Jurassic Period, 199-145 million years BP. A free-standing Promicroceras marstonense ammonite display with large Asteroceras sp. ammonite on the foreground, from the Lower Lias, Obtusum Zone of Marston Magna, Somerset, UK. 783 grams, 11cm (4 1/4"). From a Lincoln paleontological collection. This is a section of rock containing dozens of small carefully-prepared ammonite fossils and often referred to as Marston Magna Marble.