Platypus bills and some dolphin whisker pits both sense weak electric fields. What pattern is this?

Answer: Convergent evolution

Universal mammal sense — If electroreception were a universal mammal sense, dogs, horses, and humans would be using it routinely, which they do not. The dolphin result is more interesting because it is not a simple shared inheritance. It shows that water can make an old or local structure worth repurposing into a new sensory job.

Convergent evolution ✓ — Correct. Platypuses and electroreceptive dolphins are distant mammals, yet both can use passive electroreception in water. Some dolphins use transformed whisker pits, while platypuses use mucous-gland electroreceptors on the bill, so the anatomy differs. The shared pressure is the watery environment, where weak bioelectric fields can be useful at short range.

Sonar becoming touch — Sonar and touch are real dolphin themes, so this is a plausible mix-up, but electroreception is a separate channel. Dolphin clicks map objects acoustically, while the electric sense detects weak fields through sensory pits. The surprise is that a famous sonar animal also has a quiet, close-range electric sense.