Eopachydiscus marcianus (Shumard, 1854) from the Cretaceous of Texas, USA (Bill Van Deventer private collection, displayed at Coral Caverns, off Cavern Street in the small town of Manns Choice, southern Pennsylvania, USA).

Ammonites could attain large body sizes. Some fossils are so large that they cannot be picked up by one person. Here are a couple Eopachydiscus specimens that are getting to be a bit big. These are from the Duck Creek Formation (upper Albian Stage, mid-Cretaceous) of Spring Creek, Cook County, Texas, USA.

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

Ammonites get their name from the coiled shell shape being reminiscent of a ram’s horn. The ancient Egyptian god Amun (“Ammon” in Greek) was often depicted with a ram’s head & horns. Pliny’s Natural History, book 37, written in the 70s A.D., refers to these fossils as “Hammonis cornu” (the horn of Ammon), and mentions that people living in northeastern Africa perceived them as sacred. Pliny also indicates that ammonites were often pyritized.