Stegosaurus

The bony plates of Stegosaurus are among the most analyzed structures in all of paleontology — and after 150 years of study, scientists still disagree about their primary function. Three hypotheses dominate: thermoregulation (plates act as solar collectors and heat dissipators), display (plates signal species identity or sexual fitness), and defense (plates deter predators). Each has supporting evidence, and none has been conclusively ruled out.

A 2015 study by Evan Saitta examined Stegosaurus mjosi specimens and found two distinct plate morphologies in the same population — one narrow and tall, one wide and flat. Saitta argued this represents sexual dimorphism: males and females had differently shaped plates, strongly favoring a display function over thermoregulation (which should produce consistent plate shapes in both sexes). If correct, Stegosaurus plates were essentially enormous billboard advertisements for mate quality.

The thagomizer — four tail spikes named after a Gary Larson 'Far Side' cartoon — shows real battle damage. An Allosaurus caudal vertebra was found with a Stegosaurus spike-shaped puncture wound, and a Stegosaurus tail spike was found with healed bone at its base (suggesting the spike broke, lodged in an attacker, and was resorbed). These aren't hypothetical interactions — they're fossilized proof of predator-prey combat across a geological instant.