point is that it shows that sexual selection has clearly played a role in the neck morphology of an ancestral giraffe, paving a gateway for it to be equally likely in modern-day giraffe,” agrees Rob Simmons. He is a behavioral ecologist and honorary research associate at the University of Cape Town’s FitzPatrick Institute of African Ornithology. Simmons was not involved in the study.
While Meng traversed the Junggar Basin, Simmons wrote about the idea of “necks-for-sex” in a 1996 study suggesting that giraffes’ long necks are a sexually selected trait.
The complex neck and head bones of D. xiezhi helped it absorb incredible shock waves from intense headbutting.
Why it matters — By looking at an ancient giraffe ancestor’s combat equipment, there are indications that giraffes developed a long neck for more than just feeding. This proposal “opens up the door to sexually selected pathways in the giraffe lineage,” Simmons says to Inverse.
To Nikos Solounias, an evolutionary biology professor at the New York Institute of Technology, this paper doesn’t show the same novelty as earlier work. Solounias says that proposing D. xiezhi is a giraffe ancestor is like saying that since monkeys and humans have some traits in common, then humans evolved directly from monkeys.
“Every ruminant I can think of fights,” he tells Inverse. “They all do that. That’s why they have horns, as a way [for] the male to fight the other males.” He says that while this paper thoroughly describes this fossil, ultimately it presents no new information. Solounias was not involved in the study.
On the other hand, Simmons emphasizes that this finding doesn’t mean headbutting led directly to the contemporary giraffe’s long neck. Rather, it makes a case for sexual selection among giraffe ancestors. This analysis makes room for another explanation for how giraffes came to be the gentle giants we know.
Up front is a rendering of D. xiezhi males headbutting, while in the background there are the giraffes we know and love fighting with their necks.
Digging into the details — Along with a thick, keratinous head plate, D. xiezhi had a complex system of head and neck joints. The hinges and thickness of these joints were optimized for the animal to take all the shock of a full-force head butt.
Now, among ruminants, there are myriad diverse types of headgear, even within a family. This diversity is yet another point in favor of sexual selection, showing that animals with superior skulls and headgear could win in combat and mate successfully.
The neck-tastic creatures we know and love today as giraffes don’t headbutt each other, but rather swing their heads at each other, sometimes poking one another with the ossicones. They’re much smaller than the headgear on moose, rams, muskoxen, and other animals who aggressively headbutt.
What’s next — Meng’s dream is to find a fossil with teeth still intact, but for now, he and his team are content to continue broadening their knowledge with the fossils they do have. He tells Inverse that he and his team want to trace this adaptation through the evolution of different vertebrate lineages.
Solounias believes horns merit priority over teeth.
“If I had this fossil, I would study the internal structure of the horn,” he says. Prolibytherium, another early artiodactyl ungulate, had delicate horns, and he’d want to see how this proto-giraffoid’s horn compares to that species.
For his part, Simmons wants more information on the giraffe’s shorter-necked ancestors to understand how giraffes alone started reaching the sky and what their early headgear looked like. The okapi, he writes, also deserves more research.
It’s “a secretive deep forest species that we know almost nothing about in the wild,” he points out.
