Unravelling the Kraken

Nectocaris could be seen as an embodiment of the Weirdness of Cambrian life. It is (or originally was) pretty much a symbol of the Cambrian explosion as Stephen Jay Gould saw it – a festival of brand new, strange body plans that didn’t really belong with anything alive today. Gould’s is a fairly radical interpretation of the Cambrian, and I don’t think most experts share it, but that’s a discussion probably worth a whole book. This post is about Nectocaris alone.

Why did I say that Nectocaris was the embodiment of Cambrian Weirdness? It’s a really obscure creature, and other, more well-known Cambrian animals like Anomalocaris are strange enough for our icon-seeking purposes. Well, yes, but outlandish as it is, Anomalocaris makes sense. For a long time, Nectocaris didn’t.

One more Cambrian puzzle

Until very recently, only a single specimen of Nectocaris was known, and virtually no literature existed on it. What little information seeped out into public perception painted a truly baffling picture. If you believe Gould, this creature was a mongrel of creation that seemed to have the head of an armoured crustacean on the long, finned body of a chordate.

The old Nectocaris by Ghedoghedo, Wikimedia Commons

I have to say something about the family tree of animals here to explain why an animal like that is pretty much impossible unless everything we know about evolution is wrong. Animals are generally classified in 30+ different phyla, e.g. molluscs, arthropods and chordates (Chordata is our own phylum, which we share with other vertebrates, as well as sea squirts and lancelets). Phyla fall into even higher-level groups, whose more or less accepted relationships are summarised on the diagram below:

An outline of animal phylogeny, with examples of each lineage. Groups written in italic are phyla, the rest include more than one phylum. The diagram is my own, but the pictures were all taken from Wikimedia Commons. If you actually care what all the examples are, I have a list at the end of this post.

(Sometimes, biologists seem to go out of their way to make their terminology as arcane as possible. With monsters like “Lophotrochozoa”, it’s no wonder taxonomy isn’t sexy!)

Crustaceans (which are arthropods, which are ecdysozoans) and chordates (which are deuterostomes) are on completely different branches of the tree. Their last common ancestor would have shared some general features with both – but it wouldn’t have had any specific characteristics of arthropods or chordates. If an animal had both arthropod and chordate characteristics, it would violate evolution worse than a Precambrian bunny. Or, alternatively, the resemblance to arthropods, chordates or both would have to be the result of convergent evolution.

As it happens, it was neither.

The original specimen of Nectocaris isn’t that well-preserved. It’s obviously very hard to tell from it what the animal resembled in life. Simon Conway Morris, who described the fossil (Conway Morris, 1976), couldn’t really place it, though he apparently toyed with an arthropod identity*. Simonetta (1988) argued it was a chordate, interpreting what others saw as a “carapace” as the wall of the gill cavity of a primitive chordate. Then, with the discovery of good fossils of a seemingly related animal (Chen et al., 2005), another possibility arose that involved neither arthropods nor chordates – nor impossible crosses between distant lineages. Maybe Nectocaris was a secret lophotrochozoan all along?

A few years later, Smith and Caron (2010) argued that a humongous number of new Nectocaris specimens confirm the last idea. According to them, the animal (along with Chen and colleagues’ Vetustovermis) was not only a lophotrochozoan, but a relative of cephalopods – a bona fide mollusc. From the new fossils, it was obvious that the original specimen was twisted and distorted. What seemed like the vertical tail of a chordate was actually flattened horizontally, with fins on its sides, like the body of a squid. The “vertical” stripes that Simonetta (1988) interpreted as the characteristic muscle blocks of a chordate might have been gill structures, the “fin rays” fibres of connective tissue. The head sported no carapace, but there were two long, flexible tentacles. The big, stalked eyes were not faceted like those of many arthropods, but appeared to be camera eyes like those of cephalopods (and ourselves). Smith and Caron also saw a large floppy structure attached to the underside of the head, which they thought resembled the funnel of cephalopods.

The new Nectocaris by Citron, Wikimedia Commons

But despite more than ninety beautiful specimens, Nectocaris hasn’t given up its stubborn refusal to fall into place. The things that struck me as suspicious in Smith and Caron’s description are some of the same things Polish palaeontologists Dawid Mazurek and Michał Zatoń (2011) found wanting. Most importantly, Nectocaris shows no trace of either the vicious beaks of cephalopods proper (formidable-looking example halfway down this page), or the trademark feeding organ of molluscs, the radula.

