As dead as a dodo
Unambiguously and unequivocally dead.
The dodo was a flightless bird somewhat like a turkey. It was native to Mauritius; the last live specimen was seen in 1662 and they are thought to have died out completely by 1690. The extinction of the species is attributed to the introduction of domestic animals to Mauritius following the first visits to the island by the Portuguese in 1507 and the later settlement by the Dutch - although the species was thought to be then already in decline.
It's a good job we didn't choose 'as dead as a coelacanth'.
[Note: biologists believed that coelacanths had been extinct for 70 million years until 1938, when South African fishermen were found to have been regularly catching and eating them.]
There are no precise pictorial records of live dodos and paintings of them date from after 1662. Discoveries of skeletal remains of the birds have enabled biologists to reconstruct their form, which is now thought to be somewhat more slender then the familiar representations.
Lewis Carroll used the Dodo as a character in Alice's Adventures in Wonderland, 1865, and it was the popularity of this book that led to the widespread use of the phrase 'as dead as a dodo'. There are citings of the phrases 'as rare as the dodo' in the 1860s and in the 1870s we find 'as extinct as a dodo'. The earliest record I can find of 'as dead as a dodo' is a reprint of a story from a Liverpool newspaper in the Bangor Daily Whig And Courier, May 1891:
"After the next general election Mr. Parnell will have only four followers. Except as a private member of Parliament he is as dead as a dodo."
See other 'as x as y similes'.