ARM

The Tyrannosaurus rex (T. rex) is known for its bone-crushing bite, gigantic size, and famously small forelimbs. But why these large, carnivorous theropod dinosaurs evolved tiny arms has long been debated.

Some are born with three arteries in their forearm, and a new study is suggesting more and more people are holding onto this temporary embryonic artery into adulthood, which the researchers believe could be natural selection at work.