The fossils don’t interest Sherlock, in and of themselves. They’re plebian at best, and probably fake, though he doesn’t waste time examining them too closely. He’s more interested in how his fellow Tributes are interacting with their environment. It’s not the same watchful, hungry gaze of the Capitol; instead, Sherlock’s interest is academic. He’d do well to apply his knowledge to his own survival, considering that he’s been only lucky so far. But he hasn’t gotten to that point, yet.

When he spots Karkat, his mind starts filling in the blanks based on pattern. He knows how to trace lineage through a human’s physical appearance, and so he subconsciously decides that the same rules might apply to… whatever these new people are. So he takes into account horn-shape, height, the pallor of the Alternians’ skin. Kankri had not been a threat. But genetics are not a hard and fast rule, in the test of homicidal tendencies.

He’s leaning against one of the fossil exhibits, almost too casually. He waves, too, entirely at odds with his circumstance. “Is your weapon a criticism on capitalism or an announcement of your occupation as a cropper?”