Ancient Giraffe Relative Was Evolution’s Headbutting Champion, Perhaps Besting Dinosaurs

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Natural selection propels the giraffe family to absolute extremes–and it is not just about the absurdly long necks

One of contemporary giraffes’ ancient relatives, Discokeryx xiezhi, had head-neck joints more complex than any known mammal, physiology that appears to have developed for headbutting battles among males. Another example of extreme morphology is the necks of modern Giraffes. This may have been due to sexual competition. Credit: Y. Wang and X. Guo

When paleontologist Jin Meng uncovered a strange skull in the vast, dry expanse of northern China’s Junggar Basin in 1996, he immediately had a hunch about the favorite activity of the ancient animal it came from. The skull was strong and well-built, with a bony bone plate that was nearly one-inch thick around the area where the animal would have had its forehead. Meng found nearby a few neck vertebrae that were thickened. This suggests they were built to withstand tremendous force. He realized that this new species may have outperformed dinosaurs in the violent sport known as headbutting.

For years, Meng, who is now curator in charge of fossil mammals at the American Museum of Natural History in New York City, and his colleagues at the Chinese Academy of Sciences simply called their discovery guai shou, or “strange beast.” Now the strange beast has an official name: Discokeryx xiezhi. As Meng and his colleagues described today in Science, D. xiezhi lived some 16.9 million years ago and was an early relative of modern day giraffes. Unlike living giraffes, whose necks, most researchers have traditionally thought, primarily evolved for foraging at the tops of trees, D. xiezhi‘s thick skull and vertebrae were almost certainly the result of sexual competition. As the researchers surmised, D. xiezhi males butted heads over mates with a force perhaps never before seen in the animal kingdom and never seen since.

” When we talk about giraffes people immediately think of the elongation in the neck.” Meng said. “But this species is another example of extreme adaption, showing that even phylogenetically related animals can evolve in completely different directions .”

In the mid-Miocene of northern China, the desert habitat was warm and wet, making it suitable for a wide range of species to thrive. Meng and his colleagues used a variety of clues to piece together D. xiezhi‘s story at that time. They used enamel from a tooth to analyze it and CT scans of two skulls that they had recovered to see their internal structure. The researchers also compared the animal’s remains with fossils of more than 50 other species they found in the same area, most of which were ungulates like D. xiezhi. Taken together, the evidence indicated that D. xiezhi shared some morphological characteristics with modern giraffes and was likely a grazer, perhaps feeding on a mix of leafy plants and grasses.

D. xiezhi was not that large, perhaps the size of a big sheep, but Meng and his colleagues found that the species’ head and neck were perhaps some of the strongest ever possessed by a mammal–and maybe any earlier creature, too. The researchers characterized D. xiezhi as having “the most complicated head-neck joints in mammals known to date.”

As a measure of just how extreme D. xiezhi‘s headbutting morphology was, consider this comparison: Pachycephalosaurus was a dinosaur famous for headbutting–its name means “thick-headed lizard”–but dinosaur experts that Meng and his colleagues consulted with confirmed that D. xiezhi‘s unique head and neck structure probably permitted it to withstand even more force.

Fierce battles for females take place among modern male giraffes (Giraffa) as well. But while D. xiezhi shares a family tree with Giraffa, modern giraffes are not direct descendants of the ancient species. Male giraffes use the neck to fight, not the head. The authors suggested that evolution of the elongated necks could have been for fighting, not just to reach higher to get to the foliage. They noted that behavior could have had a strong impact on morphological development …, with extreme behavior leading giraffoids to extreme morphological change.

” The bottom line is that the head structure in the giraffe family has high diversity, according to the new fossils,” Meng said. These specialized morphologies reflect their diverse lifestyles .”

Advait Jkar, a Yale University paleobiologist, says that the evolutionary reasons for modern giraffes having a long neck are still a mystery. Both male and female giraffes have long limbs, as well as a long neck. He says that the evolution of modern giraffe necks was likely due to natural selection.

As for D. xiezhi, though, its “headgear almost certainly evolved as a result of sexual selection and male-male combat,” Jukar says. “If you think modern Giraffes look odd, their deeper time relatives were even stranger .”

ABOUT THE AUTHOR(S)

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    Rachel Nuwer is a freelance science journalist and author who regularly contributes to Scientific American, the New Yor

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