Below is a diagram showing transverse cross sections of skulls representative of the two subspecies (A and B) of the Trilateral* species. Is it plausible that such obvious differences – noting the rest of their morphology and physiology is essentially identical – could be found within the same species?
* A placeholder name.
Trilaterals are quadrupedal sophonts a little smaller than chimpanzees, and have triple hemispheric (left, right, and posterior, as shown) brains consistent with a body plan evolved from radially symmetrical ancestors. Their 'head' sits atop a narrower, flexible thorax without a discernable neck; a hip-like structure at the base of the trunk supports four legs and two arms.
Trilateral A is the dominant subspecies; B evolved in parallel in geographic isolation. They are capable of interbreeding and producing fertile offspring. There is no statistically significant difference in skull volume or intelligence among mature samples from either population.