A new take on an old problem.

Why tyrannosaurid forelimbs were so short: An integrative hypothesis

Abstract

The unusually shortened limbs of giant theropods, including
abelisaurids, carcharodontosaurids, and derived tyrannosauroids such
as Tyrannosaurus rex have long been an object of wonder, speculation,
and even derision on the part of both paleontologists and the public.
Two questions commonly asked are �Why did the forelimbs become so
short?� and �What did the animals use such short forelimbs for, if for anything?� Because basal tyrannosauroids and their outgroups, as well
as the outgroups of other giant theropods, had longer forelimbs, the foreshortening of these elements in derived taxa was secondary, and it ostensibly involved a shift in developmental timing of the forelimb
elements. Factors proposed to have influenced the evolutionary
foreshortening include natural selection, sexual selection, energetic compensation, ontogenetic vagaries, and rudimentation due to disuse.
Hypotheses of use have varied from a supporting anchor that allows the hindlimbs a purchase to stand from a reclining position to a pectoral
version of pelvic claspers during intercourse to a sort of waving
display during sexual or social selection. None of these hypotheses
explain selective regimes for reduction; at best, they might argue for maintenance of the limb, but in all cases a larger limb would have
suited the function better. It is likely that we have been looking the
wrong way through the telescope, and that no specific function of the
forelimbs was being selected; instead, another crucial adaptation of
the animal profited from forelimb reduction. Here I propose, in the
context of phylogenetic, ontogenetic, taphonomic, and social lines of
evidence, that the forelimbs became shorter in the context of
behavioral ecology: the great skull and jaws provided all the
necessary predatory mechanisms, and during group-feeding on carcasses,
limb reduction was selected to keep the forelimbs out of the way of
the jaws of large conspecific predators, avoiding injury, loss of
blood, amputation, infection, and death. A variety of lines of
evidence can test this hypothesis.

Open access:
https://www.app.pan.pl/article/item/app009212021.html

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On Wednesday, June 15, 2022 at 5:51:47 AM UTC-4, Pandora wrote:

A new take on an old problem.

Why tyrannosaurid forelimbs were so short: An integrative hypothesis

Abstract

The unusually shortened limbs of giant theropods, including
abelisaurids, carcharodontosaurids, and derived tyrannosauroids such
as Tyrannosaurus rex have long been an object of wonder, speculation,
and even derision on the part of both paleontologists and the public.
Two questions commonly asked are “Why did the forelimbs become so short?” and “What did the animals use such short forelimbs for, if for anything?” Because basal tyrannosauroids and their outgroups, as well
as the outgroups of other giant theropods, had longer forelimbs, the foreshortening of these elements in derived taxa was secondary, and it ostensibly involved a shift in developmental timing of the forelimb elements. Factors proposed to have influenced the evolutionary foreshortening include natural selection, sexual selection, energetic compensation, ontogenetic vagaries, and rudimentation due to disuse. Hypotheses of use have varied from a supporting anchor that allows the hindlimbs a purchase to stand from a reclining position to a pectoral version of pelvic claspers during intercourse to a sort of waving
display during sexual or social selection. None of these hypotheses
explain selective regimes for reduction; at best, they might argue for maintenance of the limb, but in all cases a larger limb would have
suited the function better. It is likely that we have been looking the
wrong way through the telescope, and that no specific function of the forelimbs was being selected; instead, another crucial adaptation of
the animal profited from forelimb reduction. Here I propose, in the
context of phylogenetic, ontogenetic, taphonomic, and social lines of evidence, that the forelimbs became shorter in the context of
behavioral ecology: the great skull and jaws provided all the
necessary predatory mechanisms, and during group-feeding on carcasses,
limb reduction was selected to keep the forelimbs out of the way of
the jaws of large conspecific predators, avoiding injury, loss of
blood, amputation, infection, and death. A variety of lines of
evidence can test this hypothesis.

Open access:
https://www.app.pan.pl/article/item/app009212021.html

Hi, Pandora!

Two days ago, I finally found enough time to start contributing again regularly to s.b.p., and it's great to see you still contributing regularly.

As usual, you've contributed another thought-provoking article. I'm still rather busy
even now, so I've only had time to skim it. One set of details is quite new to me:

"(ii) the humerus is not long
enough to extend in front of the body wall at the midline,
so the elbow would not have been able to form a 90° angle,
and as a result (iii) the hands likely would not have been
able to extend to touch each other."

If this is indeed the case, that makes the whole abstract thoroughly logical and pretty nearly convincing. One thing that I haven't seen addressed: If the K-P
extinction had not occurred, would the reduction in size have continued,
with the forelimbs likely reduced to internal vestiges as in Sirenia, or vanished altogether, as in extant Cetacea?


Before the end of this week, I will be commenting on two older posts of yours that I missed
during my half-year absence: the one on monotremes and the one on squamates.


Peter Nyikos
Professor, Department of Mathematics
University of South Carolina -- standard disclaimer -- https://people.math.sc.edu/nyikos/

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On Wed, 15 Jun 2022 11:51:43 +0200, Pandora <pandora@knoware.nl>
wrote:

A new take on an old problem.

Why tyrannosaurid forelimbs were so short: An integrative hypothesis

Related paper: "New giant carnivorous dinosaur reveals convergent
evolutionary trends in theropod arm reduction": <https://www.cell.com/current-biology/fulltext/S0960-9822(22)00860-0>

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