Because I was a top student in anatomy of the human kind, and have seen these things in the flesh, what is really creepy about this is the similarity in addition to the differences in the rib cage -- this was a strong alien, just by comparison, and whatever killed it could surely kill lil' me...
Yeah it scared the jabeezus out of me. Worst part was its blood melted my scalpel!
That was to be expected, in your line of work... there are some dangerous creatures out here ... check out these Needle Harpies...