Superclades of the Cambrian

Allow me to introduce you to a whole gigantic superclade with which many of you may not be familiar, and some other groups in the grand hierarchy of animal evolution that I’ve mentioned quite a few times before, but would like to clear the fog with some simple definitions. Consider this a brief primer in some major animal groupings. Here’s a greatly simplified cladogram; I’ve left off quite a few groups to make the story simple.


I have a frequently admitted bias: I’m most interested in the evolution and development of the Metazoa, or the multicellular animals. I don’t follow the literature of bacterial or protist (single-celled creatures) phylogeny closely, nor do I know a great deal about other significant multicellular organisms, like plants. You can tell already that by starting the diagram with the Metazoa I’ve sliced off a tiny wedge of the great circle of life (buy the t-shirt!).

The Metazoa are also a diverse group, and gave rise to the Porifera (sponges) and Radiata, animals like ctenophores and cnidarians with radial body plans, and the Bilateria to which we belong, animals with bilateral symmetry.

Within the Bilateria we have two major clades distinguished by a developmental peculiarity (the significance of which has been reinforced by molecular evidence.) During gastrulation, a key developmental process that sets up the triploblastic, or three-layer organization of endoderm, ectoderm, and mesoderm, cells migrate into the interior through an opening called the blastopore. The blastopore either becomes the anus of the animal in the Deuterostomia (“second mouth”—they have to form a second opening to make a mouth), or it becomes the mouth in the Protostomia (“first mouth”). We human beings are deuterostomes, as are all mammals, birds, reptiles, amphibians and fish, and as are echinoderms (starfish and sea urchins) and a few less familiar marine invertebrates, like sea squirts.

The Protostomia are the real success story on this planet, with a huge amount of diversity, including various exotic marine worms, insects, annelids, and molluscs. The protostomes themselves have two major branches, the Ecdysozoa (animals that molt their external cuticles, like arthropods) and another category, fairly recently formulated, that most people will have not heard of: the Lophotrochozoa. Lophotrochozoans are another complex pigeonhole that includes worms and molluscs and who know what else.

So what are these lophotrochozoans, anyway? The name itself is a portmanteau word, merging two obscure terms into a new polysyllabic monstrosity that increases the level of recondite mystery. The lophophorate phyla include creatures like the brachiopods, which possess a lophophore, a ring- or U-shaped set of tentacles surrounding the mouth, which they use for feeding. Molluscs and annelids, on the other hand, do not have a lophophore, but they do have a characteristic larval form called the trochophore, which has a band of cilia they use for feeding. When molecular data linked these two groups together, the names were merged: lophophoran + trochophoran → lophotrochozoans.

For those who are curious to know what phyla belong to each of these groups, here’s a more detailed cladogram from Valentine’s On the Origin of Phyla(amzn/b&n/abe/pwll). Note that the Lophotrochozoa, Platyzoa, and Ecdysozoa all belong to the Protostomia—we deuterostomes are greatly outnumbered.

A conservative interpretation of the relationships of metazoan phylum-level groups that are indicated by SSU rRNA studies.

Tomorrow I’ll try to find time to say more about those fascinating lophotrochozoans. For now, take note of the interesting fact that these distinctions between the superclades almost certainly evolved before the beginning of the Cambrian—these are groups we can resolve now largely on the basis of soft tissue structure in embryos and molecular differences, and these were lineages that were on separate tracks before the emergence of large scale hard anatomical features that were amenable to preservation in the Cambrian rocks.


  1. Shawn S. says

    Hey, pay your respects to the prokaryotes, without which your measly multicelled organisms (Progress? HA!) wouldn’t exist!

    Actually, as a microbiology undergrad one of my favorite classes was the basic survey of zoology. Lophotrochozoans are very cool.

  2. Laurent says

    Wouldn’t “porte-manteau” be more appropriate?

    Unless the word evolved in a punctuated manner when invading (East) American?


  3. Opisthokont says

    Nice exposition, but I am compelled to point out that the Ecdysozoa/Lophotrochozoa hypothesis is still very much in the hypothesis stage; there are a number of respected researchers still disputing it with good work (check out Creevey et al. (2005), Mol. Biol. Evol. 22(5):1175-1184, which in addition to providing evidence against the hypothesis gives a good summary of recent work in the introduction). Personally, I am inclined to believe that the E/L hypothesis is in fact correct, but I think it unreasonable not to point out that it is not yet a settled matter.

