Geo Photo of the Day: Ammonite

An ammonite specimen (Photo Credit: Natural History Museum London)

Today’s photo of the day is the fossil ammonite. They are an extinct group of cephalopods that are invertebrate marine animals. The earliest known fossils come from the Devonian and they all became extinct during the Cretaceous-Paleogene extinction. The name ammonite comes from the shape of the shape which resembles the coiled horns of a ram.

The setpa inside an ammonite is what divides the chambers inside the shells. These septa have folds which are called lobes and saddles. Ammonites also evolved to develop patterns of lines on their outer shells. There are three basic patterns for ammonite septa:

  • irregular zigzags-this is called goniatite
  • regular wavy-called ceratite
  • intricate feathery or fern like patterns-ammonite.

An artist rendition of ammonites in their environments (Photo Credit: Hooper Museum)

They were predatory, squid-like creatures that had sharp, beaklike jaws inside a ring of tentacles that extended from their shells to snare prey such as small fish and crustaceans. Some ammonites grew more than three feet across!

These shallow, warm water creatures were constantly adding on to their shells but really only lived in the outer parts of the shell. They traveled in the water by squirting streams of water from their bodies!

An ammonite fossil bed in Israel (Photo Credit: Mark Wilson, College of Wooster)