After studying the official NASA image I strongly believe that this artifact is possibly the carved top or lid of an ancient tomb. Possibly holding the remains of a Martian Knight or King.
According to Enkispeaks artwork on Mars found thus far consists almost entirely of statues of human and animal heads, intaglios carved casually on rocks featuring human and alien faces, and vast etchings carved on the ground conveying human and animal themes.
The built structures found in the photograph include an image carved on the slab that resembles an Egyptian pharaoh and several large stone sarcophagi, including the ossuary of a king in the Egyptian style laid on a cliff top.
The video below shows the carved top or lid of an ancient tomb.
Additional, some interesting objects found in the latest images captured by Mars Rover Curiosity.
Head – Skeleton of a human-like being.
Statue of dragon besides an artificially made square/ entrance?
A strange anomaly what looks like an arm/hand and a medieval knight shield symbol carved on a wall, covered with sand.
A statue resembles the ancient god Osiris and a stone block with pipe.
Alien with long arms sits on the ground. (See white arrow).
Furthermore, behind the alien you can see the faces of two persons, but it can also be a shadow that looks like two faces. (See red arrows).
Link image Medieval Knight: http://mars.jpl.nasa.gov/msl-raw-images/msss/00727/mcam/0727MR0031100000402932E01_DXXX.jpg
Link additional images: SOL 1110