A Tyrannosaurus rex rise up after lying on the ground, bracing the forepart of its body on its elbows while raising its hind part and tail into the air. The illustration is based on a fossilized tracks described in 2021.^[1] (notice the postorbital horns, based on research by Paul).^[2]

The idea that tyrannosaurids used their forelimbs to assist when raising up from the ground have been around since the first complete forelimbs of the group were described in the genus Gorgosaurus 1917 by Lambe,^[3] who imagined that the living animal would have lied on its belly while resting, and that

“[t]he fore legs were probably of assistance in minor changes of position when prone, or when rising to a sitting posture, and also for receiving the weight of the fore part of the body when bringing its full length to the ground”

A similar hypothesis was proposed for Tyrannosaurus in 1970 by Newman, who thought that its “fore-limbs acted as struts to stop the body sliding forward as the animal raised its body from the resting position”.^[4] This was subsequently tested with a biomechanical computer model by Kent Stevens with colleagues, who concluded that the animal could have extended its arms laterally, so that the palms of its hands would point away from the body with the palms facing towards the ground, making a “press-up” as it raised from the ground.^[5][6]

This idea have not been without critics: Erickson have pointed out that tyrannosaurids sometimes broke their arms, (and must have been able to survive while they healed) and would probably have been able to get up simply by placing their feet below the its center of gravity of its body.^[7] This has also been the conclusion by Andrea Cau, who interpreted the computer model by Stevens as having pronated hands^[8] (which would have been impossible for Tyrannosaurus). Healed fractures have also been raised by Kevin Padian as evidence that the forelimbs would not have been "strong enough to support quotidian functions of an animal of [T. rex] size".^[9]

In 2021, Caneer et.al. published a study of an ichnofossil which they interpreted as being made by a Tyrannosaurus rex raising from the ground by pushing its elbows to the ground, placing its foot right behind and subsequently pushing itself upward-backward.^[1] While the discovery by Caneer et.al. does not “prove” that tyrannosaurids used their arms to get up on a regular basis, it is consistent with the popular hypothesis originally proposed by Lambe 1917.

References

1. ↑ ^{a b} Caneer T., Moklestad T & Lucas S.G. "Tracks in the Upper Cretaceous of the Raton Basin possibly show tyrannosaurid rising from a prone position". NMMNH Bulletin 82 (New Mexico Museum of Natural History and Science, 2021): p. 29-37
2. ↑ Paul G.S. (August 5, 2022). "Observations on Paleospecies Determination, With Additional Data on Tyrannosaurus Including Its Highly Divergent Species Specific Supraorbital Display Ornaments That Give T. rex a New and Unique Life Appearance". bioRxiv (Preprint): doi:10.1101/2022.08.02.502517
3. ↑ Lambe L.M. (1917). “The Cretaceous Theropodous Dinosaur Gorgosaurus”. Geological Survey of Canada Memoir 100: p. 1-85
4. ↑ Newman B.H. (1970). “Stance and gait in the flesh-eating dinosaur Tyrannosaurus”. Biological Journal of the Linnean Society 2(2): p. 119-123
5. ↑ Stevens K.A., Larson P, Willis E.D. & Anderson A. "Rex, sit: digital modeling of Tyrannosaurus rex at rest". In Larson P & Carpenter K (eds.). Tyrannosaurus rex, the tyrant king (Indiana University Press, 2008). p. 192-203
6. ↑ [1]
7. ↑ Erickson G.E. “If T. rex fell, how did it get up, given its tiny arms and low center of gravity?”. Scientific American 16-4-2007
8. ↑ Andrea Cau chatting with Alessandro Chianrenza (2nd January 2011) in the comment section to Cau A. “Perché il braccio di Tyrannosaurus non é atrofico come quello di Carnotaurus?”. Theropoda 6-10-2008
9. ↑ Padian K (2022). "Why tyrannosaurid forelimbs were so short: An integrative hypothesis". Acta Palaeontologica Polonica 67(1): p. 63-76