Shark
Reference: Ampullae of Lorenzini
A skeleton of cartilage rather than bone, and a sense humans have no equivalent for: it can detect the electric field of a heartbeat.
Scored on every axis
Reading electricity
The ampullae of Lorenzini are jelly-filled pores across the snout that detect minute electric fields. Every muscle contraction — including a heartbeat — produces one. A shark can locate a flatfish lying motionless and buried under sand, with no visual, chemical or pressure cue at all.
Alongside it runs the lateral line, reading pressure changes along the flank: touch extended outward into the water.
Cartilage, and the gap in the record
A cartilage skeleton is lighter and more flexible than bone, which suits a body kept up by swimming. It also fossilises poorly, so the fossil record of sharks is mostly teeth — which is a fact about preservation, not about the animals, and is exactly the sort of distinction this app tries to keep visible.