We don't have soft tissue or a specific understanding of internal organs. Maybe they had multiple hearts with multiple vascular systems, or maybe the rate of bl

We don't have soft tissue or a specific understanding of internal organs. Maybe they had multiple hearts with multiple vascular systems, or maybe the rate of blood flow and chemistry of energy production differed from animals we do understand. All we know for sure is that the brontosaurus was way too big for its biology to be understood by similarity to any extant creatures. Maybe we'll be able to simulate dna, protein, and evolution at some point such that we can sample huge spaces to see if we can reconstruct creatures with similar features.