Paleontologists have identified a new species of giraffoid that lived in northern China during the Early Miocene epoch some 17 million years ago. Named Discokeryx xiezhi, the ancient creature had a thick-boned skull with a large disk-like headgear, a series of cervical vertebrae with extremely thickened centra, and the most complicated head-neck joints in mammals known to date. This peculiar morphology was most probably adapted for a fierce intermale head-butting behavior, comparable to neck-blowing in male giraffes. The researchers argue that selection for such combat also played a role in shaping the group’s long necks.
Male combat in Discokeryx xiezhi. Image credit: Xiaocong Guo.
The characteristic long neck of the modern giraffe — the tallest land animal and largest ruminant on Earth — has long been considered a classic example of adaptive evolution and natural selection since Charles Darwin first penned the concepts.
It’s commonly believed that competition for food drove neck elongation and allowed giraffes to browse for treetop leaves in the African savannah woodlands that were well outside the reach of other ruminant species.
However, others have argued a necks-for-sex hypothesis, suggesting sexual selection driven by intermale competition may have also contributed to neck evolution.
“Fossils of ancient giraffe species can help to clarify these evolutionary mechanisms,” said Dr. Shi-Qi Wang from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and colleagues from China, the United States, Germany, Switzerland and Austria.
The fossil community in the Junggar Basin, China, 17 million years ago. Image credit: Xiaocong Guo.
The newly-identified giraffoid species, Discokeryx xiezhi, had helmet-like headgear and particularly complex head and neck joints indicative of intense head-butting combat.
“Both living giraffes and Discokeryx xiezhi belong to the superfamily Giraffoidea,” Dr. Wang said.
“Although their skull and neck morphologies differ greatly, both are associated with male courtship struggles and both have evolved in an extreme direction.”
“Discokeryx xiezhi featured many unique characteristics among mammals, including the development of a disk-like large ossicone in the middle of its head,” said Professor Tao Deng, a researcher in the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, the CAS Center for Excellence in Life and Paleoenvironment, and the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences.
Moreover, tooth enamel isotope data from Discokeryx xiezhi suggest that the species also likely filled a specific ecological niche in the ecosystem unavailable to other contemporary herbivores.
“Stable isotopes of tooth enamel indicate that Discokeryx xiezhi was living in open grasslands and may have migrated seasonally,” said Dr. Jin Meng, a researcher at the American Museum of Natural History.
“For animals of the time, the grassland environment was more barren and less comfortable than the forest environment.”
“The violent fighting behavior of Discokeryx xiezhi may have been related to survival-related stress caused by the environment.”
According to the authors, the early evolution of giraffes is more complex than previously known, where, in addition to competition for food, sexual combat likely played an important role in shaping the group’s long and uniquely adapted necks.
Their paper appears in the journal Science.
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Shi-Qi Wang et al. 2022. Sexual selection promotes giraffoid head-neck evolution and ecological adaptation. Science 376 (6597); doi: 10.1126/science.abl8316
