Petal, the oldest African elephant in an American zoo, died Monday at age 52.

The elephant, which usually slept standing up, was found lying in her stall by Philadelphia Zoo staff around 7 a.m. Veterinarians were called immediately, but Petal died about two hours later.

A video monitoring system showed that Petal's right rear leg buckled suddenly Monday morning, causing her to collapse, said Andrew Baker, the zoo's vice president for animal programs.

Results of a necropsy are expected within the next few weeks. The animal, which had lived most of her life at the Philadelphia Zoo, had been in excellent health, showing no recent signs of illness or decline, Baker said.