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.
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.