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A strange fossil solves the evolutionary mystery of the giraffe

Un extraño fósil resuelve el misterio evolutivo de la jirafa

An analysis of head and neck fossils from an ancient giraffe ancestor, published in Science, has solved the evolutionary mystery about these animals.

Specifically, examination of a disc-shaped helmet and highly complex joints between the head and neck suggests that the long necks of modern giraffes arise because they needed them for the head-butting matchup that ensued.

This is an alternative explanation since the characteristic long neck of the modern giraffe, the tallest land animal and the largest ruminant on Earth, has long been considered a classic example of adaptive evolution and natural selection ever since Charles Darwin first wrote these concepts.

It is widely believed that competition for food drove neck elongation and allowed giraffes to forage for leaves in the canopy of trees in the forests of the African savannah, which were off limits to other ruminant species. However, others have defended the hypothesis of "neck for sex"suggesting that sexual selection driven by competition between males may have also contributed to the evolution of the neck.

In particular, giraffes use their two- to three-meter-long swinging necks to launch their heavy skulls — equipped with small ossicones and osteomas — against the weak parts of competitors. Consequently, the longer the neck, the more damage the opponent suffers.

Researchers at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and their collaborators carried out their study on Discokeryx xiezhi, a strange primitive giraffe.

This research contributes to understanding how the long neck of the giraffe, as well as to understand the extensive integration of courtship fights and feeding pressure. In fact, the size of the neck of male giraffes is directly related to social hierarchy, and courtship competition is the engine of the evolution of long necks.

The fossils in this study were found in Early Miocene strata of about 17 million years ago on the northern margin of the Junggar Basin in Xinjiang. A complete skull and four cervical vertebrae were part of the find.

"Discokeryx xiezhi had many features unique among mammals, such as the development of a large disc-shaped ossicle in the center of its head."Professor DENG Tao of IVPP, corresponding author of the study, explains in a statement, adding that the unique ossicle resembles that of the xiezhi, a one-horned creature from ancient Chinese mythology, which gives the fossil its name.

According to the researchers, the cervical vertebrae of Discokeryx xiezhi are very robust and have the most complex joints between the head and neck and between the cervical vertebrae of any mammal.

The team showed that the complex joints between the skull and cervical vertebrae of Discokeryx xiezhi were uniquely adapted to high-velocity head-to-head impacts. They found that this structure was much more efficient than that of modern animals, such as musk oxen, which are adapted to head impact. In fact, Discokeryx xiezhi may have been the vertebrate best adapted to head impact.

"Both live giraffes and ‘Discokeryx xiezhi’ belong to the Giraffoidea, a superfamily. Although their skull and neck morphologies differ greatly, both are associated with male courtship fights and both have evolved in an extreme direction."points out WANG Shiqi, first author of the study.

The research team compared the morphology of the horns of various groups of ruminants, including giraphoids, cattle, sheep, deer, and pronghorn. They found that the diversity of horns in giraffes is much greater than in other groups, with a tendency for extreme differences in morphology, indicating that courtship fights are more intense and diverse in giraffes than in other ruminants.

The research team also analyzed the ecological environment of Discokeryx xiezhi and the niche it occupied. The Earth was in a warm period and, in general, with a high density of forests, but the Xinjiang region, where the ‘Discokeryx xiezhi’ lived, was somewhat drier than other areas because the Tibetan plateau, to the south, was had risen sharply, thus blocking the transfer of water vapor.

"Stable isotopes of dental enamel have indicated that ‘Discokeryx xiezhi’ lived in open grasslands and may have migrated seasonally", says MENG Jin, another corresponding author of the study. For the animals of the time, the environment of the grasslands was more arid and less comfortable than that of the forest. The violent fighting behavior of Discokeryx xiezhi may be related to survival stress caused by the environment.

At the beginning of the appearance of the genus Giraffa, a similar environment existed. About seven million years ago, the East African Plateau also changed from a forested environment to open grassland, and the direct ancestors of the giraffes had to adapt to the new changes.

It is possible that, among the ancestors of giraffes during this period, mating males developed a way of attacking their competitors by swinging their necks and heads. This extreme struggle, supported by sexual selection, thus led to the rapid elongation of the giraffe’s neck over a period of two million years until it became the current genus, Giraffa.

Thanks to this elongation, giraffes were well adapted to the feeding niche in high foliage. However, their ecological status was necessarily less secure than that of bovids and cervids. As a result, Giraffa’s marginal ecological niche may have promoted extreme intraspecific courtship competition, which in turn may have promoted extreme morphological evolution.


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