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A Broad Brush History of the Cephalopoda<< Cephalopod Articles | By Dr. Neale Monks author of Ammonites
Cephalopods are one of the few animal groups that are both diverse and ecologically important today and yet have an extensive fossil record going back almost to the very beginnings of complex animal life during the Cambrian period around 550 million years ago. However, understanding the evolution isn't simply a case of using the fossils to build family trees culminating in the living species, because the groups of cephalopods most richly represented in the fossil record are ones with few, if any, living species! When discussing cephalopod evolution it is important to realise that they haven't evolved in a linear way from some primitive nautilus-like creature in the Cambrian through to the modern and undeniably sophisticated squids and octopuses. It is much better to think about the cephalopods as comprising three anatomically and ecologically very different groups—the Nautiloidea, Ammonoidea and Coleoidea—each of which has adapted and evolved independently of the other groups and experienced different degrees of success and failure.
Two genera of nautilus survive to the present day, Nautilus and Allonautilus, but it isn't at all obvious whether these living nautiluses are representative of their extinct kin as well. For a start the living species belong to a single family, the Nautilidae, that doesn't go back any further than the Late Triassic (about 215 million years ago); the greatest variety of nautiluses had already lived and died long before then. Ecologically the living species seem to be quite specialised
Even after their heyday in the Paleozoic, nautiluses have remained conspicuous if not actually important components of marine communities. Several new sorts evolved during the Mesozoic and Tertiary periods alongside the various ammonite and coleoid groups. Among these new types was the genus Aturia, comprising nautiluses with complex, highly folded sutures, very robust and conical siphuncles, and laterally compressed shells. These are common in Tertiary rocks as young as those as the Miocene (about 5 million years old) indicating that while the mass extinctions at the end of the Cretaceous were bad news for the
Nautiluses are currently confined to the tropical Indo-West Pacific region although the odd specimen can drift much further away (for example, a live specimen has been found on the coast of Japan). They are not particularly diverse at the species level, with about five species divided into two genera, Nautilus and Allonautilus, but individual populations around each group of islands do seem to be quite distinct. It seems probable that the various populations were able to mix more easily in the recent past when sea level was lower than it is now, but in the last hundred thousand years sea level as fluctuated significantly, and right now the populations are more or less completely isolated. In short, the nautiluses are diversifying and evolving into new kinds, like Darwin's famous finches stranded on the various Galapagos Islands.
Ammonites are the best-known cephalopod fossils and for a very long period of time, from the Devonian through to the Cretaceous (408 to 65 million years ago) they were major players in most marine ecosystems. It is tempting to think that they occupied the sorts of niches that fishes do today, but that is probably unwise. To begin with there were plenty of fishes around during this time too, particularly by the Mesozoic when many modern groups of cartilaginous and teleost fishes made their appearance. In addition, the ammonite body plan offers a different set of advantages and disadvantages compared with a bony fish, for example.
So whatever ammonites were, for the most part at least they weren't fish analogues. Much more likely is that they occupied a variety of niches comparable to those occupied by modern crustaceans such as crabs and lobsters, and molluscs like the plankton-feeding cranchid squids. There are a few ammonites that might have been active, open water predators, such as Placenticeras, which is quite smooth and streamlined, and does turn up in offshore sediments. But did it cruise the sea in schools like modern mackerel or jacks? Probably not: using its jet propulsion it would have swam backwards, with its head and arms pointing the wrong way to catch prey. Unlike modern squids it lacked fins for forwards swimming, at best it could have maintained its position using its jet, and waited for prey to swim into range of its arms.
Ammonite evolution is a parade of ostentatious success and catastrophic failures. Some groups never really amounted to much, like the Anarcestina, and only lasted a few million years before going extinct. Others were long-lived but conservative groups, such as the Phylloceratina, which lasted from the Triassic through to the end of the Cretaceous (248 to 65 million years ago). Yet others, exemplified by the Ammonitina and the Ancyloceratina, were both long-lived and very diverse. With each mass extinction even, and there have been many, some few ammonites survived and gave rise to a whole new assemblage of forms. They did of course fail to do so after the end Cretaceous extinctions for reasons that remain controversial. There were probably a combination of factors at work including the appearance of different sorts of predators better able to break through ammonite shells, climate and sea-level change, and perhaps a catastrophic collapse in the plankton on which baby ammonites appear to have fed.
