Although very erudite and scholarly and a user whose knowledge and education I greatly admire, I think "petr b" is way off base - his antitheses towards Wagner should be well known now by any and all regular patrons of this category. And I really don't mean to be pejorative: he to me, just seems to have an irrational bias when it comes to anything regarding Wagner.
Don't know whether you've googled the topic, but here's a series of articles you may find enlightening when I did:
http://www.google.com/search?sourceid=navclient&aq=8&oq=der+ring+des+n&ie=UTF-8&rlz=1T4RNTN_enUS349US350&q=der+ring+des+nibelungen+story
Remember my in the past mentioning Dr. Jane Ennis, a recognized authority on Wagner? You may find this website of her's interesting/possibly enlightening:
http://users.utu.fi/hansalmi/ring.html
My understanding is that the characters and mythological setting is primarily based on the "Nibelungenleid": http://www.bookrags.com/Nibelungenlied
But the scenario/plot is essentially that of Wagner alone. Granted, it's a very entangled one and very involved/difficult, for many if not most to make any real sense of. I once did an analysis of the time-span covered; and if one calculates it in human terms, it's that of an approximate 42 years(minimum)if I recall correctly.
To wit: the 9 valkyries are sired and birthed subsequent to the conclusion of "Das Rheingold"; Siegfried is born and if one assigns a say teen-age date to his entire life span, that's another 16-7-8 years, minimum; and so on, etc., etc.
My interpretation of the plot, is that Brunnhilde's ultimate sacrifice definitely ends the reign of the Gods; the Rheingold is purified of/freed from Alberich's original curse upon it(which is the genesis of the cycle)by her returning it to the Rhinemaidens in effect, when she immolates; and the entire world including the human race is thus purified, and can begin anew.
As to your specific question(caption), I would say that the Ring Cycle takes place during and ending a/the mythological era; thus paving the way for a human historical one, as projected/idealized by Wagner in this great epic.
Alberich
------------------------------------------------------------------------------------edit: "odzooker..": I stand before you, guilty as charged; very well said, and even better, "put". What else can I say?
I hope your response is chosen as Best; HONESTLY. "petr b" and I should be taken out behind the woodshed, and be given the "intellectual flogging/whipping", we both so richly deserve.