All of the Earth's creatures are bombarded with DNA altering radiation. One good blast of solar radiation and all our offsping might have vestigial something or other if such a DNA mutation was a dominant gene.

In the desert, you can see the some species of mutated small ants, about the size of a normal black ant's head. Radiation has shrunken them and they are significantly less intelligent and coordinated than normal ants.

A single T.Rex may have experienced a sizable dose of radiation during a solar maximum and it's offspring were born with the mini arms. The trait may have been a dominant one and over a period of time eventually all T. Rex dinosaurs had the mutant arms.

The notion that the T.Rex was king of the dinosaurs and dominant is likely a myth. It would have had a hard time getting upright again after it would lose its balance. It's mouth was about it's only defense and once down would be a sitting duck for a balanced enemy on all fours.

I'll take odds on the Triceratops, thank you very much.

;-)