Ammonites were extinct marine cephalopods (relatives of modern squid and nautilus) that thrived during the Mesozoic Era. Their shells grew in logarithmic spirals, partitioned into chambers by septa (walls). The living animal occupied only the outermost chamber; inner chambers were gas- and fluid-filled, regulating buoyancy through a tube called the siphuncle. 🔬 How this fossil formed (step-by-step): 1. Life & growth – The ammonite secreted calcium carbonate (CaCO₃) to build new chambers as it grew. 2. Death & burial – After death, the soft body decayed. Rapid burial by sediment (mud, silt) limited scavenging and oxygen exposure. 3. Compaction & mineralization – Over millions of years, sediments lithified into rock. Groundwater carried dissolved minerals that: • Permineralized the shell (filling pores with minerals), or • Replaced the original aragonite shell with more stable calcite or silica. 4. Mold & cast formation – If the shell dissolved, it left a mold in the surrounding ...