Awkward style choice or syntax error?

I'm doing a bit of copy editing on some work, and ran into a sentence that I feel is poorly constructed; however, the author insists that there is no error and that the supposed error in question is a style choice.

I've included the quote; however, I've swapped out the proper nouns for anonymity's sake, so whether or not the sentence is factual is irrelevant.

Panthera leo senegalensis are the most endangered of lions, residents of Africa.

I feel as if though something is wrong with the phrase "...of lions," but I can't put my finger on the exact rule.

Adding the definite article "the" feels as if though it would help: "...most endangered of the lions." However, from what I understand, the use of a definite article before a plural pronoun indicates the scope, and its use is optional dependent upon the meaning. Therefore, this would change the meaning of the sentence from all lions to a specific subset, "the lions."

So what I'm really wondering is if there's a grammatical rule I can cite that says "of" must be omitted in the sentence, if the definite article is not present. Or perhaps despite reading awkwardly this sentence is still grammatically correct, and it is in fact a style choice?

I also feel as if "...residents of Africa." is a dangling modifier, but in the sentence it is meant to modify lions (not Panthera leo senegalensis) so I'm not sure again if there is a syntax error, or if this is simply an awkward choice of wording.

The most endangered of lions is indeed a slightly formal or old-fashioned construction, I would say, but it is still very much alive and perfectly OK. I would not say the lions, because then you would have to be referring to a previously mentioned group of lions. You could say of all lions, though, if you really disliked the construction.

As you say, residents of Africa modifies lions, and it is therefore not dangling, just a perfectly fine apposition. However, I'm not sure the information it conveys—that lions live in Africa—is entirely correct, relevant, and presented in the right manner. What is it doing there?

One thing I would certainly object to is using a plural verb with panthera leo senegalensis, which is a singular noun phrase. Perhaps this will be acceptable to other readers, though.

I would agree that ", residents of Africa" is a dangling modifier, and would suggest you just end the sentence with "endangered Lions of Africa" or "endangered of African Lions" instead.

I don't know of a rule that says "of" must be omitted; in fact I'm quite sure it's okay to have it there without a definite article. I've read countless books where a character is described, "he was the greatest of men", etc., so that would mean all those books are just flat out wrong. Here we're just switching greatest for "endangered" and men for "lions".

I would also urge the changing of "Panthera leo senegalensis are" to "Panthera leo senegalensis is", seeing as how "Panthera ..." is referring to a singular species (replacing it with "This species" would show how silly it sounds: "This species are [...]"), but that's just my US English speaking up ;-).