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:
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.