For most of its length, the Wikipedia entry on Dollo's Law are very strict about
not being able to get *exactly* back to and earlier state.
https://en.wikipedia.org/wiki/Dollo%27s_law_of_irreversibility

However, in one place it broadens the concept:

"In Stephen Jay Gould's interpretation of Dollo's law, it would not be possible to regain a coiled shell after the coiling has been lost. Nevertheless, a few genera in the slipper snail family (Calyptraeidae) may have changed their developmental timing (heterochrony) and regained a coiled shell from a limpet-like shell.[13][14]"

My "radical reversals" are of this looser sort, but here
is one that seems to be very close to true:

"When a vertebrate goes from one environment to a radically
different one, the trends in limb length are reversed."

For example: birds evolved from archosaurs in which the trend
was towards longer hindlimbs and shorter forelimbs.
[Just look at Tyrannosaurus rex!]
This reversed dramatically as wings grew to be a bit longer
than legs already in Archaeopteryx.

Pterosaurs are an even more dramatic example: last year
there was a flurry of excitement as new evidence was found of
a fully terrestrial archosaur being the closest known relative
of the pterosaurs. Its forelimbs were only half as long as its
hindlimbs. Now look at Pteranodon, with its ca. 25 foot wingspread,
or Quetzalcoatulus.

Or look at bats -- although their ancestry is so obscure that
we cannot tell what evolutionary trends as to relative
limb lengths there were in their close ancestors.

With aquatic animals it seems to be the other way around.
I was struck long ago how *Ambulocetus* had longer hindlegs
than forelegs, but it was called "the smoking gun" by Gould for
what whale ancestors were like. Modern whales have no hindlimbs at all.

Ichthyosaurs all had fore-flippers much longer than hind-flippers,
but their ancestry is as obscure as that of bats.

We humans are a third category. Our nearest relatives, the great apes,
have longer arms than legs, especially gibbons, who can almost
walk on all fours while standing upright. And this was a marked
development from Proconsul, whose forelimbs were barely longer
than its hind limbs. This seems to be a clear case of change
from a largely arboreal life style to a more terrestrial one.

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

On 12/6/23 5:39 PM, Peter Nyikos wrote:
> For most of its length, the Wikipedia entry on Dollo's Law are very strict about
> not being able to get *exactly* back to and earlier state.
> https://en.wikipedia.org/wiki/Dollo%27s_law_of_irreversibility
>
> However, in one place it broadens the concept:
>
> "In Stephen Jay Gould's interpretation of Dollo's law, it would not
> be possible to regain a coiled shell after the coiling has been lost.
> Nevertheless, a few genera in the slipper snail family
> (Calyptraeidae) may have changed their developmental timing
> (heterochrony) and regained a coiled shell from a limpet-like
> shell.[13][14]"
Of course, slipper shells are coiled, just so loosely that they never
end up completing one whorl in the adult. This doesn't seem to me like
any sort of violation of Dollo's Law, which was intended to refer to
complex features. Further, it wasn't intended as a law in the sense used
in physics, a universal mathematical relationship. It's just an
observation of how things usually work out.

> My "radical reversals" are of this looser sort, but here
> is one that seems to be very close to true:
>
> "When a vertebrate goes from one environment to a radically
> different one, the trends in limb length are reversed."

Depends on the reason for the trend and the nature of the new
environment, one might think. And limb length doesn't seem like one of
those complex features of the sort Dollo intended.

> For example: birds evolved from archosaurs in which the trend
> was towards longer hindlimbs and shorter forelimbs.
> [Just look at Tyrannosaurus rex!]
> This reversed dramatically as wings grew to be a bit longer
> than legs already in Archaeopteryx.
>
> Pterosaurs are an even more dramatic example: last year
> there was a flurry of excitement as new evidence was found of
> a fully terrestrial archosaur being the closest known relative
> of the pterosaurs. Its forelimbs were only half as long as its
> hindlimbs. Now look at Pteranodon, with its ca. 25 foot wingspread,
> or Quetzalcoatulus.
>
> Or look at bats -- although their ancestry is so obscure that
> we cannot tell what evolutionary trends as to relative
> limb lengths there were in their close ancestors.
>
>
> With aquatic animals it seems to be the other way around.
> I was struck long ago how *Ambulocetus* had longer hindlegs
> than forelegs, but it was called "the smoking gun" by Gould for
> what whale ancestors were like. Modern whales have no hindlimbs at all.
>
> Ichthyosaurs all had fore-flippers much longer than hind-flippers,
> but their ancestry is as obscure as that of bats.
>
>
> We humans are a third category. Our nearest relatives, the great apes,
> have longer arms than legs, especially gibbons, who can almost
> walk on all fours while standing upright. And this was a marked
> development from Proconsul, whose forelimbs were barely longer
> than its hind limbs. This seems to be a clear case of change
> from a largely arboreal life style to a more terrestrial one.

And none of these seem like a big deal. Now, if modern whales were to
start becoming terrestrial and would regrow their hind limbs, we'd have
something.