This week's Nature includes a wonderful description of an exotic group of deuterostomes from the lower Cambrian, 520 million years ago. Deuterostomes are animals characterized by their embryology: when they gastrulate, the site of closure of the migrating tissues, the blastopore, becomes the anus of the animal. This is in contrast to the protostomes, in which the blastopore becomes the mouth. We chordates are deuterostomes, as are echinoderms, some marine worms called hemichordates, and the urochordates, or sea squirts.
Shu et al. have identified some animals called vetulocystids, and they are most closely related to modern echinoderms. Echinoderm evolution is confusing and complicated, largely because the modern forms are so highly derived and distinct from the ancestral forms, and because the echinoderm lineage has been spectacularly diverse morphologically. The authors jump from some somewhat speculative interpretations of the fossil anatomy (the specimens, as you can see below, are peculiar and the structures difficult to identify) to an idea that clarifies the organization of the deuterostome lineage.
Continue reading "Spectacular echinoderms from the Lower Cambrian" (on Pharyngula)