Fossil Octopuses

Fossils of octopuses are by far the most enigmatic and mysterious of all the ancient groups of cephalopods. Due to their delicate structure fossils of these animals are exceptionally rare, as the soft-bodied nature of the animal does not lend itself to fossilisation. They are so rare that there is just one known from Illinois (296million years old), one from France (164m) and just a handful from Lebanon (89-71m). Very little is known about their history, how they evolved and developed, or their lifestyle. Following is a brief look at some of the theories surrounding them, the three octopuses themselves and the sites they were found in. It should be remembered that these three forms almost certainly do not represent a single line of descent.

Evolution

A little background may be of assistance in trying to understand these fossils. If we travel back in time to the Late Devonian period (408-360m) practically all the cephalopods we would find were nautiloids. The very earliest ammonoids were starting to appear in forms known as goniatites, and living alongside them were a strange off shoot of the nautiloids known as the bactritids. These bactritids had long straight conical shells and share similarities with not only the ammonoids, but also the belemnoids; it is from this peculiar transitional group that all later forms of cephalopod including the coleoids (belemnites, vampyromorphs, squid, octopus, cuttlefish and argonauts), and ammonites are believed to have evolved. As common ancestor of the coleoids, it seems likely that at least in some forms, the shell of these bactritids may have been internal.

All these differing forms of coleoid had another very important feature in common; they all had ten arms. Each group took this in a separate direction; the belemnites, which probably represent the most 'primitive' condition, had ten equal arms covered in hooks. The vampyromorphs probably slowly reduced one arm pair into long filaments as demonstrated in the modern deep-water relic the Vampire Squid (Vampyroteuthis). The squid, spirulas, other teuthids and ultimately the cuttlefish adapted one arm pair to form tentacles and the octopus, the most 'advanced' form of cephalopod, lost the arm pair completely leaving it with eight arms. It is widely accepted that the octopods are closer to the vampyromorphs than the squid mainly due to the number of arms and the fusion of the head to the mantle. In addition, studies of the DNA of the coleoid groups have indicated that octopods and Vampyroteuthis have a closer affiliation than comparison with the squid or Spirula. The most likely scenario seems to be that during the Late Devonian the first octopods stemmed from the primitive vampyromorphs almost losing any trace of an internal shell by the time of our first fossil, Pohlsepia.

Precisely how, when and why the earliest octopods lost their shells is, as is so much with these ancient animals, unclear. There are a number of theories including the possibility that loss of a shell aided burrowing in order to evade predators and to find forms of food that would remain largely inaccessible to the fish and sharks of the time. Another theory holds that shell loss enabled the octopods to colonise deeper water than the nautiloids and ammonites as they would not have been limited to an implosion depth by a gas filled shell. Though these mechanisms are uncertain it is clear that colonisation of the deep sea was extremely successful producing the interesting and varied forms we have today. The living cirrate octopod Stauroteuthis has even modified its suckers to produce bioluminescence.

Whether or not the primitive octopods moved into deeper waters as a response to predation pressures (where they presumably lost their shells) is a debated issue. This largely depends on whether the Devonian fish and sharks shared the same diet and occupied a similar niche, but with so few fossils to examine it is very hard to determine what precisely that niche was and if competition was really an important factor. Another theory holds that the octopuses later returned to shallower waters following the end-Cretaceous extinction having colonised the deep sea where they were immune to the worst effects of the extinction event. On the other hand, the octopus is a highly tactile and visual animal with very well developed eyes capable of astounding colour changes; it is unlikely that these features would have originated in the deep where such abilities would be largely redundant.

From the fossils alone it is clearly impossible to tell if these animals had chromatophores and were capable of colour change, but it seems likely. If it is considered that both the modern squid and octopus groups have this ability then it is highly probable that their common ancestor in the Devonian amongst the bactritids could too, otherwise we would be looking at the less-likely scenario of independent evolution of colour change ability in these groups. (It is likely that the only surviving vampyromorph, Vampyroteuthis, has lost its chromatophores due to adaption to a deep-water habitat where they would be of little use).

It is interesting to note that the three forms of fossilised octopod that have been published all bear a physical similarity to some of the modern cirrate (finned) octopod forms that are alive today, mostly confined to the deep sea. In common with modern octopods such as Cirroteuthis, Opisthoteuthis and Grimpoteuthis, they all have short squat bodies and have powerful fins used to propel the animal whilst swimming. However, unlike these modern cirrates the fossil forms existed in fairly shallow warm or tropical waters, though in common with them they were probably bottom-dwellers. This is not necessarily a contradiction to the deepwater origin theories above, the lack of fossils of deep water octopods does not mean that they were not there, and not all cirrates were necessarily deep water adaptations.