  4. kmiers says

    The tears are dry, and I revert to learning. Very interesting stuff. Thanks PZ. Oh, and Shawn S., your prokaryotes are no match for the fascinating machines that are the viruses. I actually loathed microbiology until we got to the viruses. Too cool……

  5. Opisthokont says

    Ah, Cameron, you beat me to it! I was considering mentioning the startling and amazing recent changes made in eukaryotic phylogeny (I have handy about a dozen more recent papers on the subject, myself), but figured that it was too far afield (and anyway, PZ had already said that he would rather stick to animals). It is fantastic stuff, though!

  6. Rosadirame says

    Hi there,
    this post remembered me one of my dreams:
    paint the walls of my room with the “complete” philogenetic tree of life.
    Can anyone help me in this project with any link?
    I can’t figure if it’s possible (maybe I can stop at the Familia taxa for the moment? Or to different taxa for each phyla? Two dimensions would be enough, or i need 4D walls?).
    And how I can decide between different hypotesis, since my knowledge is really superficial in bacteria, fungi and plants?
    I think i’m not the only one with such a wish…

  7. Mrs Tilton says

    Ah, too cool PZ, thanks!

    This is what I love about Pharyngula. Come for the antireligious rants; stay for the cladism.

  8. says

    Can anyone help me in this project with any link?

    Rosadirame, this may not be in quite the form you could use, but check out:

    You have to follow a lot of links to a lot of pages as you traverse the tree, so it’s a bit unwieldy for the overall picture, but there’s a lot of good information on those pages.

  9. says

    Oh, I quite understand that it’s a hypothesis (but then, there are lots of hypotheses around that we use as a consequence of their utility.) Think of this more as a short description of some terminology that has common currency in the evo community, as a sort of preface to some other things I’m hoping to get to soon (still grading, but I hope to complete that all in short order).

  10. David Marjanović says

    Rosadirame, come back in 20 years.

    2D walls are enough, but they’d need to be incredibly huge….

  11. David Marjanović says

    Rosadirame, come back in 20 years.

    2D walls are enough, but they’d need to be incredibly huge….

  12. Bob O'H says

    “Superclades of the Cambrian” sounds to me like a great title for a Hollywood blockbuster. Who’ll play the evil Protostomic overlord, always talking out of the “wrong” end?


  13. says

    Rosadirame: I’m not sure you can cover your wall with this, and it’s three orders of magnitude short of being complete (3,000 species, based on rRNA survey), but here’s a ginormous PDF of a tree-of-life in circular format, complete with cute “You Are Here” pointing to H. sapiens, which is right next to M. musculus. Because I’m not a biologist, I don’t recognize most of the species names, but hey, it’s pretty staggering, isn’t it?

    (It’s recommended to print it at roughly five feet by five feet, also, which, if it won’t cover all your walls, will cover quite a bit.)

  14. says

    I was fortunate to be a student in the late 70’s-early 80’s when Systematic Zoology teemed with vituperative correspondence between three schools of systematics: Traditional (e.g., Mayr), Phenetics (e.g., Sokal), and the upstart Cladistics (e.g., Wiley, Farris). Each had their talking points, each made their share of ad hominum attacks (I think it was Farris that referred to Mayr’s Animal Species and Evolution as “Uncle Ernies’ Joke Book”). It was a fascinating introduction to the primary literature for a sophomore. And it was clear that the cladists were on a mission to bring clarity, rationality, and Popperian falsifiability to a discipline they viewed as peopled by cults of personality and hand waving.

    An attitude not unlike some “anti-religious rants” that we see in these parts of late. ;-)

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  15. Opisthokont says

    Pretty much all of the evidence, pro and con, is molecular (analyses of DNA sequences). There are a number of characters that are mentioned as possible homologies, but most if not all of the arguments are based on fitting them into molecular phylogenies, as the latter are seen as more objective. Unfortunately, which phylogeny one gets is very much a consequence of which genes one uses, from which and how many groups one samples, and what methods one uses to generate trees, and so the arguments made in favour of one hypothesis over another can get very technical. It will be a while before things quiet down, I suspect. I have not checked the most recent editions of the relevant textbooks (and will not be able to until next term starts), but I think it likely that they present both hypotheses at present.