Nectocaris: mystery fossil was actually a 500-million-year-old squid relative

By Ed Yong | May 26, 2010 1:00 pm

Nectocaris

In the Canadian Rockies, a horde of 91 squid-like animals have risen from the depths, millions years after their demise. This isn’t the plot of a terrible B-movie; it’s the doing of Martin Smith and Jean-Bernard Caron from the University of Toronto. Together, they have solved a mystery some 500 million years in the making.

Smith and Caron have been giving a makeover to an enigmatic creature called Nectocaris. Until recently, only one specimen had ever been found. Its poor state and puzzling combination of features made it nigh impossible to classify. But not anymore – by finding a staggering 91 extra specimens, Smith and Caron have revealed that Nectocaris is the earliest known cephalopod. It’s the great-great-great-(etc)-granduncle of today’s octopuses, squids and cuttlefish.

Nectocaris pteryx or “swimming crab with wings” was first described by Simon Conway Morris in 1976. It’s one of the stars of Canada’s Burgess Shale formation, arguably the planet’s most important collection of fossils. Its rocks preserve an extraordinary diversity of animals from the Cambrian period, some 505 million years ago. It was a time of great evolutionary experimentation, when the ancestors of all of today’s animal groups mingled with bizarre creatures that have left no living descendants.

Until now, Nectocaris’s allegiances have shifted all over the place. Conway Morris himself had no idea where to place it. Some scientists suggested that it was an early arthropod, a relative of crabs, shrimp and the like. Others placed it within the chordates, the group that includes us and all other back-boned animals. But Smith and Caron think that both of these possibilities are unlikely. Their new specimens reveal a host of features that are distinctly cephalopod-like.

Nectocaris_fossil

Around four centimetres in length, Nectocaris had a soft, flattened, kite-shaped body with two fins running down its sides. Its small head was adorned with two long tentacles and two stalked eyes. Unlike the compound eyes that were common among Cambrian animals, probably had the camera-like structure that modern cephalopods use. From its neck protruded a flexible funnel, which opened into an internal cavity containing pairs of gills.

The funnel lay behind some of the earlier confusion about Nectocaris. In the original specimen, it was flattened so that it looked like a shield-like plate behind the eyes, reminscent of a crustacean’s body armour. The new specimens put paid to that interpretation. The structure is clearly a funnel, similar to those used by modern cephalopods. Nectocaris probably used it to swim the same way, giving it an extra boost of jet propulsion to complement the beating of its large fins.

It was either a predator or a scavenger, grabbing small, soft-bodied animals with its long tentacles. And it probably spent most of its time close to the seabed; some specimens had sediment-filled gill chambers, suggesting that they were caught by a sudden fatal mudslide. The sediment helped to preserve their bodies with such quality that 500 million years later, their position in the animal tree of life has suddenly become clearer.

Nectocaris’s new status pushes back the rise of the cephalopods by 30 million years, telling us that this popular group arose far earlier in earth’s history than previously thought. Smith and Caron think that two other Burgess Shale oddities – Vetustovermis and Petalilium were also members of the same family.

The revised family tree also repaints our picture of the group’s origins. Until now, scientists had thought that the group’s first representatives – the nautiloids – evolved from a group of creeping snail-like creatures called monoplacophorans, whose backs were covered with cap-like shells. These casings were gradually modified so that the animals could float. The living nautiluses and the extinct (but frequently fossilised) ammonites belong to the same shell-bearing group.

But Nectocaris had no shell despite being the earliest known cephalopod and an active swimmer. If Smith and Caron’s interpretation is right, the cephalopods didn’t inherit hard coverings from a monoplacophoran ancestor. These shells were a later innovation all their own.

There are a few parts to the puzzle that haven’t been fitted yet. For example, did the cephalopods start off with two tentacles as in Nectocaris only to evolve more over time, or were Nectocaris’s arms formed by fusing multiple pairs? Also, all modern cephalopods have a sharp, horny beak and a nightmarish, rasping tongue called the radula; it’s unclear if Nectocaris shared these features, for its mouthparts have never been well preserved.

The radula is a particularly big deal – it’s a uniting feature of all molluscs (the group that includes cephalopods, monoplacophorans, snails and others), including some that are supposedly more primitive than Nectocaris. Finding a radula would be the clincher for Smith and Caron’s argument; failing to do so puts their analysis in a tricky position.

