There really isn't just one signature artifact for Spiro, because so much art was found there.  Almost 75% of all elite art in the Mississippian Culture came from Spiro. Some of the items that are recognized as special are the fabrics (especially the lace), the bird effigy copper celts, the embossed copper plates, the incredible conch shell engravings (some of which were used as a writing system), the effigy pipes like the Resting Warrior, and the cedar masks that had copper covering and shell inlays.  Spiro was plundered during the depression (starting in 1933) by a mining company that was set up to hunt for treasure.  Fortunately, some of the art has been recovered.  If there had been a systematic dig, who knows what we would have discovered and learned. 
Let us hope that something like the looting of Spiro never happens again. 
An engraved conch shell and a sketch of the engraving. 
A cedarwood ceremonial mask with
shell inlay in the form of a deer's head.  
A ceremonial hafted
bird effigy copper celt
about two feet long.
Click on the celt for a
larger view.

Spiro was a major site for copper artifacts. Over 250 copper plates with various images and 25 celts were discovered there.  Copper was an indicator of great wealth.
An embossed copper plate from Craig Mound, Spiro.
The bird figure, identified as a peregrine falcon, has a distinctive "weeping eye" motif that appears in many falcon depictions on  religious paraphernalia associated with the Southern Cult.
SPIRO ARTIFACTS
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