An Introduction to Zoology: Chapter 4

A more recent animal phylogeny, based on molecular data.

Molecular data shows that some of the “traditional” taxa are true clades, while others are only grades and are thus in need of revision. The molecular data also strongly suggest that the choanoflagellate protists were the ancestors of all members of the Kingdom Animalia.

According to the molecular data, the Parazoa and the Eumetazoa are true clades. Evidently, animals split into these two groups very early in their history.

The Radiata and the Bilateria are also true clades, according to the molecular data. This is another split that occurred very early in animal history.

Things get more complicated after this point, however, as the molecular data show that the Acoelomates, Pseudocoelomates and Coelomates are not true clades. As such, though the terms are sometimes used to describe animals, they’re no longer used to classify them, since they don’t accurately indicate phylogenetic relationships.

The molecular data indicate that the division of the Bilateria into the Protostomia and the Deuterostomia is justified and that these are true clades. Again, the separation between protostomes and deuterostomes occurred very early in animal history, when the most complicated living animals would have resembled worms. (That’s why that episode of Star Trek: The Next Generation in which Lieutenant Barclay “devolved” into a spider-like creature is so funny. Spiders are protostomes; humans are deuterostomes. Therefore, humans most-definitely are not descended from anything that ever looked even remotely like a spider!)

The molecular data show that the Protostomia are divided into two major groups, the Lophotrochozoa and the Ecdysozoa. We’ll discuss each of these groups in later chapters.

Finally, the molecular data show that, as had been suspected from morphological and developmental evidence, the two major phyla of the Deuterostomia – the Echinodermata and the Chordata – are indeed closely-related. Perhaps it’s strange to think of a sea star as a fairly close relative of yours, but that is indeed the case.

Heh, heh. Yeah ... right.