The gigantism of lamniform sharks, including the extinct megalodons, may be associated with warm-bloodedness and the practice of eating fellow sharks while still in the womb.
Megalodons are considered to be one of the largest fish ever to exist on Earth. It is believed that these ancient sharks reached 16 meters in length, although it is difficult to accurately estimate their size. Cartilaginous skeletons are poorly preserved in the fossil record, and in such calculations one has to rely on the size of individual mineralized vertebrae, as well as the teeth of megalodons, which are found in many.
Kenshu Shimada and his colleagues from the US De Paul University compared the size and shape of the teeth of the entire order of lamniform sharks, including 15 modern species, such as the great white, and 13 fossils, such as the megalodon. In an article published in the journal Historical Biology, biologists note that megalodons and their teeth stand out even among other giant lamniforms, which grow "only" to five to seven meters.
As the authors of the study note, both extinct and modern giant sharks are endothermic (warm-blooded), which allows them to be less dependent on the temperature of the surrounding water and maintain a high swimming speed for active hunting. However, the development of a massive warm-blooded body requires a lot of resources, and "intrauterine cannibalism" could become an incentive for this.
Most sharks practice ovoviviparity, in which fertilized eggs develop in the mother's body and are released into the world as fry. Sometimes by this time they reach a fairly decent size, feeding on eggs that did not have time to hatch, and then on less fortunate large brethren. As a result, only the biggest and strongest survive.
This, in turn, makes special demands on the mother's body, which is forced to produce large amounts of nutrients for raising large offspring, and also stimulates the development of gigantism. Thus, the colossal size of megalodons could arise due to the peculiarities of their reproductive cycle, and then be maintained thanks to endothermicity, which made it possible to nourish a huge body.