That is correct. In fact, one could say that alien lifeforms need to
NOT conform to Earth taxonomy. Because evolutionary theory says that all Earth lifeforms are related to all other Earth lifeforms, and that members of any specific taxonomic groups are more closely related to each other than they are to members of all other taxonomic groups.
And obviously alien lifeforms cannot be members of any Earthly taxonomic group, no matter how much they are similar due to convergent evolution.
Biologists on Earth believe the evidence indicates a strong probability that all present lifeforms on Earth are descended from early lifeforms which existed on Earth billions of years ago, and thus that all lifeforms on Earth are related.
In "Return to Tomorrow":
http://www.chakoteya.net/StarTrek/51.htm
Which indicates in that
Star Trek as in real life, the lifeforms native to Earth are all related, and and are probably totally unrelated to lifeforms on planets in other star systems.
A reptile is defined as an Earthly lifeform that is a member of the taxonomic class
Reptilia. Thus all reptiles are descended from primitive single celled lifeforms existing on Earth billions of years ago and related to all other lifeforms on Earth.
It is considered scientifically impossible for a lifeform that evolved naturally on another planet in another star system to be related to any Earthly lifeforms. Therefor if a lifeform native to an extrasolar planet seems to belong to a species that evolved on Earth, there would be a paradox. A paradox which can only resolved by either proving that lifeform actually belongs to that Earth species and was introduced to that planet sometime previously by space travelers, or by proving that lifeform has differences from the Earthly lifeform it seems identical to.
It seems totally impossible for any extrasolar lifeform to belong to the same species, genus, family, order, class, phylum, or kingdom as any lifeform native to Earth. Therefore, any similarities between lifeforms on Earth and lifeforms in other solar systems must be due to accidental convergent evolution producing similar lifeforms on different planets.
Therefore no
Star Trek lifeforms on other planets can be members of the same species, genus, family, order, class, phylum, or kingdom as any Earthly lifeforms star. Which means that neither Gorn, nor Voth, nor Cardassians can be reptiles, no matter how similar they are to Earthly reptiles due to convergent evolution providing similar developments in similar situations.
Of course in
Star Trek there have been advanced civilizations that traveled interstellar space thousands and millions and maybe even billions of years ago. Thus lifeforms from various planets could have been taken to other planets and spread on those other planets and evolved into new species related to the lifeforms on the planets there ancestors came from.
But in
Star Trek there seems to be no evidence that this happened on Earth. And so, unless for some reason Earth is very, very, very special in this regard, there must be many other planets in the
Star Trek universe without any evidence that any life from other planets and star systems was ever seeded on them. So even if some reptile-like alien lifeforms actually are reptiles descended from Earth reptiles, there should be many other reptile-like alien lifeforms which are not reptiles descended from Earth reptiles, but which have developed separately by convergent evolution to resemble Earth reptiles.
Therefore, it seems inaccurate to describe alien plants, animals, and people as being rodents, or dolphins, or reptiles, or starfish, or elephants, or clovers, or sponges, or viruses, or as members of any other Earthly group of organisms, no matter how much they resemble them.
