Rebuilding Ailurarctos

Giant Panda’s ‘Incredible’ Feature Developed At Least Six Million Years Ago

Rebuilding Ailurarctos

An artist’s reconstruction of Ailurarctos of Shuitangba. The grasping function of its false thumb (shown in the individual on the right) has reached the level of modern pandas, while the radial sesamoid may have protruded slightly more than its modern counterpart during walking (seen in the individual on the left ). Credit: Illustration by Mauricio Anton

Eat bamboo? It’s all in the wrist.

When is an inch not really an inch? When it is an elongated wrist bone of the giant panda which is used to grasp the bamboo. In its long evolutionary history, the panda’s hand has never developed a truly opposable thumb. Instead, it evolved a thumb-like digit from a wrist bone, the radial sesamoid. This unique adaptation helps these bears to subsist entirely on bamboo despite being bears (members of the order Carnivora, or meat eaters).

In a new paper published today (June 30, 2022), scientists report the discovery of the first ancestral bamboo-eating panda to have this “thumb”. Surprisingly, it is longer than its modern descendants. The research was led by Los Angeles County Vertebrate Paleontology Curator Xiaoming Wang and colleagues.

While the famous false thumb of contemporary giant pandas (Ailuropoda melanoleuca) has been known for over 100 years, it has not been understood how this wrist bone evolved due to an almost total absence of fossils. A fossil fake thumb of an ancestral giant panda, Ailurarctos, dating from 6 to 7 million years ago was discovered at the site of Shuitangba in the city of Zhaotong, in the province of Yunnan, in the south of China. It gives scientists a first look at the early use of this extra digit (sixth) – and the first evidence of a bamboo diet in ancestral pandas – helping us better understand the evolution of this unique structure.

Chengdu panda eating bamboo

Chengdu panda eating bamboo. Credit: Photo reproduction courtesy of Sharon Fisher

“Deep in the bamboo forest, the giant pandas traded an omnivorous diet of meat and berries to quietly consume bamboo, a plant abundant in the subtropical forest but of low nutritional value,” says curator Dr Xiaoming Wang. of NHM Vertebrate Paleontology. “Holding bamboo stalks tightly in order to crush them into bite-size pieces is perhaps the most crucial adaptation to consuming a prodigious amount of bamboo.”

How to walk and chew bamboo at the same time

The discovery could also help solve a lingering mystery about pandas: Why do their fake thumbs look so underdeveloped? As an ancestor of modern pandas, Ailurarctos One might expect them to have even less well-developed false ‘thumbs’, but the fossil Wang and his colleagues found revealed a longer false thumb with a straighter end than the shorter, hooked finger. of his modern descendants. So why did pandas’ fake thumbs stop growing to a longer number?

“Panda’s fake thumb has to walk and ‘chew’,” says Wang. “Such a dual function serves as a limit to how big that ‘thumb’ can become.”

Panda Grab vs Walk

Panda grabbing vs walking (the white bone is the fake thumb). Credit: Courtesy of LA County Museum of Natural History

Wang and his colleagues believe that the modern panda’s shorter false thumbs are an evolutionary compromise between the need to manipulate bamboo and the need to walk. The hooked tip of a modern panda’s second thumb allows it to manipulate bamboo while allowing it to carry its impressive weight until the next bamboo meal. After all, the “thumb” performs a dual function as a radial sesamoid – a bone in the animal’s wrist.

“Five to six million years should be enough time for the panda to develop longer false thumbs, but it seems evolutionary pressure from the need to travel and bear weight kept the ‘thumb’ short – strong enough. to be useful without being big enough”. to get in the way,” says Denise Su, associate professor at the School of Human Evolution and Social Change and researcher at Arizona State University’s Institute of Human Origins, and co-leader of the project that recovered the panda specimens.

“Evolving from a carnivorous ancestor and becoming a pure bamboo eater, pandas have to overcome many obstacles,” says Wang. “An opposable ‘thumb’ of a wrist bone may be the most amazing development against these obstacles.”

Reference: “Giant Panda’s First False Thumb Suggests Conflicting Locomotion and Feeding Demands” by Xiaoming Wang, Denise F. Su, Nina G. Jablonski, Xueping Ji, Jay Kelley, Lawrence J. Flynn, and Tao Deng, 30 June 2022, Scientific reports.
DOI: 10.1038/s41598-022-13402-y

The authors of this article are affiliated with the Natural History Museum of Los Angeles County, Los Angeles, CA, USA; Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; Arizona State University, Tempe, Arizona, USA; Pennsylvania State University, University Park, Pennsylvania, USA; Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China; Yunnan Institute of Cultural Relics and Archaeology, Kunming, Yunnan, China; Harvard University, Cambridge, Massachusetts, USA.

Funding was provided by the National Science Foundation of the United States, the Yunnan Natural Science Foundation, the National Natural Science Foundation of China, the governments of Zhaotong and Zhaoyang, the Institute of Paleontology and Paleoanthropology of vertebrates.

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