Students of the Cambrian Explosion have had much to think about this year. In this blog, and others to follow, several of these will be featured. We start with Nectocaris, one of the 'weird wonders' of Stephen Jay Gould in Wonderful Life. There have been many attempts to locate it within the traditional taxonomic framework. Some have placed it in the arthropods and others have considered it a chordate. However, research published earlier this year, based on 91 specimens (rather than one), has concluded that the animal is a mollusc.
The (reconstructed) ancient squid hunted using its two long tentacles (Source here)
Martin Smith and Jean-Bernard Caron have suggested that Nectocaris displays characters that put it into a relationship with cephalopods. They point to paired camera-type eyes, flexible tentacles and jet propulsion via a 'nozzle'. The reconstructed animal has the appearance of a squid but with two rather than eight or ten tentacles. They suggest that the animal is a stem-group rather than a crown-group cephalopod. It lacks features they consider to be more advanced: there is no shell, only two tentacles and no obvious beak or radula (although the mouthparts are poorly preserved). Their analysis has been accepted by most commentators. The relevance to the Cambrian Explosion is that, before now, the earliest fossil remains of cephalopods are Late Cambrian. Since the studied samples all come from the Middle Cambrian Burgess Shale Formation, Nectocaris "extends the cephalopods' fossil record by over 30 million years".
"The findings make the ancestors of modern squid and octopuses at least 30 million years older. Evolutionary biologist Martin Smith, the main author of the study, told PA news agency that the findings bring cephalopods much closer to the first appearance of complex animals. "We go from very simple pre-Cambrian life-forms to something as complex as a cephalopod in the geological blink of an eye, which illustrates just how quickly evolution can produce complexity," said Mr Smith."
The above quote from Martin Smith illustrates both the significance for the Cambrian Explosion and the conundrum the evidence provides for evolutionary theory. The problem is that everything occurs "in the geological blink of an eye" - whether it be the origins of the phyla with radically different body plans, or whether it be the origins of different classes within a phylum (such as the origin of cephalopods within the mollusca). In a News & Views essay for Nature, Stefan Bengtson provides an insight into these animals.
"To most people, molluscs are rather dull creatures: slugs, snails, clams, mussels and such, at times good for eating but otherwise uninteresting. Yet everyone harbours a fascination for cephalopods, which are also molluscs: the octopus, the chambered nautilus, the cuttlefish and the squid, not least the mythical giant Kraken that Alfred, Lord Tennyson pictured in "ancient, dreamless, uninvaded sleep" in the ocean abyss. Cephalopods are not like other molluscs.
Anything but sluggish, they are capable of instant and rapid movement. Far from being mindless filterers or grazers, they are active predators possessing the most advanced nervous system known among invertebrates. Their brain-to-body ratio exceeds that of most vertebrates (although we have not been smart enough to figure out exactly how smart they are). They are masters of camouflage, changing shape, surface pattern, texture and colour in the blink of an eye - and they do have good eyes. When threatened, they escape by means of a built-in hydro jet that can even send them squirting through the air like little rockets on a tail of water."
At very least, then, if the identification of Nectocaris as a cephalopod is valid, the time available for evolutionary transformation is reduced by 30 Ma. The credibility of gradualist mechanisms, already at breaking point, simply vanishes. It is a cop-out to say that the data "illustrates just how quickly evolution can produce complexity" because it begs the question that evolution can generate complexity at all. Martin Smith's words are the evolutionary biologist's equivalent of the 'god-of-the-gaps' argument. He has no mechanism to explain how it could ever happen, but since evolution is regarded as a 'fact', then evolution must have done it.
It is worth considering whether Nectocaris is primitive or derived. The authors entertain this idea when they write: "Nectocaridids' single pair of tentacles may originate via the fusion of multiple pairs, or represent the primitive state". They also articulate the puzzle of having mineralised ancestors and mineralised descendants - but opt for Nectocaris being primitive because "no obvious precursors" for cephalopods have been found. They resolve their conundrum by postulating that there must have been non-mineralised precursors:
"Given that the highly plastic molluscan secretome has convergently produced similar shell microstructures in unrelated lineages, we suggest that nautiloids evolved from a nonmineralized, coleoid-like ancestor related to the nectocaridids."
Not all are convinced. Christopher Taylor finds reasons for thinking that Nectocaris is not such a primitive animal, but a specialised organism (and that, it should be said, raises even more puzzles for the Darwinian, gradualist paradigm).
"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."
Primitive soft-bodied cephalopods from the Cambrian
Martin R. Smith and Jean-Bernard Caron
Nature, 465, 469-472, (27 May 2010) | doi:10.1038/nature09068
Abstract: The exquisite preservation of soft-bodied animals in Burgess Shale-type deposits provides important clues into the early evolution of body plans that emerged during the Cambrian explosion. Until now, such deposits have remained silent regarding the early evolution of extant molluscan lineages - in particular the cephalopods. Nautiloids, traditionally considered basal within the cephalopods, are generally depicted as evolving from a creeping Cambrian ancestor whose dorsal shell afforded protection and buoyancy. Although nautiloid-like shells occur from the Late Cambrian onwards, the fossil record provides little constraint on this model, or indeed on the early evolution of cephalopods. Here, we reinterpret the problematic Middle Cambrian animal Nectocaris pteryx as a primitive (that is, stem-group), non-mineralized cephalopod, based on new material from the Burgess Shale. Together with Nectocaris, the problematic Lower Cambrian taxa Petalilium and (probably) Vetustovermis form a distinctive clade, Nectocarididae, characterized by an open axial cavity with paired gills, wide lateral fins, a single pair of long, prehensile tentacles, a pair of non-faceted eyes on short stalks, and a large, flexible anterior funnel. This clade extends the cephalopods' fossil record by over 30 million years, and indicates that primitive cephalopods lacked a mineralized shell, were hyperbenthic, and were presumably carnivorous. The presence of a funnel suggests that jet propulsion evolved in cephalopods before the acquisition of a shell. The explosive diversification of mineralized cephalopods in the Ordovician may have an understated Cambrian 'fuse'.
Bengtson, S. A little Kraken wakes, Nature, 465, 427-428, (27 May 2010) | doi:10.1038/465427a
Moskvitch, K. Mystery fossil is ancestor of squid, BBC News (27 May 2010)
Taylor, C. Nectocaris: Largely Irrelevant to Cephalopods? Catalogue of Organisms (27 May 2010)
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