"On a cool evening in Hell Creek, a king surveys his domain. Stan
has been trailing this Edmontosaurus herd for a few hours now, keeping a sharp eye and keen nose while he remains in the cover of the forest. So far, they are unaware of his presence. As the darkness advances, so does the tyrant. Soon he is near the edge of the trees, his eyes locked on target.
"A juvenile Edmontosaurus is near the edge of the herd. She will do nicely. As soon as she meanders away from the cow that stood between Stan and the juvenile, the tyrant makes his attack. There is no roar. No warning of his assault. Pads on his feet make Stan silent as he rushes from the foliage. By the time the young hadrosaur sees him, it's too late. As she turns to run, Stan's jaws clamp down on her hips, sacral vertebrae instantly fracturing or breaking on impact. The juvenile's herd scrambles in chaos, and she can do nothing but watch as the silent king dispatches his prey.
"After gulping down her severed head, Stan kneels down and latches his arms around his meal in a death-grip. He stands and retreats to the wood. The herd is reorganizing, and he's not interested in sticking around for when they get their bearings. For an adult Tyrannosaurus rex, one Edmontosaurus isn't a problem, but just a few years ago he watched his brother trampled and beaten to death by three bulls. Besides, there are four little mouths to feed back at the nest
, and Stan's mate does not like to be kept waiting..."
The function of T. rex's infamously tiny arms is the source of a lot of debate in the paleo community. A study in 1990 suggested that each bicep could curl 400+ pounds, and combining that with the massive shoulder and chest muscles means that these comparatively-tiny arms could actually generate a tremendous amount of force. The most common speculation in books when I was young was that the arms would assist the tyrant in standing. It's also been proposed that they helped facilitate mating or held prey in place as they delivered their devastating bite. Recently Dr. Stanley caused quite a bit of media attention when he suggested that T. rex used their powerful arms to wound prey in conjunction with their bite.
I think a few of these proposals have merit. I think the arms may have been helpful in standing, but I think they'd be better off standing like penguins*, using their snouts and hands together to stand. Even the highest estimates of force don't seem sufficient to push them to a standing position using just the arms. Facilitating mating makes perfect sense to me. As for the arms taking a role in active hunting, I'm not so sure. Dr. Holtz remarked that they would have to compromise the effectiveness of their bite in most situations in order to employ their arms. For a theropod like Allosaurus or Suchomimus that invested in those nasty meat-hook hands that's a reasonable trade (more on that for my Suchomimus picture), but any wound that an adult T. rex could deliver with their arms would be 10x worse with a bite.** In most situations I just don't see the point.***
Now, employment of the arms post-hunting seems a lot more reasonable to me. Carrying their kills would make use of the strong arms without putting them under tremendous stress. Holding food in their arms rather than their mouths not only frees up the mouth to bite anything that tries to sneak a meal, but leaves their eyes free to look about, and also brings the payload closer to the center of balance (the hips) thereby making it easier to carry.****
*Penguin standing up: www.whaletime.org/2016/10/how-…
**Juvenile T. rexes may have used their arms a lot more in active hunting than their parents. If Bloody Mary is a juvenile T. rex, for example, those hands are definitely large enough that they might have been used in place of a bite in some circumstances. Even more conventional reconstructions of juvenile rexes have proportionally larger hands so I wouldn't be surprised if their hunting strategy was more hand-oriented.
***An examination of Sue's arm bones in 2016 found that, while there were significant muscle attachments on the arm bones of T. rex, there was minimal stress applied to these joints. At least in the case of Sue, there doesn't seem to have been frequent use, at least not frequent enough to push her limbs beyond their (albeit impressive) limit. However, a study in 2008 by Lipkin et al. showed at least 3 specimens of Tyrannosaurus rex that did show pathologies on the forelimb bones and furculae. I was unable to determine the source of these pathologies, but if they were stress-related, then that would be an interesting contrast to Sue's study.
****I remember reading that Dr. Horner suggested the arms would be used to carry scavenged remains as part of his proposal that T. rex was an obligate scavenger, I couldn't find any reference connecting him to the theory so I'm either misremembering or I missed it in last nights search. What I found actually implied that he thought the arms were useless so I might be giving him too much credit. The only other suggestion I found of T. rex using their arms to carry meat was Duane Nash in a fascinating post about theropods using their arms to dispatch prey. This was a fascinating article, and 100% inspired my next piece: antediluviansalad.blogspot.com…