You could say that these organs were just not preserved. After all, fossils are never quite complete; scavengers, decay and the vagaries of geological history see to that. That would sound reasonable, except that radulae and beaks are quite durable. One of the first things anyone learns about fossilisation is that hard parts are preserved much more easily than soft parts; beaks, bones and shells don’t rot rapidly like skin and flesh do. But soft tissue is all that remains of Nectocaris, in all ninety-two known specimens. Chances are it never had a radula. The “funnel” is also suspect, Mazurek and Zatoń point out, since its shape is completely wrong for what cephalopod funnels do (squirt water for jet propulsion). Without a funnel, all that’s really left of Nectocaris’s “molluscness” is a superficial resemblance to a flattened squid. Fins and tentacles are hardly defining characteristics of any one group of animals. (Here’s another lophotrochozoan with some lovely tentacles. Don’t click if freakish-looking worms give you bad dreams ;))

So, what on earth IS Nectocaris?

Mazurek and Zatoń (2011) very tentatively go back to the arthropod hypothesis, comparing the creature to anomalocaridids, which are close relatives of true arthropods (Budd and Telford, 2009). But to me, the resemblance to Anomalocaris is as superficial – if not more – as the similarity to cephalopods. Just as fins on the side don’t make Nectocaris a cephalopod, they don’t make it an anomalocaridid either. The slim, supple tentacles are nothing like the sturdy, jointed, clawed, hardened head appendages of Anomalocaris and its kin. While Nectocaris has no molluscan radula, it also lacks the unique pineapple-slice mouth of an anomalocaridid.

Much as it pains me, I still don’t think we know what Nectocaris is. I think the mollusc people and Chen et al. (2005) were on to something. By its general appearance, the creature seems more lophotrochozoan than anything else. Maybe it was a stem mollusc, not quite a mollusc but related, just like Anomalocaris was not quite an arthropod. Or maybe it was related to another lophotrochozoan phylum, say, flatworms, or not particularly close to any living phylum at all.

Until there are even better fossils, we can’t know – and I think that’s the take-home message of this post (insofar as it has one). The problem with fossils is that you can have literally thousands of them, and be no closer to the truth (Shu et al., 2003 and the responses to it are a case in point). To move beyond reasonable speculation, you need clear details of diagnostic traits – those that actually tell you where a creature belongs. In soft-bodied creatures, such details not only decay, but they may decay in a downright misleading way (Sansom et al., 2010). Vertebrate palaeontologists have it easy with their bones and teeth.

*Alas, I don’t have access to that paper, so I’ll have to take Simonetta’s and others’ word on it.

***

References:

Budd GE and Telford MJ (2009) The origin and evolution of arthropods. Nature 457:812-817

Chen J-Y et al. (2005) An Early Cambrian problematic fossil: Vetustovermis and its possible affinities. Proceedings of the Royal Society B 272:2003-2007

Conway Morris S (1976) Nectocaris pteryx, a new organism from the Middle Cambrian Burgess Shale of British Columbia. Neues Jahrbuch für Geologie und Paläontologie 12:705-713

Gould SJ (1991) Wonderful Life. Penguin.

Mazurek D and Zatoń M (2011) Is Nectocaris pteryx a cephalopod? Lethaia 44:2-4

Sansom RS et al. (2010) Non-random decay of chordate characters causes bias in fossil interpretation. Nature 463:797-800

Shu D-G et al. (2003) A new species of yunnanozoan with implications for deuterostome evolution. Science 299:1380-1384

Simonetta AM (1988) Is Nectocaris pteryx a chordate? Italian Journal of Zoology 55:63-68

Smith MR and Caron J-B (2010) Primitive soft-bodied cephalopods from the Cambrian. Nature 465:469-472

List of animals pictured with the phylogeny:

Sponges: ??? Ctenophores: sea walnut (Mnemiopsis). Placozoans: Trichoplax (I didn’t have much choice there – that insignificant blob is the only known placozoan). Cnidarians: sea nettle jellyfish (Chrysaora) and beadlet sea anemone (Actinia). Deuterostomes: acorn worm (Balanoglossus), a hemichordate; common starfish (Asterias), an echinoderm; and poison dart frog (Phyllobates), a chordate. Lophotrochozoans: garden snail (Helix), a mollusc; serpulid tube worm (Protula), a segmented worm; and freshwater planarian (Dugesia), a flatworm. Lophotrochozoa is actually the largest of the three bilaterian “superphyla”, but I didn’t have space to do its diversity justice. Ecdysozoans: Trichinella, a nematode; Heliconius butterfly, an arthropod; and a penis worm (Priapulus).

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