American Indian casinos aren't exactly new to the game—people were playing dice in the New World as early as 5,000 years ago, preliminary research suggests.

Mysterious holes arranged in c shapes—punched into clay floors at the Tlacuachero archaeological site in Mexico's Chiapas state (see map)—may have been dice-game scoreboards, according to archaeologist Barbara Voorhies.

If so, Voorhies added, the semicircles are the oldest known evidence of games in Mesoamerica, a region that stretches from Mexico to Costa Rica.

Previously, the oldest known evidence of games in Mesoamerica was a 3,600-year-old ball court located not far from Chiapas.

Voorhies first found one of the arcs in 1988, when she discovered a buried floor within a Chantuto shell mound, a large ancient pile of discarded seafood shells and other debris. The Chantuto people were foragers who lived along the coast of what is now southern Mexico between about 3,500 to 7,500 years ago.

In 2009 she found another clay floor just below the first floor—as well as portions of nine other arcs. The upper floor has been radiocarbon dated to about 4,300 years ago, the lower to about 4,800 years ago....