Secondarily Plantigrade Limbs

Question

So, this came about when I was attempting to construct create a unified Kaiju universe, and wondering about a hypothetical evolutionary path of Godzilla. However, I figured that it could also open up more possibilities when building hypothetical other worlds of mine.

Sometimes, in biology, a feature becomes too derived to reasonably revert i.e. fingers in birds. With that being said, however, is it possible for a creature with digitigrade limbs to re-evolve Plantigrade limbs? If it is possible, are there any real life examples, and what would the circumstances most likely be?

Any answers would be greatly appreciated.

Answer 1

The reason a creature would evolve back an old trait is because it is important to have and increases the organism's ability to survive and reproduce.

The reason an organism would lose a trait is that it costs too much energy to maintain. In Carlos Zamora's answer, the reason that the blind fish lost their eyes but the dolphins kept their hips is because the energy cost of growing eyes that weren't needed was large enough to matter. Dolphins have hips because it is pretty low-cost to have them.

Answer 2

Yes.

In the same way nature evolves some physical characteristic in an species, it can "involute" (retrieve the characteristic) and change to a previous state. For example, the blind fish that live in deep lakes inside caves (cavefishes). They once had eyes, but the eyes rendered useless in the new environment, so Mother Nature took them away. Here is the link:
https://www.leisurepro.com/blog/explore-the-blue/blind-fish-species/

In the same way, dolphins and whales still have pelvic bones. They evolved from land-dwelling ancestors more than 40 million years ago, and although they have legs no more, they preserve vestigial hips.

So the short answer is "Yes": If the ecosystem where your digitigrade limbs creature lives changes enough, Nature will make your creature change in order to adapt to the new conditions, and if the new "best fit" characteristic involves plantigrade limgs, then so be it.

So... the answer to your second question: What changes in my ecosystem can make a digitigrade creature evolve into a plantigrade creature?

Let us start reviewing very briefly each one:
Digitigrade: They can execute quicker and faster movements.
Plantigrade: Generally can carry a heavier load, have a greater surface area, but are slower.

I can think right now of 3 changes in the ecosystem that could lead to such a change:

1) Shortage in the sources of food, so the animal needed to have more fat reserves, therefore started to weight more, and the feet needed to tradeoff their dexterity versus carry their own weight.

2) Predator extintion: If the animal has no need of running away faster or being more agile than the extinct predator, then the need of the agile feet will eventually dissappear.

3) Prey change: The prey once hunted dissappeard from the ecosystem, and were replaced by others more slow and easy to hunt (but of course with a higher reproduction ratio, to compensate that). So your animal needed its agility no more.