are different taxa according to Tschopp et al. (2015)...
So, the Manuals could have been 75 years behind or
almost 40 years ahead
of their time...
However, I do not think you have to do a Brontosaurus
icon yet - I am not sure if the division is going to last, and the icon would probably look like your Apatosaurus
I am quite interested in doing the water spider. I read to my surprise that in D&D, there are marine versions which have 20 ft leg span diameter and 10 ft body diameter. Not sure how such a creature is supposed to work, particularly concerning the breathing method, but it sounds funny!
What I wondered is if the real sea spiders (pycnogonids) mentioned in the monster manuals... If not, the marine giant spider could perhaps be depicted as something ancestor-like (kind of mix of pycnogonids and araneaeids).