"Da Zhuang", the square-faced monkey at Hefei wildlife park, passed away Wednesday afternoon after suddenly falling ill that morning, according to the park's announcement at night from its official WeChat account.

Despite extensive treatment, the black-capped capuchin could not be rescued.

A team of veterinary experts conducted a thorough examination and preliminarily determined the cause of his death as heart and lung failure, the park said.

The monkey had gone viral since early May. His distinctive squared face earned him the nickname "guo-character-face monkey" or "square-faced monkey", as the Chinese character "guo" has a square-shaped structure.

Netizens expressed their love and regret over his sudden passing. "It's such a sudden loss," said one netizen. Some netizens called for more care for the park's animals.