This is a Mangrove Lobsters from Gunn Point, Darwin, Northern Territory, Australia.  The fossil was formed after the area was catastrophically buried by an intense cyclone, with Mangrove trees in Calcium rich mud.  The original exoskeleton of the lobsters was replaced by calcium carbonate, magnesium, strontium and iron creating the fossil.  This particular specimen, although not complete, is well preserved and measures 4″ x 1 1/2″ x 1″.  It would make a great addition to any fossil collection.