Strange fossil defies grouping

I've got to hand it to paleontologists. Go look at the artist's sketch of the creature that is the subject of the linked article, then look at the image of the creature itself. That someone can imagine such a creature from such a fossil is simply amazing. Nevertheless, there exists a creature that does not readly fit current models. The story gives rise to useful points about subject identity.
The trouble is the animal, named Vetustodermis planus, did not possess a set of features, or characters, which placed it clearly within any known group.

I am interpreting the word "characters" to mean characteristics. This creature identity issue is telling in the sense that it suggests open issues for topic maps subject identification processing. How does ISO 13250 address subject identification? Section 5.2.1 "Topic Link Architectual Form" of ISO 13250 suggests this:
The optional subject identity attribute refers to one or more indications ("subject descriptors") of the identity of the subject (the organizing principle) of the topic link.

There exist numerous interpretations of 5.2.1, which are manifest in XTM, TMDM, and TMRM. Is it appropriate to revisit the assumptions inherent in those interpretations?

I am indebted to Patrick Durusau for long and productive discussions centered around the subject identity issues related to topic maps implementations. I'd like to see such discussions in greater depth, in public.