It takes very rare and special conditions to preserve a specimen like this in both positive and negative forms. First the dead creature must be covered by sediment very quickly, usually at the bottom of a body of water, and then this sediment must harden to form a nodule wholly enclosing the animal but remaining distinct from the sedimentary layers building up around it. Once the organic material has decayed away, the cavity is filled w/some other mineral - calcite, siderite or similar - while retaining both positive and negative impressions. This is not dissimilar to the way all fossils are formed, but if the nodule does indeed remain distinct from the rock that forms around it, then once removed whole it can be split along the naturally weaker plane on which the original creature lay to expose two mirror images as here; the positive and negative record of an 80 million year-old fish. Such specimens are, of course, rare and highly sought-after, and this example preserves the remains of a 2-3/8" long Diplomystus, from an extinct genus of freshwater fish distantly related to modern-day herrings, alewives, and sardines. This Diplomystus specimen came from Cretaceous Period deposits in Southwest Wyoming. The example we are offering is wonderfully preserved, w/individual vertebrae visible on left side of the as well as well-defined tail and fins on both specimens. A superbly prepared example of a highly rare fossil. From the Robert M. Overstreet Collection and comes with COA.