Reference: Nature http://dx.doi.org/10.1038/nature09068

Images: reconstruction by Marianne Collins; fossil photo by Jean-Bernard Caron

More on cephalopods:

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Comments (15)

  1. I am not at all certain that they have Nectocaris in the right part of the tree. For example, they do not seem to have addressed an alternative explanation: that this is a poorly-mineralized anomalocaridid (or other “great appendage”-type panarthropod), with the tentacles –> feeding arms and the “siphon” –> the round mouth part. This would also be consistent with the lack of a radula.

  2. It’s… beautiful!

  3. Even though we did briefly consider an anomalocaridid affinity at early stages in the research, there are a few reasons to discount this intriguing possibility. For one, Nectocaris lacks the segmentation that is characteristic of the arthropods. More importantly, Nectocaris has large internal gills — whilst the anomalocaridids have an external gill blade associated with each segment. Neither do Nectocaris’s continuous fins and their structure match up with what we see in the anomalocaridids. So whilst at first glance the two do look quite similar, they differ substantially in the details of their anatomy.

  4. I absolutely love blogging for stuff like this. Thanks to Thomas for raising an interesting question and to Martin for answering it.

  5. Rich Holm

    Wow… very cool!

  6. Just looking at online images, Vetustovermis seems very much like this beasty.

  7. gribley

    Not having access to the paper at home, I am forced to speculate — but the eyes have it, I would guess. A true camera eye would place it evolutionarily very far from the anomalocaridids, wouldn’t it? I wonder how much fossil detail they have on eye structure — guess I’ll read the paper tomorrow!

  8. DB

    Very interesting stuff. I’m also glad to see the Burgess Shale getting some good press, I saw the place several times as a child and it is probably what first sparked my interest in science as a whole. I heartily recommend seeing it to anyone who gets the chance.

  9. Gribley – this is what the paper says on the eyes:

    “Unlike probable compound eyes in the Burgess Shale, which preserve in the same fashion as body tissue (that is, as carbon coated by diagenetic aluminosilicates), the eyes of Nectocaris preserve as a conspicuous carbon film that envelopes a thick layer of bedding-perpendicular muscovite crystals (Supplementary Fig. 8). These crystals are evidence that a structure or a void within the eyes was replaced during diagenesis, consistent with a camera-type construction.”

  10. Not exactly a comment on Nectocaris, but there will be plenty more fascinating fossil stuff like this at the International Palaeontological Congress in London next month (www.ipc3.org). We are keen to encourage some good quality blogging from the meeting, and I may be able to offer one or two complementary ‘bloggers passes’.

  11. David Marjanović

    Are anomalocaridids known to have compound eyes?

    What are the “eyelashes” on the fins that are missing from the reconstruction?

  12. Ed

    Connective tissues, apparently. From the paper:

    “Repeated bar-like elements cross the entire width of the fins at regular intervals. A longitudinal element extends along the base of fins, where they connect to the body… We interpret the bars and lineations to represent connective tissues, which were presumably associated with musculature”

  13. Brian Too

    The Burgess strikes again!

  14. Aaron Baldwin

    I have some great doubts that this is a cephalopod. The main problem I see (based upon a probable relationship with Vestuvermis where the gut is illuminated) is that the anus is terminal. If Nectocaris is related and has a straight gut then it can be neatly eliminated as a cephalopod. The U-shaped gut of cephalopods is probably a relic of their ancestry, a means of ‘fixing’ a molluscan design flaw (having the gills and osphradium near the anus).

  15. Cephalopod Lover

    In the meantime several researchers have raised serious questions concerning the cephalopod (or even mollusc) interpretation of these critters:
    Mazurek and Zaton 2011: http://dx.doi.org/10.1111/j.1502-3931.2010.00253.x
    Kröger et al. 2011: http://dx.doi.org/10.1002/bies.201100001
    Runnegar 2011: http://dx.doi.org/10.1111/j.1502-3931.2011.00296.x (the latter is a reply to Smith and Caron 2011’s reply to Mazurek and Zaton 2011: http://dx.doi.org/10.1111/j.1502-3931.2011.00295.x)

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