Tuesday, 16 November 2010
Fossils - Part 2: Orthoceras and Belemnites
Orthoceras (Phylum Mollusca, Class Cephalopoda) lived from the early Ordovician 495 million years ago) until the Triassic Period which ended 206 million years ago so the species was around for about 300 million years!
Orthoceras was carnivorous possibly even preying on trilobites and would have swum in the ocean or crawled on the sea floor. They ranged in size from a few inches to over 14 feet and their soft bodies lived in the last open-ended segment of their shell.
The name orthoceras means "straight horn".
It is the long conical shell that the animal lived in that is preserved as a fossil as can be seen in the photos of polished fossils below.
Belemnites lived during the Jurassic and Cretaceous Periods and were entirely marine being cephalopods that became extinct during the mass extinction event at the end of the Cretaceous 65 million years ago.
They looked very similar to today's squid and today their closest relatives are squid, cuttlefish and octopuses. It is believed they were carnivores and probably move through the water in a similar fashion to squid.
Very rarely whole belemnite fossils have been found but the vast majority are pieces of the bullet-shaped external skeleton which was made of calcite and called the guard (see photo below). The guard which was located at the rear of the animal may have acted as a counterbalance to the head at the front and helped keep the organism level in the water.
Belemnite fossils are often washed out of Jurassic and Cretaceous clays and occasionally large numbers occur together which may represent what was a mass mortality following mating, as happens with squid in current times.
Belemnites were name from the greek word belemnon meaning dart. During the medieval period it was believed that belemnites were thunderbolts which had been turned to stone.