Thoughts about T-Rex’s short arms

Inspired by the exhibition of the T-Rex-skeleton Tristan at the Naturkundemuseum Berlin

Essay , 2016 , 7 Pages

Biology - Miscellaneous

Some exercises in logical thinking regarding the question: Why has the T-Rex such short arms and what where they used for?

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As far as I now there are several theories out there, of what use the short arms could have been,

acknowledging that they seem to have been very muscular (able to push 200 ­ 300 kg each) these use

cases come to mind:

1. Use during mating rituals

2. Pushing the body upwards while standing up

3. Killing and dissection of prey

4. Care for young T-Rex / eggs

Regarding the size, proportion, location and form of the t-rex's arms and especially it's claws, I highly

doubt those purposes, as they just seem inappropriate to me for such use.

1. Use during mating rituals

The arms and claws as they are basically two teeth on a short ranged limb with limited variability in

motion would have left significant marks on the mating partner's body if not leaving marks on their

bones or hurting them to extreme extend. The proposed mating position makes much more sense in

terms of mating behavior of today's birds and one of the many use of T-rex's hip flexor muscles

(source: http://www.earthtimes.org/nature/bringing-t-rex-baby/1512/).

(This picture has been removed due to copyright issues.)

Abbildung 1: Mating T-Rex couple (source: https://i.kinja-img.com/gawker-media/image/upload/s--GDZnECTs--

/c_fit,fl_progressive,q_80,w_636/18ky9nznkwzvmjpg.jpg)

2. Pushing the body upwards while standing up

The position and the strength of the arms just don't feel sufficient for me to provide T-rex help to

stand up. Especially not in the way, as I found illustrations of it during desk research.

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Abbildung 2: T-Rex in a fictional stand-up process (source: http://www.dinosaurier-interesse.de/web/Bilder/Hille/Zusatz/T-

rex-stehtauf-450.png)

The arms, although they were strong, do not provide enough stability because of their lack of a contact

surface. Weighing approximately 5 ­ 7 tons, the pressure for T-rex's arms would have been too much.

The danger of severe injuries seems unreasonable.

Considering the position of T-rex's legs right in the middle of the body with head and tail as counter

weights, the use of its' head as proposed in the drawn picture seems reasonable.

3. Killing and dissection of prey

Regarding the killing of prey, the T-rex's arms just do not seem to have a specific use here, mainly

because they seem so short, that the t-rex never actually saw, what his claws did or where they were.

According to studies the neck musculature was sufficient and efficient for killing prey. The use of the

arms would have led to and an extremely vulnerable position for the t-rex as its prey might have been

shorter than he was and could have hurt him in the chest or on the underside of its jaw.

Maybe the arms could have been used for dissection of dead prey, while t-rex was defending it with

his jaws or it was good for pulling the final blow to its prey as in breaking their neck or precisely

cutting specific areas open. The inward turn in his claws though seem counterintuitive to this behavior.

4. Care for young T-Rex / eggs

In fact the position of its claws seems to be perfect for carrying eggs, nest parts or other childcare

related stuff. Despite the fact, that T-rex would have had a hard time picking anything up, its claws

with just two "fingers" seem to be an evolutionary flaw for such purpose. Especially when regarding

the limbs of a "professional egg carrier" such as Oviraptor.

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Abbildung

3: Oviraptor (source:

https://qilong.files.wordpress.com/2011/06/oviraptor_philoceratops_skeleton.jpg?w=599&h=317)

A more likely theory?

Having stated what I think T-rex's arms weren't used to, what were they used for?

I think the secret lies in the T-rex's growth.

As all carnivore's T-Rex has to spend a long time as a young being smaller and easy prey for bigger

dinosaurs, all that to become a much bigger predator with an exceptional growth rate. As a small

youngster, he had to use not just his jaw, not yet grown to be the impressive weapons as they are in

adultery, for defense, hunting and dissection but its claws as well.

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Abbildung 4: T-Rex growth chart (source: http://www.nature.com/nature/journal/v430/n7001/images/nature02699-f2.2.jpg)

Pictures indicate a much more reasonable ratio between body and limbs. This manly had the use to

keep up with other dinosaurs and lead kind of a self-supporting life (source:

http://www.livescience.com/41486-smallest-intact-baby-ceratopsid-found.html).

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Abbildung 5: T-Rex size comparison

(source: http://pre03.deviantart.net/9fee/th/pre/i/2015/106/b/4/don_t_mess_with_t__rexes_by_scotthartman-d7t58oi.jpg)

As those longer limbs became successively expendable when the jaws became bigger, they just

stopped growing, still showing signs of excessive, such as features to support muscles, as they were

extensively used during growth.

This theory is backed up by the finding, that "during development, the torso became longer and

heavier whereas the limbs became proportionately shorter and lighter. The results very successfully

show the change in mass of the forelimbs. "The forelimbs clearly became proportionately lighter from

juveniles to adults (by as much as a factor of 10)"" (source: http://www.earthtimes.org/nature/

bringing-t-rex-baby/1512/#sthash.sWUOHxQ3.dpuf).

This article indicates as well, that the so far called Nanotyrannus is in fact juvenile T-Rex, which gives

us a better comparison of the body-arm-ratio of the T-Rex during his lifespan.

This theory even leads some hobby-paleontologists to the belief that juvenile T-Rex was actively

hunting probably in a more raptor-esque style, while the adult specimen focused on scavenger hunting.

Some see the proof in longer thigh bones and therefore their ability to run faster (source:

http://myjurassicpark.proboards.com/thread/150/rex-hunter-scavenger-thread) but on the other hand

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one should consider, that a carnivore of approximately 7 tons actually shouldn't leave out any situation

for a meal.

Another interesting hint seems to be that the chest plates of an adult tend be more stuck together in the

middle of the body (regarding the pictures of Tristan), in contrast to the juvenile, whose chest plates

and arms seem a little more side wards bent providing him a better range and flexibility.

(This picture has been removed due to copyright issues.)

Abbildung 6: Juvenile T-Rex Jane in the Burpee Museum Rockford Illinois

(source: https://media.mnn.com/assets/images/2013/06/burpee_jane_tyrannosaurus_rex.jpg.638x0_q80_crop-smart.jpg)

However I think all this points in the clear direction, that the relatively shorter arms of the juvenile T-

Rex were vital for him, but became more and more expandable, as he grew bigger and got equipped

with are more ferocious jaw.

Thanks for reading my thoughts. I would be happy to discuss more about it via [e-mail]

• Author: Felix Zappe (Author)
• Publication Year: 2016
• Pages: 7
• Catalog Number: V338311
• ISBN (eBook): 9783668280991
• ISBN (Book): 9783668281004
• Language: English
• Tags: T-Rex, Paleonthology, dinosaurs