Field of Science

Nectocaris: Largely Irrelevant to Cephalopods?

Nectocaris pteryx as reconstructed by Marianne Collins in Smith & Caron (2010).

Today's issue of Nature sees the publication of a paper presenting a radical reinterpretation of the Middle Cambrian nektonic animal Nectocaris pteryx (Smith & Caron, 2010). Previously only known from a single specimen, Smith & Caron increase the hypodigm of Nectocaris by a whopping 91 specimens, an absolutely mindblowing advance. Unfortunately (and, I'm sad to say, not uncommonly for a Nature paper), the authors then take this amazing discovery and use it to make some decidedly unwarranted inferences.

Smith & Caron reconstruct Nectocaris as a small squid-like animal with two anterior tentacles, broad lateral fins and a ventral cylindrical funnel close to the head. Based on the similarity of the funnel to the siphon of living cephalopods, the authors infer a relationship between Nectocaris and cephalopods and suggest that the former is representative of the ancestral morphology of the latter. One problem with that - Nectocaris doesn't have a shell and cephalopods have always been assumed to have evolved from shelled ancestors like other mollusc classes. Smith & Caron suggest that this assumption is incorrect and that each of the living mollusc classes acquired shells independently.

This is the representation given by Smith & Caron (2010) of molluscan evolution and the known fossil record of each of the classes:

Smith & Caron (2010): "Arrows indicate the crown groups of 1, molluscs; 2, conchifera; 3, cephalopods. Stars represent the earliest record of mineralization in each lineage (after ref. 23). Clade divergence times (dotted lines) are unconstrained. Early branches follow previous phylogeny (after ref. 20)."

Simple, straightforward and very misleading. The diagram only shows the living classes of mollusc but omits all lineages not directly relatable to one or another of the recent taxa - a category that includes most Cambrian molluscs, including many that are directly relevant to cephalopod ancestry. The phylogenetic positions of Tryblidiida (including modern 'monoplacophorans') and Polyplacophora (chitons) as sister group or serial* sister groups to other molluscs, together with features of putative stem molluscs such as Wiwaxia and their possible nearest living relatives the annelids, suggest that serially-repeated structures were part of the ancestral ground plan for molluscs. The absence of indications of serial structures in many Cambrian 'monoplacophorans' such as helcionelloids suggests that they were (at least) part of the clade including bivalves, gastropods and cephalopods, and the fossil record for helcionelloids extends back to the very earliest Cambrian (Runnegar & Jell, 1976). The supposed absence of an early fossil record for scaphopods overlooks good support for a derivation of scaphopods from the Rostroconchia, another Palaeozoic mollusc group (Peel, 2006) which may take the scaphopod lineage back to the early Cambrian. Smith & Caron dismiss the possibility that Nectocaris may have secondarily lost an ancestral shell by claiming that it is too early in the fossil record and lacks likely predecessors; however, shells have been lost on a large number of occasions in molluscan history; shelled molluscs appeared in the fossil record some twenty million years or so before the earliest known nectocarids; and the relative rarity and simplicity of early molluscan fossils (early molluscs were generally small and fairly delicate) means that it is quite possible that a direct nectocarid ancestor may not have been preserved, nor is there any guarantee that it would be recognised as such if it had.

*No pun intended.

As described in an earlier post, the earliest known stem cephalopods (from the Late Cambrian) possessed shells with large numbers of very tightly packed septa and were unlikely to have been very buoyant. Their generally short conical shape would have been ill-suited for jet-propelled swimming as in modern cephalopods and they were most likely benthic. As other molluscan classes were also ancestrally benthic, it seems unparsimonious that the actively swimming Nectocaris represents the ancestral cephalopod lifestyle.

If Nectocaris is a stem cephalopod (which essentially depends on how strong the siphon is as a supporting apomorphy), then the most likely scenario is that its shell loss and squid-like form is an independent convergence on modern shell-less cephalopods rather than representing the ancestral form for cephalopods as a whole. Nectocaris would not be an ancestor, but a highly specialised side branch of its own.


Smith, M. R., & J.-B. Caron. 2010. Primitive soft-bodied cephalopods from the Cambrian. Nature 465: 469-472.

Peel, J. S. 2006. Scaphopodization in Palaeozoic molluscs. Palaeontology 49 (6): 1357-1364.

Runnegar, B., & P. A. Jell. 1976. Australian Middle Cambrian molluscs and their bearing on early molluscan evolution. Alcheringa 1 (2): 109-138.


  1. I'm sooo glad someone else agrees with me. I had no idea this paper was coming so have only had an hour or so to come to grips with it. But my initial reaction is that yes it looks a bit like a squid but given that squid appear to be very derived within cephalopoda its highly incongruent with everything else we know about mollusc evolution. I rather suspect these guys are nothing at all to do with cephalopds. The funnel expands distally, would this work functionally? As far as I can see all modern cephalopods have a distally tapering funnel which makes sense if you want to create a powerfull jet of water. I like Thom Holtz's suggestion that this is actually a proboscis-like mouth of a poorly mineralised anomalocarid.

  2. I was a little bemused by the expanding funnel as well but wasn't sure if that couldn't have been distortion due to specimens being flattened. Looking at the paper again, the authors seem to be explicitly identifying it as expanding distally.

    I'm willing to buy the idea that it is a mollusc though I don't know that I'd be willing to call it anything more specific than 'Cyrtosoma' due to the presence of cephalic tentacles.

  3. This whole mess reminds me immensely of a a recent article from Cladistics on bad palaeontological systematics (in reference to the Heteroptera). It seems that instead of actually conducting cladistic analyses of characters, palaeontologists are content to produce the tired old just so stories with such fallacies as "common equals primitive" and "older equals primitive".

