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The ancestral state of terrestrial Spondylozoa is to have a backwards knee. However, this was misinterpreted several times independently to be the heel of a digitigrade leg. The original backwards knee is depicted correctly in iconic species such as the Nomad, so it's baffling that this has happened so many times without being caught--some instances being as recent as Week 24 (and it's a miracle that the correct backwards knee survived that long at all). Only a few species are left extant that have the correct backwards knee, mostly snappers and some dwellers (though at least one living branch of dwellers got misinterpreted).

Because this happened so many times and so inconsistently that it would be impossible to retcon and undesirable to plague or mass-replace, the only thing I can think of is to replace or redesign the most recent instances (such as in flunejaws and grubnubs) and find a way to explain the rest. I think a possible option would be that the "heel" is still a knee, and the additional uppermost leg segment is derived from part of the pelvis. Maybe the default hindlimb setup is something between a hip and a shoulder, having both flexible and inflexible parts, unlike the inflexible pelvis of tetrapods on Earth.

I could re-draw the flunejaws' and grubnubs' knees, provided the art has a fairly simple style.

Perhaps some of them had birdlike limb arrangements? The conspicuous "knee" of a bird is actually its ankle.

It would help to have diagrams of the correct Spondylozoa knee arrangements, relative to knee/ankle arrangements of real animals.

Gonna put it out that by "recent" I mean after the snowball event. If the knee turned into a heel before week 22 it's not worth trying to revise and an explanation like the one I proposed should be used to justify the "digitigrade" legs.

This post has been edited by Disgustedorite: Mar 4 2021, 10:02 PM

It's becoming clear that we also need a list of which living lineages have two knees and which have one knee. Here's ones I know so far:

Should have just one backwards knee (Like the externally visible part of a bird's leg):

• Soriparasites
• Dwellers
• Flunes
• Skysnappers, amazingly
• Shrogsnapper

Should have both a backwards knee and a forward knee (Resembles most tetrapod legs):

• Living furred shrews
• Bubbleskins
• Tilebacks
• Glowsnappers
• Earbacks

@OviraptorFan

For the "Should have just one backwards knee (Like the externally visible part of a bird's leg)" category, don't forget to include the Shrogsnapper, which also retains the backward knees