So Nice When People Agree With You

Nectocaris pteryx

Some of you may remember this post from back in May, in which I critiqued Smith & Caron's (2010) interpretation of Nectocaris as an early cephalopod. I was not convinced. Nor am I the only one: a paper recently released by Mazurek & Zatoń (in press) comes to much the same conclusions. They point out that Smith & Caron's proposed model of cephalopod evolution conflicts strongly with what we previously knew from the cephalopod fossil record (and that's no small amount—there are few groups of organisms whose fossil record has been as intensely studied as cephalopods), and that most of the characters supposedly shared between Nectocaris and cephalopods are in fact only shared between Nectocaris and coleoids (modern octopods and squid) that did not appear in the fossil record until during the Mesozoic, considerably later than the Cambrian Nectocaris. Smith & Caron (2010) suggested that the absence of a shell in Nectocaris indicated that the cephalopod shell had been evolved independently to that of other molluscs, but Mazurek & Zatoń point out that the only known cephalopods to completely lack any trace of a shell are octopods, and that octopods are secondarily shell-less is indicated, not only by their phylogenetic position, but also by the presence of a remnant shell in Cretaceous stem-octopods.

What I find particularly interesting about Mazurek & Zatoń's paper, however, is how much it brings up the same points already raised by commenters here. The primary feature cited by Smith & Caron (2010) as connecting Nectocaris with cephalopods, the presence of a funnel, is contradicted by the apparent difference in functional structure between a cephalopod siphon and Nectocaris' 'funnel', as noted by Adam Yates. Aydin Örstan commented on the absence of a radula. Just goes to show that I've got some pretty clever readers here. Mind you, I'm not happy with everything in Mazurek & Zatoń's paper. They make the argument that Nectocaris could not have evolved from a shelled and radula-possessing ancestor because it was 'too early', but the known fossil record of molluscs pre-dates Nectocaris by about twenty million years, more than long enough for shell loss to potentially occur. The absence of a beak in Nectocaris is also of doubtful significance as this is a cephalopod autapomorphy.

I am also not swayed by Mazurek & Zatoń's (provisional) alternative placement for Nectocaris as a dinocarid. Though it is tempting to compare the funnel of Nectocaris to the proboscis of dinocarids such as Opabinia, and dinocarids have the advantage over coleoid cephalopods of being coeval with Nectocaris, no dinocarid has cephalic tentacles like Nectocaris. Also, some of the figures in Smith & Caron (2010) appear as if the pharynx might pass through the funnel; in Opabinia, the proboscis was a separate structure in front of the mouth. I can think of two groups of animals for which cephalic tentacles are definitely known: annelids and molluscs (presuming that the tentacles of Nectocaris are not a unique autapomorphy of its own). The lack of obvious segmentation makes it unlikely that Nectocaris is an annelid. That leaves us with mollusc. While Smith & Caron's identification of a pinhole camera eye in Nectocaris could still connect it to cephalopods, I believe that this is outweighed by the arrangement of the gut. The presence of an apparent through-gut, opening with a terminal anus, was identified by Chen et al. (2005) in Vetustovermis (now recognised as a synonym of Nectocaris)*. Cephalopods, however, have a U-shaped gut, opening in the mantle cavity not too far from the head. This is related to the 90° shift that the cephalopod body plan has gone through during its evolution: the apparent "front-back" axis of a squid actually represents the "top-bottom" axis of other molluscs. A similar U-shaped gut is present in scaphopods, the probable living sister group of cephalopods, so a molluscan Nectocaris would have to sit outside the scaphopod-cephalopod clade. As far as is known, cephalic tentacles are a possible synapomorphy of the clade uniting cephalopods, scaphopods and gastropods**, so a molluscan Nectocaris would probably have to be either a stem representative of this clade, or just possibly a stem gastropod.

*My apologies to the commenter somewhere whose identity I've forgotten who brought my attention to this point.

**Cephalic tentacles are definitely absent in polyplacophorans and tryblidiids***. They are also absent in bivalves, but bivalves don't have a head to have cephalic tentacles on in the first place, so the absence of tentacles is probably best treated as ambiguous for bivalves.

***Or whatever Neopilina and its ilk are going by these days.

REFERENCES

Chen, J-Y., D.-Y. Huang & D. J. Bottjer. 2005. An Early Cambrian problematic fossil: Vetustovermis and its possible affinities. Proceedings of the Royal Society of London B 272: 2003-2007.

Mazurek, D., & M. Zatoń. in press. Is Nectocaris pteryx a cephalopod? Lethaia.

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