The final of the three groups of cephalopods appeared at about the same time as the ammonites and diversified alongside them. It is absolutely crucial to recognise that coleoids didn't replace ammonites or that ammonites in some way out-competed coleoids during the Mesozoic. While the two groups do seem to have some anatomical and perhaps developmental traits in common that set them apart from the nautiluses, in many ways they are very different groups. While ammonites seem all to have relied on their shells for defense was well as buoyancy, coleoids had internal shells useless as a defense. A few coleoid groups have also discarded the shell as a buoyancy aid. While these groups—the squids and the octopuses—happen to be the dominant cephalopods in the modern seas, coleoid evolution didn't lead inexorably towards this state. The belemnites, one of the most successful groups of coleoids, had well developed chambered shells, and were very abundant in the seas of the Jurassic and Cretaceous (213 to 65 million years ago) right alongside the ammonites. In the Tertiary there appeared several different groups of coleoid that each had highly modified but still chambered and buoyant shells. Among these, two groups survive to the present day and both are ecologically important and unquestionably successful: these are the shallow water cuttlefishes, and deep-water spirula.
We noted at the beginning of this essay that the buoyant shell is what allowed cephalopods to become active predators at a time when most other
Octopuses (and their close relatives the vampyromorphs) seem to have lost their shells independently of the shell reduction seen in squids. Vampyromorphs, represented today by Vampyroteuthis, were moderately diverse during the Jurassic and many of the so-
The squids have shells that no longer provide buoyancy but unlike the situation with octopuses, the squids have retained their shell for other purposes. Most crucially the shell provides rigidity to the body and support for the musculature, essential functions in these powerful, fast moving animals. Confusion exists over whether or not the two groups of squids, the Oegopsida and the Myopsida, are more closely related to one another than either are to the cuttlefishes or spirulas. The
But why are octopus and especially squid shells unchambered when neutral buoyancy would be useful to active, midwater animals? There are three obvious possibilities. The first is that the early squids and octopuses inhabited deep water where maintaining a gas-filled shell is difficult: the gases needed to fill the chamber won't diffuse out of the blood fast enough, and water pressure tends to crush any gas-filled shell anyway. Nautilus can't survive at depths in excess of 500 m (it implodes), and the spirula is found at depths of 800 to 1000 m. Many squids and octopuses inhabit far deeper waters than this, and if this was the habitat they evolved in, then perhaps there was no way the shell could be kept. Interestingly, many deep-water fishes have no swim bladder, probably for the same reasons. Another possibility is that they passed through a bottom-dwelling stage in their evolution. Many modern octopuses are active burrowers and perhaps a shell is redundant, or even a handicap, to an animal that inhabits burrows and caves. Also, being crawlers rather than swimmers, a buoyant shell was probably unnecessary, in the same way that benthic fish like gobies and flounders have reduced swim bladders. Finally, it may be that squids and octopuses lost their buoyant shells in response to predation pressure from echolocating predators like dolphins. A hollow shell will return a very obvious echo, making the bearers of such shells easy targets. The extra costs involved in having to swim without the benefit of neutral buoyancy may be offset by the improved survivability against these sorts of predators. It is certainly noticeable that until the Miocene when echolocating cetaceans diversified, there were many more kinds of cephalopod with chambered shells than there are today.
A crucial question that needs to be answered is did the squids and octopuses evolved in deep water and expanded into shallow water later on? This could explain why they have unchambered shells, and might also explain why they survived the Cretaceous-Tertiary mass extinctions when so many shallow water groups (including the ammonites) succumbed. In a deep-water refuge they could have been protected from extreme changes in temperature or pH, and perhaps able to use food sources less sensitive to collapses in phytoplankton or zooplankton populations. A deep-water stage in their evolution raises an interesting contradiction though: the complex eyes and visual modes of communication employed by squids and octopuses aren't typical of deep-water invertebrates or even fish, which tend to emphasis the senses of touch and olfaction instead.
If they did evolve in deep water, then what was the cause of their expansion into shallow water habitats? Squids and octopuses are often talked about as "invertebrate fish" but if this were true, then surely the bony fishes would have already occupied the niches the squids and octopuses might occupy? That the squids and octopuses are in shallow waters would seem to imply that there isn't that much overlap between them and the fishes, however attractive such an idea might be. So what is it that squids and octopuses had going for them that allowed them to carve out entirely new ways of making a living in coastal waters and the upper levels of the open sea? Their extraordinary physiology, combining rapid growth and large size with plastic behaviour and a sophisticated sensory system, must be significant, but how?
Cephalopods may have a good fossil record, and may be among the most studied invertebrates for a variety of reasons from fisheries management through to neurophysiology, but in many ways they remain a complete enigma. Just how does evolution turn a floating slug into a racing snail? And why?
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The Cephalopod Page (TCP), © Copyright 1995-2016, was created and is maintained by Dr. James B. Wood, Associate Director of the Waikiki Aquarium which is part of the University of Hawaii. Please see the FAQs page for cephalopod questions, Marine Invertebrates of Bermuda for information on other invertebrates, and MarineBio.org and the Census of Marine Life for general information on marine biology.