T. rex's short arms may have lowered risk of bites during feeding frenzies

Kevin Padian taught a freshman seminar called The Age of Dinosaurs over two decades ago and one question asked frequently by undergraduates was: Why are the arms of the rex so short?

He would usually list a range of hypotheses, but his students were skeptical. Padian thought that scholars who had proposed a solution to the problem came from the wrong perspective.

Padian said the question should be what benefit those arms were for the whole animal.

Padian believes that the T. rex's arms shrank in length to prevent accidental amputation when a pack of T. rexes descended on a carcass. The equivalent of a 6-foot human with 5-inch arms is a 45 foot long T. rex.

What if adult tyrannosaurs converge on a carcass? You have a bunch of huge skulls, with powerful jaws and teeth, right next to you. What if your friend thinks you are getting too close? Padian, a professor at the University of California, Berkeley, and a curator at the UC Museum of Paleontology, said that they might warn you away by severing your arm.

He said that severe bite wounds can cause death.

The predecessors of tyrannosaurid had longer arms, so they must have been reduced in both size and mobility. The African and South American abelisaurids from the mid-Cretaceous and the carcharodontosaurids, which ranged across the globe, would have been affected.

Padian said that all of the ideas that have been put forward about this are either untried or impossible. If the arms had not been reduced, all of the proposed functions would have been more effective.

66 million years after the last T. rex died, it will be hard to prove any hypothesis.

The T. rex has arms.

The arms of the first T. rex fossils were too small to be part of the skeleton, which was discovered in 1900. Thepectoral claspers, which hold the female in place during copulation, may have been the reason why T. rex had short arms. This is similar to some sharks and rays, which have modified fins. Padian noted that the T. rex is too weak to exert any control over a mate.

The short arms were thought of as an anchor to allow T. rex to get up from the ground, as well as waving for mate attraction or social signaling, and even pushing over a sleeping Triceratops at night. Padian said to think cow-tipping. The arms have no function, so we shouldn't be concerned with them.

Padian asked what benefit shorter arms might have for the animal's survival. Evidence that some tyrannosaurid hunted in packs, not singly, as depicted in many paintings and dioramas came to him.

Several important quarry sites preserve adult and juvenile tyrannosaurs together. They were buried together. When you find several sites with the same animals, that is a stronger signal. The possibility that they were hunting in groups has already been raised by other researchers.

He thought that the arms might have shrunk to get out of the way. T. rex youngsters would have been wise to wait until the adults were done.

Padian looked at speculations by other paleontologists, but none of them appeared to have been fully tested. The first thing he did was to measure the lifesize T. rex cast outside the doors of the UCMP.

He said that the arms are too short and that they can't reach the mouth or touch each other. The kind of death machine you saw in "Jurassic Park" is in front of them.

The arms of T. rex were analyzed 20 years ago by two paleontologists and they believed that it could have bench pressed 400 pounds.

Beware of Komodo dragons.

Padian has a hypothesis about some fearsome animals. The giant Komodo Dragon lizard of Indonesia hunts in groups and when it kills prey, the larger dragons leave the remains for the smaller ones. crocodiles feeding can cause moolings. T. rex and other tyrannosaurids, which first appeared in the Late Jurassic and reached their peak in the Late Cretaceous before becoming extinct, could be the same.

If museum specimen were checked for bite marks, a correlation could be found. He admitted that it would be quite a challenge.

Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs.

Padian doesn't think his idea will be the end of the story.

The first thing I wanted to do was to show that the prevailing functional ideas do not work. We can think about social organization, feeding behavior and ecological factors apart from mechanical considerations.

The problem in establishing the hypothesis is that there were several groups of dinosaurs that reduced their fore limbs in different ways.

The proportions of the limb bones in these groups are different, but so are other aspects of their skeletons. This is true for the large, living, flightless ratite birds, like the rhea and the emu. They evidently took different evolutionary paths for their own reasons.

There is a common thread in the history of explanations of short arms.

The study of the arms is interesting because of how we tell stories in science. Is the problem being looked at the right way?

Padian's paper is part of a Festschift honoring Richard Cifelli, a long-time head of the Oklahoma Museum of Natural History and Presidential Professor of Biology at the University of Oklahoma.

More information: Kevin Padian, Why tyrannosaur forelimbs were so short: an integrative hypothesis, Acta Palaeontologica Polonica (2022). DOI: 10.4202/app.00921.2021 Journal information: Acta Palaeontologica Polonica Citation: T. rex's short arms may have lowered risk of bites during feeding frenzies (2022, April 1) retrieved 1 April 2022 from https://phys.org/news/2022-04-rex-short-arms-lowered-frenzies.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.