Archaeologists say excavations for a Mexico City subway extension have turned up what appears to be an unusual Aztec offering.

Excavators found a dog's skull with holes that indicate it was displayed on a ritual skull rack normally reserved for human sacrifice victims.

The find has left researchers scratching their heads. They also found a woman's skull with similar perforations around the temple. The perforations allowed skulls to be mounted on the racks.

The find dates to between 1350 and 1521, the date of the Spanish Conquest.

Mexico's National Institute of Anthropology and History says several offerings were uncovered during the construction of the subway line.

That work was performed between 2008 and 2012, but the finds were announced Tuesday.