An almost complete Tyrannosaurus Rex fossil was sold at auction for a record $31.85 million - making it the most expensive dinosaur fossil. The fossil is more than 67 million years old.

Appraisers at Christie's auction house initially estimated it would fetch around $8 million. However, a 20-minute bidding saw prices nearly four times that. The identity of the buyer was not disclosed, but the company did reveal that it was a telephone bidder.

The T-Rex skeleton, which was nicknamed Stan, is 13 feet high and 40 feet long. It has 188 bones - making it one of the most complete T-Rex fossils. It is one of only about 50 semi-complete T-Rex fossils discovered. Most others are displayed in museums and only a handful are held by private owners.

Before the auction Stan was on display at the Black Hills Institute of Geological Research in Hill City, South Dakota. Owing to its pristine condition, Stan was often used as the model for various depictions and figurines.

The fossil was discovered in 1987 by amateur paleontologist Stan Sacrison, hence the name. It was unearthed in the Hell Creek Formation in South Dakota's Cretaceous Badlands. Paleontologists initially misidentified the fossil as a common triceratops. In 1992, the Black Hills Institute of Geological Research properly identified Stan as a T-Rex.

Stan would have weighed around 7 tons to 8 tons. Based on its skull and neck vertebrae it was believed to have lived a very difficult life, surviving numerous attacks from its own species.

The last T-Rex skeleton auctioned was bought by a consortium that included Disney and McDonald's in 1997. The fossil, nicknamed Sue, was bought by the group from collector's items broker Sotheby's for $8.36 million. Sue is on display at the Field Museum of Natural History in Chicago.