Ammonites

Ammonites

  

Ammonites are perhaps the most widely known fossil, possessing the typically ribbed spiral-form shell as pictured above. These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs. The name 'ammonite' (usually lower-case) originates from the Greek Ram-horned god called Ammon. Ammonites belong to a group of predators known as cephalopods, which includes their living relatives the octopus, squid, cuttlefish and nautilus. 

Ammonites began life as tiny planktonic creatures less than 1mm in diameter. In their infancy they would have been vulnerable to attack from other predators, including fish; however, they quickly assumed a strong protective outer shell that shielded their soft interior from damage. Evidence suggests that they gained in size rapidly, with females growing up to 400% larger than the males.

Nautilus shells comprise of individual chambers, each growing in size as the creature grows. These chambers are secreted by the creature at a rate of one every four weeks, equal to 13 each year. Using this as a guide an ammonite shell containing 26 chambers could be assumed to have housed the creature for two years. Like the nautilus, ammonites retained their original shell throughout their life. However it's worth noting that in comparison to modern day nautili which live in cold, deep water, ammonites preferred warm shallow waters and may have had a higher metabolism. Consequently, it's possible that ammonites could reach larger sizes far quicker than modern day nautili.

Ammonites were the predators of their time, feeding on most living marine creatures including molluscs, fish and even other cephalopods. By analogy to modern cephalopods, their method of attack probably comprised of silently stalking their prey, then rapidly extending their tentacles to grasp the target. Once caught the prey would be devoured by the ammonite's powerful jaws, located at the base of the tentacles, between the eyes.

Much of the ammonite's life was spent in shallow waters. The evidence to support this includes their diet, which could be found in the greatest volumes in the warm shallows. It is also unlikely that their shells could withstand the high pressures present in deep water (over 100 meters). Other theories based around their social behaviour suggest their shells were decorated by an array of patterns, indicating that colour/good light played a large part in their lives.