Earlier in the 1920s, several archeologists set out to the mountains of Montana in search of fossils. These professionals were interested in finding fossils of the deadliest species of all time. A specie that killed hundreds and thousands of dinosaurs some 7 million years ago. Initially, the expenditure was seemingly unattainable and weird in and of itself.
Why? Because these paleontologists were searching for a mystery. They had no crystal ball whether or not a fossil - that later turned out to be the most popular T. Rex - existed. Even if it does, it was a herculean task to find the remains of the deadly species. Plus, these historians and archeologists had assumed that this deadly species was notoriously responsible for the extinction of dinosaurs.

National Geographic / Finding the T. Rex fossil was a herculean task in and of itself back in the 1920s.
Later on, as this challenging expenditure progressed, archeologists came to the conclusion that they were near to finding the most mysterious fossil of all time. Something that experts were not very sure about.
As they found the T. Rex fossil, the question that bothered paleontologists was the purpose of the mysterious arms of the T. Rex fossil. Since the long and weird teeth of T. Rex added to its popularity worldwide, its tiny little arms added to the uncertainty of paleontologists. What is the purpose of these tiny little arms, asked paleontologists. Most importantly, they were curious about the mismatch of the long teeth of T. Rex with its tiny little arms.

Quora / While the long teeth of T. Rex added to its popularity worldwide, its tiny little mysterious arms made paleontologists doubt that the species was deadly by nature.
Whether right or wrong this assumption, paleontologists assumed that the actual arms of T. Rex disappeared as it continued to swallow dinosaurs some 7 million years ago.
The Tiny Little Withered Arms of T. Rex Remain an Unsolved Mystery
Archeologists wonder how T. Rex survived the deadly extinction that claimed the lives of millions of dinosaurs. Plus, paleontologists wonder if the T. Rex fossil shows signs of survival back then, is it because of its massive body? And was T. Rex deadly enough to claim the lives of dinosaurs? Thus, this remains an unsolved mystery for archeologists and paleontologists.

History Wonders / T. Rex’s mysterious little arms and long teeth remain an unsolved mystery for archeologists and paleontologists.
However, it is essential to note here that the arms are withered and bizarre. One of the archeologists argues that if you look at the tiny little arms of T. Rex, you will feel that these are so different.
Deep inside, these arms seem to be attached to the body parts like dinosaurs. Popular archeologist at the University of Idaho, Krumenacker, suggests that the arms of T. Rex remain an unsolved mystery of our time. Not only the withered arms are “ludicrous,” but they are also unrelatable to other parts of the fossil.
Plus, the missing bones in the body parts of T. Rex are also other mysteries that remain unsolved for archeologists.