That is not a spider


Grrr. The CBC got me excited with a headline about “the granddaddy of spiders”. It’s not a spider. It’s a Cambrian chelicerate, which ought to be cool news enough without pretending it’s some kind of familiar organism. At least it wasn’t SciTech, which called it a frightening 500-million year old predator” or LiveScience, which called it a “nightmare creature”. C’mon, people. It was a couple of centimeters long. I do not like this pop sci nonsense that has to jack up the significance of a discovery by pretending it was scary. Does this look scary to you?

a–c, Reconstructions. a, Lateral view. b, Dorsal view (the gut has been removed for clarity). c, Isolated trunk exopod. an, anus; lam, lamellae.

At least the article by the discoverers is sensible. This is an early Cambrian chelicerate with those big old feeding appendages at the front of the head (which spiders also have) and with modified limb appendages that resemble book lungs (also a spider trait), but they are most definitely not spiders. They are their own beautiful clade, and cousins of Mollisonia plenovenatrix might have been spider ancestors, but calling them spiders is like excavating an ancient fish and calling it a mammal. Very misleading.

Yes, I’m being pedantic. It matters. Let’s not diminish the diverse chelicerates by calling them spider wanna-bes.

Here’s the abstract for the paper.

The chelicerates are a ubiquitous and speciose group of animals that has a considerable ecological effect on modern terrestrial ecosystems—notably as predators of insects and also, for instance, as decomposers. The fossil record shows that chelicerates diversified early in the marine ecosystems of the Palaeozoic era, by at least the Ordovician period. However, the timing of chelicerate origins and the type of body plan that characterized the earliest members of this group have remained controversial. Although megacheirans have previously been interpreted as chelicerate-like, and habeliidans (including Sanctacaris) have been suggested to belong to their immediate stem lineage, evidence for the specialized feeding appendages (chelicerae) that are diagnostic of the chelicerates has been lacking. Here we use exceptionally well-preserved and abundant fossil material from the middle Cambrian Burgess Shale (Marble Canyon, British Columbia, Canada) to show that Mollisonia plenovenatrix sp. nov. possessed robust but short chelicerae that were placed very anteriorly, between the eyes. This suggests that chelicerae evolved a specialized feeding function early on, possibly as a modification of short antennules. The head also encompasses a pair of large compound eyes, followed by three pairs of long, uniramous walking legs and three pairs of stout, gnathobasic masticatory appendages; this configuration links habeliidans with euchelicerates (‘true’ chelicerates, excluding the sea spiders). The trunk ends in a four-segmented pygidium and bears eleven pairs of identical limbs, each of which is composed of three broad lamellate exopod flaps, and endopods are either reduced or absent. These overlapping exopod flaps resemble euchelicerate book gills, although they lack the diagnostic operculum. In addition, the eyes of M. plenovenatrix were innervated by three optic neuropils, which strengthens the view that a complex malacostracan-like visual system might have been plesiomorphic for all crown euarthropods. These fossils thus show that chelicerates arose alongside mandibulates as benthic micropredators, at the heart of the Cambrian explosion.

I think this diagram illustrates the relationship of M. plenoventrix to spiders well.

a, Simplified consensus tree of a Bayesian analysis of panarthropod relationships. This tree is based on a matrix of 100 taxa and 267 characters. Extant taxa are in blue; dashed branches represent questionable groupings. Asterisk shows that the radiodontans resolved as paraphyletic. This analysis excludes pycnogonids, but this had little effect on the topology. The letters A to D at the basal panchelicerate nodes refer to boxes on the right, and summarize the appearances of major morpho-anatomical features: (1) extension of cephalic shield, including a seventh tergite; (2) cephalic limbs all co-opted for raptorial and masticatory functions, and reduction of some trunk endopods; (3) dissociation of the exopod from the main limb branch; (4) presence of chelicerae; (5) trunk exopods made of several overlapping lobes; (6) some cephalic limbs differentiated as uniramous walking legs; (7) multi-lobate exopod covered by sclerite (operculum); (8) reduction of seventh cephalic appendage pair; and (9) all post-frontal cephalic limb pairs are uniramous walking legs. b, Life reconstruction. Drawing by J. Liang, copyright Royal Ontario Museum

Not a spider, but still cute and adorable.


Aria C, Caron J-B (2019) A middle Cambrian arthropod with chelicerae and proto-book gills. Nature https://doi.org/10.1038/s41586-019-1525-4.

Comments

  1. Owlmirror says

    Isn’t that rather like calling birds ‘dinosaurs’?