    So it goes.


  4. Nectocaris also lacked a radula, which is hard to explain if it was indeed a stem-group cephalopod.

  5. this was very enlightening!

    having read the paper, but not being very versed in mollusc systematics, i had wondered if all known molluscs fit into known (extant) groups. as it seemed a bit of a stretch to me...

    now i know. thanks!

    which is too bad. i'd learned about the cephalopod interpretation of nectocaris back in novemember (they put the abstracts up for all the talks at the burgress shale 100th anni conference last year), and that one in particular stuck out. i've always been fascinated by nectocaris, and the short version of it as a cephalopod made a lot of sense.

    i wish they'd just stuck to a pure description in this paper...

    that theory on thomas holtz's part is very interesting.

  6. Being a mere hobbyist in the fossil business I can only describe my reaction when I first saw the pic:

    "Nectocaris another type of Anomalocarid!"

    Then I read the article and was disappointed.

    Suppose the swimming lobes of an Anomalocarid fused to a single fin? Then this would look an awful lot like several other big-appendage critters from the Cambrian. Then the "siphon" could actually be an everted esophagus, squished out by the compression of the body in the mud.
    On a side note, all of the discussion reminds me of a trick I use all the time in my work. I'll put out a reasonable, but incorrect or incomplete, statement in order to get my audience to react. I get the real information from them in their discussion of how wrong I am.

    I wonder if that crossed the minds of the authors in this case?

  7. The authors claim a number of features pointing to the neighborhood of cephalopods, eg, axial chamber with internal gills, funnel, and especially camera eyes with internal lens. It appears that the eyes are well enough preserved on a number of specimens to support the "camera-eye" reading, and if most of these features hold, it's NOT anomalocarid...

    Still, the "shrimp-lancet" restoration always gave me vertigo. The present drawing at least looks like a real animal, and also pretty much like the "best case" photo.

    We'll all want to see the Nature article, I'm sure.

  8. I certainly agree that a cephalopod affinity for Nectocaris is somewhat surprising!
    I've mentioned elsewhere the problems with an anomalocaridid affinity, and whilst the figure you mention is certainly no cladistic analysis (which I'm hoping that a longer, more in-depth description will eventually produce), it does illustrate the most likely position of an organism with some, but not all, crown-group cephalopod characteristics. Molluscan phylogeny is so unresolved, and early fossils so difficult to interpret, that I suspect that it will be some time before we are able to use cladistic methods in a more systematic fashion.

    Differences are often more obvious than similarities; the absence of a radula needs explaining (was it too small to see in the fossils? was it secondarily lost, as in some modern cephalopods? was it just not preserved, as in squid fossils?), and although the funnel doesn't seem well suited to jet propulsion, and combined with the large size of its fins it seems unlikely that this represented its primary mode of locomotion.

    In any case, scepticism is all to be encouraged; it seems to me that whatever the case with Nectocaris, the evidence aligning some of these late Cambrian chambered monoplacophorans to the cephalopods (or indeed the Neopilinidae) is rather weak – see for instance a paper demonstrating that one of these "cephalopod ancestors" is in fact a brachiopod! (Dzik 2010, freely available PDF)

    Given that the fossil evidence can only ever tell part of the story, we have to be very careful not to read too much into it – and I'm excited to see what a combination of renewed scrutiny of the fossils and other techniques (such as molecular methods) will bring to the table.

  9. Aydin: As you probably know, radulae have been lost by various mollusc groups (bivalves, some nudibranchs, etc.) so I don't see its absence in this case as a definite strike against.

    Martin: I did read the Dzik paper, but while I do agree that the evidence either way is pretty slim, I don't see that Dzik's arguments for Hypseloconus being a brachiopod are really so much more rock solid than the case for it being a mollusc (a microstructure study would be nice).

    Either way, there's still the issue of the early cephalopods (plectronocerids, ellesmerocerids, etc.) for which the picture is reasonably clear, and which don't sit well with a Nectocaris-like ancestor.

  10. Why on earth is my old intro to protists post 'linking' here? Weird...

    Glad to know metazoan phylogeny is still about as messed up as any other =P

  11. I was of the impression that molluscan shells were clearly developmentally homologuous across the various (modern) classes. Is this wrong?

  12. I think the most significant illustration was kept out of the main paper, that is fig. 4 of the supplementary information. It clearly shows the that in continuation of the putative siphon, there's tube that passes through the head, and connects to the "axial cavity".

    To me that is highly similar to anterior digestive tract as reconstructed for anomalocaridids. It would turn the "axial cavity" into the posteiror digestive tract and it would be thus unnecessary to conjure those inhalant slits out of ink. The expanded funnel works if see as proboscis perhaps adapted to capture very small prey, near the bottom.

    @Adam Yates: I'm also very happy that there those unimpressed by this "cambrian squid".



  13. i just wonder, how much reliable are other articles from nature or science, where its poor quality is less obvious.
    i afraid it's not the first case in nature. not being new and actual, similar case is an article Ancestral echinoderms from the Chengjiang deposits of China, describing genus Vetulocystis. it seems to me, that authors a priori decided what it have to be, and then they made their interpretation of preserved structures. i'm aware that it is the only way, how to suggest the nature of of that particular structure, but it could not be considered as a evidence for proposed hypothesis on specimen's phylogenetic relationship.

  14. I agree with your post entirely. When I first heard about this paper, I was confused. I was convinced that it was my own ignorance that made it seem like this hypothesis was not sensible.

    Given that every other cephalopod we know appears to have internalized and/or lost a shell after a long period of differentiation from a shelled ancestor, and that there is no way to gather more than rough morphological information about this species, the paper's assertions now seem solidly like a step beyond the evidence to me.

    Thanks for the post!


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