I was thinking about this before the Tholian wave, but it’s apropos.

It unscientifically appears to me that TOS had a far higher incidence on non-humanoid aliens than later series. Tholiens, Horta, the flying neural parasites on Deneva; while there were many bipedal aliens sometimes differing only by skins color, many were non-bipeds or were bipedal but radically different from humans, like the Gorn and the salt vampire. In later series, it seems nearly all aliens were reduced to bumpy head species.

TOS ran for only three seasons, and truly different aliens are expensive; I understand the economics of going the prosthetic forehead route. And it’s difficult to have recurring truly alien biology in a series.

My question is whether anyone’s done a statistical analysis covering the originality of aliens, per series, based on divergence from the humanoid base. Does it only seem like TOS had more different types of aliens (intelligent and non) because it was so short, or was the universe really more diverse in TOS?