1. Origin of high-rank groups of organisms

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It is outlined how seemingly fundamental differences between high-level groups such as phyla and classes may have been introduced in fairly simple and rapid ways. Examples of such ways are development of endosymbiosis, heterochrony involving change of adult mode of feeding with associated changes of basic life habits and body plan, development of asymmetry, and turning upside-down. The latter modification has resulted in fundamental misinterpretations of so-called deuterostomes (or notoneuralians). The basal deuterostomes, the hemichordates, are much more similar to protostomes than textbook authors have been willing to admit. In fact, it is difficult to understand why they are at all considered as deuterostomes. For instance, protostome characteristics found in hemichordates include a main nerve cord on the ventral side, a circum-oesophageal nerve ring, a larva surprisingly similar to a protostome trochophora, blood circulation in the same direction as in protostomes, and (in all pterobranchians and many enteropneusts) schizocoelic formation of the coelom. New consideration of morphologies and life postures indicates that deuterostomes (notoneuralians) are not upside-down compared with protostomes (gastroneuralians). Instead, it is vertebrates that are upside-down compared with all other animals, including other deuterostomes. Fossils very poorly reveal the changes in body plans, but at least indicate that in general these changes are of (at least) Cambrian age. The oldest known group with vertebratetype orientation is the conodonts.
日本古生物学会の論文
1997-04-30

1. Origin of high-rank groups of organisms

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