Yellowstone National Park, Ptychopariid trilobite

YELL-165706 and YELL-165715 (pictured here): Many times when fossils are collected they are only casts of the specimen, such as the pictures above of a few nearly complete Ptychopariid trilobite (YELL-165706 and YELL-165715) casts. As an organisms becomes buried in sediment, its body decays, leaves an open space in the shape of the animal and then gets filled in with sediment creating an external cast of the organism. If an open space or cavity remains after the animal is buried and become fossilized then it is referred to as a mold. ;
Megan Norr;
March 2016;
Catalog #20800d;
Original #USNM_165715