I have been down my work hole for weeks. Apologies, plain people of Crooked Timber. Also, I haven’t worked on On Beyond Zarathustra for months. Maybe that’s even worse. Gotta get back into the good stuff over the summer. Here is a downpayment. I’ve found the first occurrence of a Dr. Seuss-style tree in Western art. It’s from the Vienna Genesis, which is pretty awesome proto-comics and you should check out all the pages at Wikipedia.
I don’t recall Scott McCloud saying anything about this in Understanding Comics. If you want to read a confusing scholarly discussion, try Franz Wickhoff on Roman Art. I think it’s the earliest occurrence of ‘continuous narrative’, also ‘illusionism’. And his use of the latter is eccentric, so you are sure to be the life of the party discussing his ideas!