Ward, Peter. "3 Evolving Respiratory Systems as a Cause of the Cambrian Explosion." Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. Washington, DC: The National Academies Press, 2006.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
and more recently from the interesting new fossils found in Early Cambrian strata in Chengjiang, China.
Compared to other phyla, ours is very different in several anatomical characteristics. Most animal phyla that show bilateral symmetry have a nerve chord running the length of the body, but this nerve cord lies beneath the gut. In chordates, it is dorsal to the gut. Getting to this reversal of anatomy required some major evolutionary rearranging. One of the oldest and most elegant hypotheses came from the evolutionist W. Garstang, who noted the similarity between the presumed anatomies of ancestral vertebrates (and the fish-like but boneless Lancelet, Amphioxus, which is often used as a model of what the first chordates might have looked like) and the larval stage of the common tunicate, commonly called a sea squirt. While tunicates are sessile filter feeders and look nothing at all like any sort of living chordate, their larva strongly resemble small fish. It was thus theorized by Garstang, followed later by a slew of anatomists, that true vertebrate chordates arose from the larva by a process called paedomorphism—where evolution causes larval characteristics to appear in adults. Later DNA studies have supported this view. We now have a pretty good idea about the “tree” of evolution leading to us chordates: our nearest nonchordate ancestor appears to be the phylum Urochordata, the phylum containing the sea squirts, as so long ago suggested by Garstang.
Is there a connection between this phylogeny, or proposed evolutionary pathway, and levels of oxygen in the latest pre-Cambrian, when this split of the tunicate group into tunicates and vertebrates probably occurred? There has never been any published suggestion that oxygen levels had anything to do with the evolution of the phylum Urochordata. Let’s change that now:
Hypothesis 3.4: The phylum Urochordata—ancestor of thechordate—evolved in response to low oxygen by producing abody plan with a highly powerful and efficient pump gill, whichbecame co-opted for feeding as well. It was this body plan thatled to the larva, which became a template for the chordate bodyplan. The chordates thus came into being because of the bodyplan of their immediate ancestors.