Which fossil observation supports Pitx1 involvement in pelvic spine loss in ancient lake populations?

• A. Right pelvic vestiges were larger than left
• B. Pelvic vestiges were completely absent in all fossils
• C. Pelvic vestiges were larger on the left in more than 75% of cases
• D. There was no pelvic vestige observed

Answer: (C)

Pitx1 directs left-right patterning in pelvic development, and reduced Pitx1 expression is linked to pelvic spine loss. If ancient lake fossils show pelvic vestiges that are larger on the left side in more than three-quarters of cases, that reveals a consistent left-side bias in how the pelvis develops. This directional pattern fits with Pitx1 operating to produce asymmetry, making one side more developed than the other and thus supporting its involvement in the evolution of pelvic spine loss. Other scenarios—like no vestige on all fossils or a random or right-side bias—would not as clearly point to a left-right regulatory role for Pitx1.