What is the biggest invertebrate that is a dinosaur?

Answer 1

Dinosaurs are subset of the Archeosauria.

This means Dinosaurs are Exclusively Reptilian.

And Reptiles are All vertebrate animals with no exceptions.

If what you mean is "what is biggest Paleo-invertebrate"

Then the answer could vary depending on what time period you

are talking about.

Their have been some very large ancient cephalopods in the Ammonite and belemnite clades allong with many others which the names currently escape me.

The Eurypterids "Sea scorpions" featured animals the size of Golf carts or larger.

However if you were wanted to know the very biggest off all, it would likely fall into the Cnidaria "jellyfishes and corals" and the Poriferans "the Sponges"

In particular the sponges are capable of reach truly titanic proportions

and they "the sea sponges" were some of the very first Animals to ever exist!