It seems clear that cirrate octopuses are clearly an ancient and primitive form; unfortunately it is currently impossible to determine when the more familiar incirrate (non-finned) octopuses evolved due to the lack of fossils. We cannot even be certain whether the incirrate octopuses gave rise to the cirrates, or vice versa, though the latter theory seems more likely given the recent discovery of Pohlsepia.

Pohlsepia mazonensis

clearly had its own ancestors and even at this early date had clearly defined cirrate-octopus features. The true origin of the octopods must have happened a few million years before even this remarkable fossil.

The fossil hails from the Upper Carboniferous deposits at Mazon Creek in Illinois, a source of extensive coal deposits. Many other cephalopods have been found in these deposits including nautiloids and the shelled torpedo-shaped ten-armed coleoid known as Jeletzkya. Specifically Pohlsepia comes from the Francis Creek Shale Member, this site of exceptional preservation consisted of rapid deposition of silt and sediments believed to have been at the mouth of a river delta where it met the sea. It is believed that storm surges following heavy rains swept masses of sediment down the river and out to sea burying coastal and marine animals and vegetation extremely rapidly. Concretions of ironstone then formed around the dead animals very quickly. Pohlsepia originates from the 'Essex' marine deposits and is preserved as a carbon film resembling a compressed stain inside one such nodule; this is typical for most fossils from Mazon Creek.

Just one example of Pohlsepia is known; as it is in a primitive condition the octopod actually has ten arms, two of these were modified but the other eight were approximately of the same length. The animal is small and is estimated to have had a Mantle Length of just 25mm long by 35mm wide. The animal lacks an internal shell much as with modern cirrate octopuses. The animal is sack shaped, has no clearly defined head and has very short arms. It also had two fins on its mantle, which are longer than they are wide, much like modern cirrate octopuses. The fossil has been preserved in a ventral aspect, eyes, a funnel, mandibles and a radula are identifiable and there is an indistinct feature that may represent an ink sac (extant cirrate octopods do not have these). No arm hooks or suckers are present. Peter Doyle and Joanne Kluessendorf published the fossil in 2000 and they have concluded that Pohlsepia should be assigned to the order Cirroctopoda.

Pohlsepia is housed at the Field Museum of Natural History, Chicago, Illinois.

Proteroctopus ribeti

Moving forward in time 132 million years to the mid Jurassic, at a date of 164 million years ago (Lower Callovian) we find our next ancient octopus, Proteroctopus ribeti. Again, this consists of just one specimen and was discovered in 1982 in the marl deposits at Voulte-on-Rhone in France by the director of the local palaeontology museum, Bernard Riou. Luckily the fossil is exceptionally well preserved in three dimensions allowing detailed studies to be made of its anatomy.

The amazing preservation of Proteroctopus is due to the unique conditions at Voulte. During the mid-Jurassic the whole of France, excepting Brittany, was covered in a shallow tropical sea and Voulte lay in a basin in the seabed. An underwater current propelled animals from surrounding areas into the lower oxygen conditions inside this basin. It is believed that animals were then buried very rapidly thus preventing the rapid decay of tissues by bacterial action. Most unusually, mineralisation consisted of a mixture of apatite, calcite, pyrite and galena, each mineral forming at a different stage slowly replaced the soft tissues. Each mineral replaced a different part of the fossil, and in the case of Voulte, at a molecular level. Thus the bedding planes contain incredibly preserved fish, worms, starfish, sea urchins and lobsters amongst others. Other notable cephalopods from Voulte include the small dibranchate coleoid Gramadella piveteaui that is believed to be on the lineage that led to Spirula and the teuthids, and one of the earliest known vampyromorphs, Vampyronassa rhodanica.

Examining Proteroctopus, we now have more familiar looking animal. Gone is the modified arm pair of Pohlsepia, Proteroctopus has eight arms containing traces of suckers. The sac-like body appears to have been powerfully muscled and the head is not distinct from the body. A funnel has also been identified. In addition there are two large powerful blade-like fins at the rear of the mantle indicating that the animal was probably a powerful swimmer. As with many soft-bodied coleoid fossils the precise position of Proteroctopus is not clear and some researchers have argued that it is actually a vampyromorph rather than an octopod, but this is not a universally accepted view.

Proteroctopus is currently on display at the Musée de Paléontologie de La Voulte-sur-Rhône.

Palaeoctopus newboldi

Moving on again, our next octopus is Palaeoctopus newboldi (aka Paleoctopus) which is known from a handful of specimens at a Late Cretaceous date of roughly 89-71 million years ago. It was originally described in 1896 by Woodward and was published under the name Calais newboldi. The name was revised to Palaeoctopus when it was realised that the name Calais had already been assigned to an insect! Unlike Pohlsepia and Proteroctopus, fossils of this animal are not unique but they are still extremely rare. Palaeoctopus is known from Lebanon from the Mount Hajoula region.

