Strange Fossils Reveal The Evolution Of Giraffes

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Giraffes are distinguished by their extremely long necks. In fact, their necks can grow to 7.9 feet (2.4 meters) Although there are various hypotheses about the evolutionary origin of these giraffes, they do not have enough evidence, which makes them an unsolved mystery

Charlesdarwin put forward the "competing browsers" hypothesis, which basically means that giraffes evolved because they could expose them to food that competitors could not touch. It makes sense, but is it true?

According to a new study led by researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS), a strange early giraffe fossil reveals the key driving force of giraffe evolution.

The study was published in [Science] on June 2, 2022( https://www.science.org/doi/10.1126/science.abl8316 ) 》In magazines.

How the giraffe's long neck evolved has long been an evolutionary mystery. Although there have been different views on the process of giraffe neck elongation, scientists have never doubted that the power of neck elongation is high leaf.

However, with the increase of observations on giraffe behavior, scientists began to realize that giraffe's elegant long neck is actually a "weapon" in male courtship competition, which may be the key to the mystery of giraffe evolution.

Specifically, giraffes use their swinging necks two to three meters long to throw their heavy skulls -- equipped with small bones and osteomas -- to attack weak parts of competitors. Therefore, the longer the neck, the greater the damage to the opponent.

IVPP researchers and their collaborators studied discokeryx Xiezhi, a strange early giraffe. This study will help to understand how giraffe's long neck evolved, and also help to understand the extensive combination of courtship struggle and feeding pressure. In fact, the neck size of male giraffes is directly related to the social hierarchy. Courtship competition is the driving force of giraffe evolution.

The fossils in this study were found in the early Miocene strata about 17million years ago in the northern margin of Junggar basin, Xinjiang. A complete skull and four cervical vertebrae are part of the discovery.

Professor dengtao of IVPP, the corresponding author of the study, said: "the deer has many unique characteristics in mammals, including the development of a large disc-shaped bone in the middle of its head." Deng Tao said that this single bone plate is similar to the insect. It is a single horned unicorn in ancient Chinese mythology. Therefore, the fossil was named the insect.

According to the researchers, the cervical vertebra of the animal is very thick. Among all mammals, it has the most complex joints between the head and neck and the cervical vertebra. The research team proved that the complex joint between the skull and cervical vertebra of the deer is particularly suitable for high-speed head-to-head impact. They found that this structure is much more effective than existing animals, such as musk deer, which are adapted to head impact. In fact, the Cervus nipponensis is probably the most adapted vertebrate to head impact in history.

"Both the living giraffes and the giraffes belong to the giraffe family, a superfamily. Although their skulls and necks are very different in shape, they are related to the male courtship struggle, and both have evolved in an extreme direction," said wangshiqi, the first author of the study.

The team compared the horn morphology of several groups of ruminants, including giraffes, cattle, sheep, deer and giraffes. They found that the diversity of giraffes' horns was much greater than that of other groups, and there was a trend of extreme differences in morphology, which showed that giraffes' courtship struggle was more intense and diverse than that of other ruminants.

The research team further analyzed the ecological environment and the niche it occupied. At that time, the earth was in a warm period with dense forests, but the Xinjiang region where the deer lived was drier than other regions, because the Qinghai Tibet Plateau in the South had been rising sharply, which hindered the transfer of water vapor.

Mengjin, another corresponding author of the study, said: "the stable isotopes of the enamel show that the deer lived in open grassland and may have seasonal migration. For the animals at that time, the grassland environment was more barren and uncomfortable than the forest environment. The violent fighting behavior of the deer may be related to the survival related pressure caused by the environment."

In the early stage of giraffe, there was a similar environment. About 7million years ago, the East African plateau also changed from a forest environment to an open grassland. The direct ancestors of giraffes had to adapt to the new changes. It is possible that in the giraffe ancestors of this period, mating males developed a way to attack competitors by swinging their necks and heads. Therefore, with the support of sexual selection, this extreme struggle led to the rapid elongation of giraffe's neck in 2million years, becoming the existing genus giraffe.

Based on this elongation, giraffes are very suitable for high leaf niche. However, their niche is not as safe as that of cattle and deer. Therefore, the marginal niche of giraffe may promote extreme intraspecific courtship competition, which in turn may promote extreme morphological evolution.

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