Actually, if you compare the marauder to the basilisk, you'll see that the head shape, claw position, and limbs are very similar. Also, both aliens have those little hooks on the ends of their rear limbs. The marauder is just bulkier, has more armor, and its posture is different. Both the basilisk and marauder concepts were done by the same artist.
You can look at the sequence this way:
Dretch -> Basilisk: limbs become longer, forelimbs are now used for cutting instead of movement, head separates from the body.
Basilisk -> Marauder: more body mass, armored plates on body, posture is changed so that the head is in front of the body instead of stuck upwards.
Marauder -> Dragoon: even more body mass, armored plates are more pronounced.
Dragoon -> Tyrant: huge increase in body size, armored plates now cover the whole body, limbs retain same general shape but are modified.
To be honest though, you can't really expect all the aliens to look like they are closely related to each other. That would be hard to do without putting a huge emphasis on realism, and we're more interested in things looking cool than strictly realistic. For a comparison, look at the old aliens. Seriously, look at Tremulous. Back then, you had what, this sequence?
Brown spider -> white reptilian thing missing a lower jaw -> red reptilian thing with both jaws -> dark blue bulky thing that now bites instead of slashes -> huge blue bulky thing that has an entirely different body shape and goes back to slashing
At the very least, we're going to be coloring our aliens on the gradient I described in the article, and they all have features going along that gradient too. They're not going to look like they all descended from the same organism, but as I've already covered, we're not going for that. We're prioritizing cool over realism. In the end, isn't that really all that matters?
