File:Nacreous aragonite shell of Baculites ammonite (Cretaceous) 3 (49073196601).jpg

Baculites sp. - fossil ammonite with nacreous aragonite shell from the Cretaceous, probably from western North America. (~5.4 centimeters across at its widest)

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.

Seen here is a fragment from a straight-shelled heteromorph ammonite called Baculites, which was common in Cretaceous seas. The rainbow colors of the shell are "mother-of-pearl", technically known as nacreous aragonite, or just "nacre". At a micron level, nacre is interlayered tablets of aragonite and organic material. Nacreous aragonite occurs in the shells of many molluscs and is the cause of iridescence in pearls.

Classification: Animalia, Mollusca, Cephalopoda, Ammonoidea, Ammonitida, Baculitidae

See info. at: <a href="https://en.wikipedia.org/wiki/Baculites" rel="noreferrer nofollow">en.wikipedia.org/wiki/Baculites</a> and <a href="https://en.wikipedia.org/wiki/Ammonitida" rel="noreferrer nofollow">en.wikipedia.org/wiki/Ammonitida</a> and