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Everybody loves squid, cuttlefish, and octopuses…and we all love all of the crazy belemnites, ammonites, nautiloids, and orthocones which came before them. But, if you are like me, you have probably been sitting around wondering what came before that. How old are cephalopods, really, and what were the first ones like? Yet, although cephalopods are amply represented in the fossil record from the Ordovician onward, their very earliest origins are shrouded in controversy and mystery. Although there are various fossils which might be cephalopods (or their antecedents) at present the oldest animals to be indisputably classified as cephalopods are the Ellesmerocerida. This order of nautiloids flourished at the end of the Cambrian and into the Ordovician 9approximately half a billion years ago).
Although they were definitely cephalopods, the Ellesmerocerida were somewhat mysterious themselves. They were typically quite small—or even minute. They seemingly had ten arms–although this is a conjecture based on where the muscles attached to their shells (and based on what we know of their descendants). The soft parts of the first cephalopods were not preserved and so we don’t exactly know.
Their shells reveal close-spaced septa–closed off interior spaces within the shell, which provided buoyancy. The Ellesmerocerida also had relatively large ventral siphuncles—tissues which pass longitudinally through the septa to allow buoyancy control. So the first cephalopods we know about were more or less built on the same line as the subsequent ones (until belemnites internalized the shells). I wonder what else we will find out about the origins of this fascinating group of animals as we learn more about paleontology.