First appearing in the Devonian period and descended from an animal called a Bactrite, Ammonites roamed the seas of the earth from around 400 million years ago, and didn’t die out until around 65mya, a staggering period given the relatively infinitessimal time mankind has been around. At every one of the world major extinction events only a few species of ammonites survived, but they always bounced back until their luck finally ran out, along with the dinosaurs, at the end of the Cretaceous period.
Ammonites were predatory mollusks, very mobile and with tentacles. Very close in appearance to the still-living Nautilus, they were in fact more closely related to the octopi. Usually spiral in shape, although straight species aren’t rare, they remained buoyant using a siphuncle, basically a biological pump and siphon system. Each of the segments in the shell were a chamber that the animal resided in and the pattern of the edge of each chamber, called a a suture, is what marks out each species.
Many species probably carried ink sacs for defence, at least some were plankton feeders, and many were munched on by huge undersea reptiles called Mosasaurs. Fossils are plentiful and range from the tiny up to a colossal two meters in diameter! Some, especially in Europe, are so beautifully preserved that the original mother-of-pearl sheen is still fully intact. Others are less well preserved but show extraordinary internal detail with some having quite amazingly complex suture lines. Regardless, fossils are plentiful and make a great way to date rock formations.