I, for one, welcome our new cephalopod overlords ... but what are they?
Aug 18, 2016
With the apparent rise in the numbers of some cephalopod species and perhaps with a disdain for human governance systems, many people are expressing their enthusiasm for welcoming our new cephalopod overlords. If we are to toil under our new suckered rulers, it may be prudent to examine where they came from, especially the octopuses who will likely make up the ruling class in the new mantled hierarchy.
Living cephalopods include roughly 1000 species of octopuses, cuttlefish, nautiluses and various distinct groups of “squid”; ram’s horn squid, bobtail squid, bottletail squid, teuthidian squid and the not-even-a-squid vampire squid.
Cephalopods are molluscs, and weird ones at that when compared to the other mollusc groups: chitons, solenogastres, caudofoveatans, bivalves, scaphopods, gastropods and monoplacophorans. With the exception of nautiluses, living cephalopods lack an external shell. They are free-living active hunters, possess a well-developed brain and nervous system in addition to a famed reputation for predicting the outcome of sporting events. Their intelligence has earned them a level of protection under EU law as “honorary vertebrates”.
Just as cephalopods are weird molluscs, octopuses are likewise weird cephalopods. With the exception of the beak, small rod-like shells in some species and a toothed radula, they lack hard parts like the cuttlebone and guard found in cuttlefish and squid respectively. Despite what popular culture may have you believe, octopuses do not have tentacles (tentacles are appendages in addition to the eight arms that only have suckers at the tips). Their bodies are gelatinous and are famously versatile, allowing them to change in colour, texture and overall shape, making identification of living octopuses from morphology alone very difficult.
So where did octopuses come from? How do they fit in with the rest of the cephalopods?
Cephalopods have a long fossil record, the earliest certain cephalopod fossils are loosely coiled or straight shelled nautiloids found in the Cambrian of China 515 million years ago. It is hypothesised that all other cephalopods diverged from these ancestors. Nautilidan cephalopods, represented today by the six living species of nautilus are thought to have diverged from other nautiloids in the Devonian period. Around the same time as this split, geologically speaking, the straight-shelled cephalopods Bactritida diverged. Bactritidans are thought to be the ancestors of the ammonites, goniatites, ceratites, belemnites and all of today’s major groups.
Surprisingly, however, the palaeontology textbooks are silent when it comes to the appearance and evolution of the octopuses. My undergraduate text book, Invertebrate Palaeontology and Evolution by E.N.K Clarkson, only mentions octopuses in passing. The work on invertebrate palaeontology, the 50 volume series Treatise on Invertebrate Palaeontology started in 1953, still doesn’t have a complete octopus section. This is because fossils of soft-bodied octopuses are incredibly rare. So rare are octopus fossils that one of the few people to write an accessible history of fossil octopuses, Phil Eyden, suggested in 2004 that the entire fossil record of octopuses would fit inside a suitcase.
The oldest fossil octopus at 300 million years old is Pohlsepia mazonensis from Carboniferous Mazon Creek fossil beds in Illinois. Frustratingly, the only known specimen resembles modern octopuses with the exception of possessing eight arms and two tentacles (Kluessendorf and Doyle 2000). Until the discovery of this specimen, the oldest octopus fossils were from the Jurassic period. The discovery of a much older octopus with many features of modern octopuses indicates that they have an even longer history, albeit one that is fossil poor.
From this isolated data point, the presence of tentacles in this specimen could mean that this was the ancestral state of the group (all octopuses derived from ancestors with tentacles). Alternatively, tentacles were lost or evolved multiple times in the group.
The second oldest species is Proteroctopus ribeti a single specimen from the Jurassic of France was published in 1982. This specimen does not possess tentacles but does have clear fins leading to speculation that it may be related to deep sea species such as today’s dumbo octopuses or alternatively, this is a member of the vampire squid group, Vampyromorphida.
2009 was a bumper year for fossil octopuses, with three new species from five specimens collected from Cretaceous outcrops in Lebanon (Fuchs et. al 2009). Styletoctopus annae, Keuppia hyperbolaris and Keuppia levanter were interpreted as the oldest unambiguous octopus specimens. These three species are differentiated by the position and shape of paired ‘gladial vestiges’ supposed to be vestigial remnants of the gladius found in modern squids and which may have supported fins found in modern groups.
Palaeoctopus newboldi from the same region in Lebanon was described in 1896, the first fossil octopus discovered, and slightly younger in geological age than the aforementioned Lebanese species. Palaeoctopus newboldi is known from a number of specimens and was thought to be ancestral to modern octopuses until the 2009 Lebanese specimens showed there was already a diversity of octopus forms in the Cretaceous. A second species, Palaeoctopus pelagicus is known from a single gladius vestige from Mexico (Fuchs et al. 2008).
Only one other genus of octopuses is known, two species of Paleocirroteuthis from the Upper Cretaceous of Canada and Japan have been described from fossil jaws alone (Tanabe et al. 2008).
This material, alongside a few ambigious remains which defy classification and some tricky egg case trace fossils thought to made by extinct argonauts is the sum total of the palaeontological evidence of these groups.
Returning to the original questions about where octopuses came from, this is one group where, to date, the fossil record raises more questions than it does provide answers. It’s possible to dig through the (mostly freely accessible) entire literature in a day and be as informed as the experts.
For groups of organisms like these, the lack of our knowledge about their evolutionary histories can be as interesting as what we do know about them from less than a suitcase full of fossils. At the borders of scientific knowledge, that’s where the fun (educated) speculation has a part to play. Are octopuses distinct cephalopods or a weird kind of squid? What events in the past caused the extinction of most externally shelled cephalopods but lead to the modern diversity of squishier animals? Lastly, if we’re not happy with life under the new cephalopod overlords, what tools might we use to bring them down?