Osteolepis macrolepidotus. Age: Devonian (380 million years ago) Location: Old red Sandstone, Scotland, UK
Okay.. this is actually pretty cool.
This fossil is of a lobe finned fish. It belongs to the Rhipidistia: the group that both lungfish and tetrapods are derived from. Although this animal is probably more related to a ceolocanth than to a human, the family is likely to be close to the base of the tetrapods. All mammals, birds, reptiles and amphibians share a common ancestor, which had a lot of important characteristics found in this fish.
This photograph is of the actual item that has been offered for sale. Note that an information card has been attached to the rock and this has left an area which is lighter than its surroundings.