    No, absolutely not.

    If all dinosaur descendant species are graphed as a phylogenetic tree, all bird species are deeply nested within that tree.

    The whole point is that chelicerate species are all outside of the arachnid phylogenetic tree.

    A better analogy would have been “like calling pterodactlys (or mosasaurs, or plesiosaurs, or ichthyosaurs) dinosaurs”. (Pterodactyls are closest of that list to dinosaurs, but are still outside of the dinosaur phylogenetic tree)

  2. Owlmirror says

    Whoops, sorry, I screwed up.

    Arachnids are chelicerates, but not all chelicerates are arachnids. Mollisonia, as a chelicerate, is outside of the arachnid phylogenetic tree.

    The analogy I suggested still works, though. Dinosaurs are archosaurs, but not all archosaurs are dinosaurs. Pterodactyls were archosaurs that were not dinosaurs.

    (So too are crocodilians, FWIW.)

  3. Owlmirror says

    And mosasaurs, plesiosaurs, and ichthyosaurs were reptiles that were not archosaurs (and thus even more distantly related to dinosaurs).

  4. monad says

    I have heard Pikaia described as our – in the sense of the one very particular mammal Homo sapiens – ancestor often enough that calling this a spider granddaddy doesn’t seem so strange. Yes, it’s also an offshoot on the way to scorpions and mites and horseshoe crabs and more, but still. The real problem is that it’s actually a grand uncle.

    This tree is a better one than what I saw previously, in that it gives a place for trilobites (in Artiopoda) fit, something that seems particularly relevant because their body plan is so often compared to chelicerates like this. But that article’s question of where the sea spiders go is still a good one.

  5. John Harshman says

    In the reconstruction, the most cranial little pair of yellow feeding appendages sure looks like mandibles. Just sayin’.

  6. PaulBC says

    The lowermost picture is pretty scary, or would be if you were a few mm in size. For reasons that don’t make much sense, I’m reminded of https://ultraseries.fandom.com/wiki/Gabadon from an Ultraman episode I remember well from my childhood. Probably, just because everything reminds me of Ultraman, but I digress.

    The bottom picture could be the result of mischievous kids fancying up a sketch of Gabadon that comes to life the next day as a frightening monster that the Science Patrol is incapable of countering until Hayata slips off to use the beta capsule, transforming into Ultraman.

    In fact, the monster they actually made from the harmless Gabadon looks nothing like the creature above, but I think this one is better.

  7. nomdeplume says

    The writers of media headlines on science articles must share a huge proportion of the blame for the success of the anti-science crusades of the Right. Every report of research has to involve the biggest, fastest, scariest, oldest, earliest, weirdest. Every discovery is “rewriting” the science books, or rewriting evolution, or physics. All invertebrates are “creepy-crawlies” or “critters”. Nothing can be reported without bottom of the barrel tabloid reality tv hype. And yet, the people who are likely to be interested in scientific findings will be turned off by this style, and those unlikely to be interested will at most see the misleading headlines.

  8. chrislawson says

    nomdeplume@14–

    I find these overhyped headlines as distasteful as you, and I agree that they cause huges problems in public understanding of science, but there’s no reason to blame them on anti-science crusades. Those crusades have been going on since long before there was even such a thing as a printing press.

  9. nomdeplume says

    @17 Quite right Chris, I wasn’t blaming them so much as suggesting they have helped creare a suitable environment. Creationists for example often refer to the “rewriting evolution” kind of headlines as meaning nothing in evolution is certain.

  10. PaulBC says

    Just don’t compare Bret Stephens to one of those critters. At least not until you are safely tenured.

  11. birgerjohansson says

    Goddammit!
    Tualha @ 15 beat me to it! -But if these critters lived in the ocean, they would come close to the description of some of the smaller Lovecraftian entities, just add some wings.

  12. birgerjohansson says

    You could base a proper B-film on the reconstruction. Unfortunately, the Hammer film company is no longer around.
    At “God Awful Movies” -a podcast that rips apart (mostly) christian films (but also New Age and islamic films)- they have dealt with all kinds of evil critters on the screen. In “Shark Exorcist” there was apparently a shark (in a freshwater lake!) but you never see it clearly. Maybe the “shark” that was summoned by a curse was actually a swarm of these aquatic… thingies. (There is not much need for scientific accuracy in the medium)
    Too bad its ancestors branched off before Mandibulata, it would have made a nice ancestor to every insectoid entity to emerge from a low-budget film.

Leave a Reply