At the time of Palaeoctopus Lebanon enjoyed a hot tropical climate and was covered in a series of swamps, rivers and deltas that met the sea. Palaeoctopus would have died in a hot shallow coastal region where above the stagnant seabed there existed a clearly defined oxygen-deficient layer. Any marine animal that dived too deeply would quickly find itself in trouble, but it seems likely that most fossils, such as our octopus, were animals that probably drifted down after dying from natural causes and were buried quickly. Animals lying on the seabed would have been largely free from predation and bacterial action due to these environmental conditions helping to explain the exceptional preservation of the fauna.

In the Late Cretaceous the whole area was uplifted due to compression effects when the Africa-Arabian plate collided with the Eurasian plate thrusting up the old seabed to eventually form the mountainous regions in Lebanon.

Palaeoctopus is preserved as a film, or tissue impression, in sandstone. It is a short squat eight-armed octopus with an indistinct head. Much as with Pohlsepia and Proteroctopus, Palaeoctopus has a pair of triangular fins on either side of its head though these are smaller than Proteroctopus. A faint trace of a web uniting the arms is visible and the presence of suckers on the arms has been identified. Due to its similarity to the cirrate octopuses the specimen has been assigned to the order Cirroctopoda, and granted its own family, the Paleoctopodidae.

The Palaeoctopus illustrated here is Woodward's 1896 specimen found at below the Old Covent, Sahel-el-Alma, Mount Lebanon, Lebanon and is in the collection of the British Museum of Natural History in London.

Post-Cretaceous Octopuses

Following the Cretaceous extinction our knowledge of octopuses actually gets even worse. Evidence for octopuses consists of drill-holes in marine gastropods and these have been extensively studied. Despite the lack of an actual fossil of the creature in 1993 a name for the missing 'phantom' octopus was raised following extensive work on Miocene-period drill holes in European scallops, Oichnus ovalis. By 2002 at least seven separate Oichnus species have been identified differing from each other by minute variations in the borings. One Caribbean species, Oichnus excavatus has had its unique form of drill holes traced back to the Late Cretaceous in echinoids and probably thrived at least until the Middle Miocene. Such an animal that has been assigned a name for which we only have trace evidence and no body fossil is called a 'ichnogenus/ichnospecies'.

Lastly, a quick word about argonauts. The secreted shell of the female argonaut has a superficial resemblance to the ammonite and one researcher has suggested that argonauts are ammonites that have lost their shells and have somehow re-learned to secrete a similar looking structure to hold their eggs. However, this similarity is almost certainly coincidental as the argonaut shell differs from the ammonite not only in form but also function and mineral composition. There is also a 40 million year gap between the extinction of the ammonites and the earliest recorded argonaut fossil in the Oligocene. The origin of the argonauts, and how and when they split from the octopuses, is shrouded in as much mystery much as the origin of the octopuses themselves.

References:

Blissett, DJ, Pickerill, RK. 2003. Oichnus excavatus Donovan and Jagt, 2002 from the Moneague Formation, White Limestone Group, Jamaica. Caribbean Journal of Science, Vol.39, No.2, 221-223. University of Puerto Rico.

Clarkson, ENK. 1998. Invertebrate Palaeontology and Evolution (4^th ed). Blackwell.

Kluessendorf J, Doyle P. 2000 Pohlsepia mazonensis, an early „Octopus" from the Carboniferous of Illinois, USA. Palaeontology 43(5): 919-926

MacLeod ,N. 2003 PalaeoBase Macrofossils pt.2: Mollusca. Blackwell.

Any bits and pieces from many other websites, too numerous to mention!

Useful links:

Dr Theo Engesers' Fossil coleoidea Page

Official website of the Musée de Paléontologie de La Voulte-sur-Rhône http://www.musee-fossiles.com/jurassique_lv.html

Tree of Life web pages http://tolweb.org

Dr Neale Monks' A Broad Brush History of the Cephalopoda

Picture Credits:

Proteroctopus and Ammonite by the author

Timeline by the author

Grimpoteuthis from http://www.exploretheabyss.com/index.htm

Pohlsepia adapted from Kluessendorf and Doyle (2000) as above

Proteroctopus from the Musée de Paléontologie de La Voulte-sur-Rhône website

Palaeoctopus from the British Museum of Natural History

Cirroteuthis courtesy Paul H. Yancey, Whitman College

Thanks to:

Dave Lindo, TPOTH, Bernard Riou, Paul Yancey, Joanne Kluessendorf and Peter Batson