Jayne Mansfield | Autopsy Report

The most plausible explanation for the myth is a visual one. After the crash, the upper portion of Jayne Mansfield’s skull was so depressed that her recognizable features were gone. In the dark, with blood everywhere, seeing a crushed face and a separate body might have looked like a decapitation. Coupled with the fact that Sam Brody was decapitated, it is likely a case of mistaken identity at a gruesome scene.

What actually happened was a “decapitation by proxy” of legend. The impact occurred because the tractor-trailer, owned by Tri-State Trucking, had slowed down behind a mosquito-control fumigator truck spraying fog. The Buick, traveling at an estimated 70 mph, failed to see the trailer’s rear. Because the trailer’s underride guard was defective, the car slid under the truck. The top of the Buick was sheared off at the level of the front seat headrests. jayne mansfield autopsy report

It's worth noting that Mansfield's death was attributed to the improper handling of her vehicle, which was being driven by her friend and passenger, Ronald H. Hyatt. The accident occurred when the vehicle was being driven at a high speed and crashed into a truck, causing Mansfield to be thrown from the vehicle and suffer fatal injuries. The most plausible explanation for the myth is a visual one