I don't know why... I feel I've seen this very argument more than recently...
Since my position is the same as at the other place, and it is worth argumenting, here I go again.
a) we'll first state that "normal" Atlanteans, who have not become Tattooed Men, despite their various superhuman traits, are still not yet to be considered as supernatural creatures, which would in and of itself make them ineligible for Cosmo Knighthood and render the point moot
b) the Atlanteans are immune to any change of their physical shape ( shape, consistence/material, size) but not necessarily of their physiology. that is : their appearance can't be changed by so much as the thickness of the skin of their teeth (metamorphosis, transformation ritual, turning into a vampire); they are flesh and will never be anything else (turning to mist, petrification) ; nor can their size be altered. BUT their flesh can be made Mega Damage and their PPE augmented through the action of magic (when they are tattooed) and their system responds to such mutagenic treatments as the juicer process.
c) Since the transformation into a C K does not in the slightest alter the knight's physical appearance, only his "function" (the cosmic armor and weapon are not after all physical transformations of the person, but outwardly visible manifestations of his life force and Cosmic Power) it then become arguably possible that an Atlantean could become one.
d) the "Overwhelming Divine Power" of the Cosmic Forge and its ability to overcome even as near an absolute as an Atlantean's immunity to transformation can also be considered, but I deem this unnecessary, especially as TD, upon raising this argument, put another problem into the light :
e) does a C K lose his natural, or acquired racial abilities : of course, for an Atlantean, that represents a lot( marks of heritage, immunity to transformation?, vampire sense, knowledge of dimensional pyramids, ley line powers) but that's because Atlanteans are a special and powerful race. If you take more classical cases, would you deprive a Noro knight of their sensitive psionics? or a Wolfen of his nightvision and keen smell? (I forget the Wolfen's extra attack/rd and initiative bonus, but maybe they should be put in the smae package)
f) personnally, after a thorough re reading of the CK section in PhW, I see no
logical reason to deprive the character of such... then again, a C K is already a powerful character, if you add powerful racial abilities... but we're going on into game balance, not pure logic. but, talking of game balance, remember that the sword cuts both ways... you just might *want* that fallen knight villain to have these Atlantean abilities...
g) Gadrin mentioned the possibility of mutant powers... well, if the rules officially allow for tattoos and juicer powers, and I'm willing to permit C K powers, lesser ones should be allowable too, but, just to stay safe and avoid tweaking the "no transformation" rule too much, I'd forbid those that entail gross physical change (APS, alter limbs, lycanthropy, etc)
h) Boras, you speak of "Atlanteans from Skraypers"... well, I'll be shooting myself in the foot there, but please note that while it is written that it is *believed* that seeronian humans are from a lost Atlantean clan, there is no hard data confirming this.: their attributes and life span are standard human, and their special abilities/powers (which only came about with the Tarlok plague) are , at least for the 7% that have intangibility or APS, not in keeping with the "no physical transformation" trait common to Atlanteans and the precise extent of which is the core of all this discussion ; nor do they have the high proportion of psionics or the affinity for magic prevalent among Atlanteans.
i) well... to finish about Mike's comment... there's the question about whether a tattooed man (vamp slayer or sunaj) is or not supernatural enough to forego eligibility to become a Knight... personally, I'd say yes, the same as for dragons, or even draconids phantoms or silhouettes... the same as I doubt the Forge would ever call on one of the Machine People... whell... those of course are my opinions, to be discussed later, and possibly in pv or on another forum