Intrauterine cannibalism:
The large birth size of O. megalodon suggests that the young sharks, like many present-day sharks, ate unhatched eggs in the uterus to survive – a phenomenon called intrauterine cannibalism.
“The consequence is that only a few pups will survive and develop, but each of them can become large in body size at birth which gives [them] an advantage as already large predators,” says [Kenshu Shimada at DePaul University. [“Megalodon sharks grew 2 metres long in the uterus by eating eggs,” Karina Shah, NewScientist (16 January 2021)]
A related phenomenon, I suppose, is the occasional occurrence of a human twin absorbing the other of the pair into its body. When the twins are fraternal, I believe the result is called a chimera, as different cells may exhibit different DNA signatures.
