Madrid. The first fossil evidence of a thumb-like sixth finger used by giant pandas and their ancestors to grasp bamboo indicates that bamboo was their diet six million years ago.
In addition to the five digits of the front legs, modern giant pandas (Ailuropoda melanoleuca) they have an enlarged wrist bone with a thumb-like structure that they use to manipulate bamboo. Previous research documented the existence of this phalanx-shaped structure only between 100,000 and 150,000 years ago.
Xiaoming Wang, along with colleagues from the Los Angeles County Museum of Natural History, examined the wrist bone of an individual from the ancestral panda genus. ailurctos which was discovered in Shuitangba, a site near the city of Zhaotong, in the Chinese province of Yunnan, and which dates from the late Miocene (around 6 to 7 million years ago).
As published in ScientificReports, compared the shape and size of this bone with previously published bone data on the wrist of modern giant pandas and indarctos arctoides, an ancient specimen that lived nine million years ago and could share the same common ancestor as these animals.
hooked structure
The researchers found that the modern giant panda’s thumb-like structure has the same distinctive shape as the wrist bone of the giant panda. ailurctos, but not the I arctoides, which was bigger, wider and more hooked.
This indicates that, although the thumb-shaped sixth digit was not present in the latter or in the common ancestor it shares with pandas, it has been present in the lineage of this animal – and has been used to grasp bamboo – for at least 6 million years.
Although the sixth digit was present in both modern giant pandas and ailurctos, the researchers noted differences in their size and shape. That of the former is significantly shorter than that of the latter in relation to its body size and has a hook at its end and a flattened outer surface, while that of the latter. ailurctos does not have it.
The authors propose that the hook may help modern pandas better grip bamboo, while the shorter length and flattened outer surface may help distribute weight when walking. These weight-bearing limitations could be the main reason why the giant panda’s thumb-like structure never evolved into a full digit, they add.
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