1) location of death (or final resting place, anyway) matters a ton for likelihood of fossilization,

2) not all animals die in the same places,

3) if sex has an influence on the distribution of likelihood of dying (or ending up in—water carries corpses, predators and carrion-eaters may drag them) in fossilization-friendly locations, we'd expect to see a difference in the rates of fossil discovers of that species by sex, different from whatever actual distribution there was of male & female in the original population.