A neat example of an a posteriori necessity

Recall Kripke's examples of a posteriori necessities in science were of the sort:

Gold = the element with atomic number 79
Water = the substance composed of two atoms of hydrogen and one atom of oxygen

LaPorte (Natural Kinds and Conceptual Change) provides a neat example that is more analogous to the well-known 'Hesperus=Phosphorus':

The statement 'Brontosaurus = Apatosaurus' has a history quite like that of 'Hesperus=Phosphorus'. 'Brontosaurus' was coined as a genus-term in 1874 by O.C. Marsh who thought he had discovered a new genus of dinosaur in Wyoming. As it turns out, the fossils he discovered were fossils of a dinosaur genus that he himself had already discovered and named 'Apatosaurus'. Marsh supposed that the specimen he associated with the name 'Brontosaurus' and the specimen he associated with the name 'Apatosaurus' could not be from the same genus because there was such a difference in size between the two specimens. He did not realize that the reason for the difference in size was only that one of his specimens was not fully grown. later, another scientist, Elmer Riggs, straightened out the matter, determining that Marsh had applied two names to the one genus. Riggs discovered empirically that 'Brontosaurus=Apatosaurus' is true. (p. 37)

The story about Brontosaurus comes from Gould's Bully for Brontosaurus: Reflections in Natural History, 1991, pp. 79ff.