When a comma and a coordinating conjunctions are used, both sides of the comma should be independent clauses. However, I have seen many examples in reputable sources where this is not the case. Is this occurrence due to writers treating coordinate conjunctions as part of a parenthetical phrase? Is it correct to do so? Any guidelines in regards to this in American English? Thanks.
Example: "She was the mother of three other foals, and was inducted into the AQHA Hall of Fame in 2009."
Example: "So profound is our ignorance, and so high our presumptions,that we marvel when we hear of the extinction of an organic being."
Shouldn't there be no comma before "and" in both examples?