I wondered just how "etcetera" is defined as a part of speech. Apparently few others have thought of that, because I got plenty of examples of usage and only Merriam Webster says it is a noun. However, it isn't just a noun, it is a placeholder in a list of similar nouns, to be used like an ellipsis (....) for something missing. The list needs to have items that go together, conceptually. If you say "dogs, cats, etc.," the implication is domestic animals, or pets. If you say "the cat and dog etc.," then the "etc." is implying a missing list of--what? More dogs? Dog toys? Now if "etc." can be called a modifier, then it refers to "dog" which is the nearest prior noun, i.e. the "antecedent." Getting antecedents wrong (lost antecedent) can totally change the meaning in a sentence and paragraph, and is probably my pet grammar gripe. (Insert picture of lost and confused Auntie Sedent doing Mr. Magoo-like things) Anyway, in the paucity of information about this universal shortcut, I did find that it must always be preceded with a comma.