@dalehileman,
dalehileman wrote:
Quote:….without that starting clause, then a comma is necessary.
Dk, I doubt this, can you give us an example
Would you consider in Al's OP the rest a complete sentence
Yes.
"This Plesiosaur swam the seas where now stands the Dallas-Fort Worth Airport."
The clause above is an independent one and therefore can stand as a complete sentence.
"About 70 million years ago" is a dependent clause that modifies that independent clause.