One of the greatest features of Kathy Wankel's T. rex is that it has all the arm bones, shoulder to fingers. Only the claws are missing. That's the first time we've ever had all those parts from a T. rex. And those bones allowed my co-workers to draw some conclusions about how those arms worked, conclusions that surprised me.
What didn't surprise me was how small T. rex's arms were—no longer than mine. In evolution it's an amazing reduction over earlier tyrannosaurs.
Once you look at the actual bones, it's obvious that T. rex's arms were not just dangling uselessly at its side as it walked along. The arm bones have very distinct muscle scars—sometimes smooth surfaces, other times indentations in the bone made by the attachment of large muscles, or projections to which tendons attached.
T. rex's arm bones are only three feet long on an animal forty feet long. But T. rex's arm bones are about three times as thick as mine.
Matt Smith, a sculptor and former fossil worker at the Museum of the Rockies, and Ken Carpenter, a researcher and fossil mounter who is now a preparator at the Denver Museum of Natural History, made a special study of our T. Rex's arm bones. Ken detailed the shape of the arm bones. Matt looked at the muscle scars in the forearm of T. rex, the pits and grooves in the bone where muscle had attached. By measuring these scars they could estimate the size of the muscles and tendons. For instance, the bump on the humerus where T. rex's biceps attached is about the size of a nickel. It doesn't sound big, but that's a lot larger attachment point than you'd see on a weightlifter's upper arm bone. The biceps didn't attach to the same point on T. rex's arm as matt smith recreated t. rex's arm musculature by measuring the origin and insertion points for arm and shoulder muscles, except for the biceps which corresponds in size to the results of a bi omechan ical study matt and ken carpenter did, the rest is educated guesswork. so matt kept the muscles at the conservatively small end of the range of possibilities.
our muscles do on our arms. Our biceps attaches on our forearm, inside our elbow and along our upper arm. T. rex's biceps attached farther down its forearm. That gave T. rex more leverage. Matt compares it to a drawbridge. If you try to lift a drawbridge with a rope, you want the rope to go out as close to the end of the bridge as possible. That way you can lift more of the bridge's weight with a given amount of force.
The muscle scars for the shoulder and upper back muscles, which on us are called the teres and the latissimus dorsi, are also massive on our T. rex. Emerging from a huge shoulder pad of muscle, T. rex's arm would have looked even stubbier than it was.
From this kind of data, Matt and Ken worked out the range of motion that arm could move through. They also reconstructed the forelimb musculature of T. rex with comparative anatomy to figure out T. rex's range ofmotion. But the comparisons weren't easyto find. No birds have forelimbs like those of T. rex. Birds' limbs allow for a lot of rotary movement that helps them fly.
The closest arms to T. rex's among living animals belong to a crocodile. But crocodiles walk on all fours,
left to right: the bones of the arm and shoulder, the humerus is as massive as the upper leg bone of a racehorse.
the biceps and brachialis flex the forearm, bringing it up and in. the triceps extends the forearm.
the supras capularis and deltoid move the arm out and back while the pectoralis provides the opposite movement, rotating the arm forward and inward.
the popeye-like forearm extensors work the hand and wrist.
unlike birds or T. rex. T. rex arms didn't move quite like anything alive today. When Ken and Matt put muscles on T. rex's forelimbs, the arms were capable of manipulating in front and back movement, and up and down motion, as well as lateral motion. But that motion was limited. The elbow end of T. rex's upper arm was flat, not rounded like ours, so T. rex's arm wasn't very flexible. T. rex couldn't extend its arm much past a ninety-degree angle between the forearm and the upper arm. Matt says no animal today has such a limited range of movement. And no other meat-eating dinosaurs, even T. rex's closest relatives, the other tyrannosaurids, were so restricted in how they could move their arms.
T. rex didn't have much arm motion, but it did have a lot of arm strength. Its biceps muscle would have been six inches in diameter, three times as thick across as most of ours. Imagine our arm bones worked by muscles the size of our thigh muscles. Ken figured T. rex could hoist four hundred pounds toward its body at one time. On average, we can pull in only nineteen pounds. And Sue, Pete Larson's T. rex, had even sturdier arms. Small wonder Ken called T. rex "the Schwarzenegger of dinosaurs."
While those figures surprised a lot of us, Ken and Matt weren't surprised by their findings. They thought a bone as thick as T. rex's upper arm could lift a lot of weight. They were surprised that T. rex's lower arms were not nearly as muscular. T. rex's strength, Matt says, would have been concentrated in its contracting muscles that pulled the arm up. It wouldn't have been able to hold much weight way out in its claws. As for what T. rex might have done with those littie arms, Ken and I disagree (see Chapter 9).
Was this article helpful?