Spadella cephaloptera

In any case, the preliminary results of the molecular analysis are in agreement with those of the morphologic analysis: the mosaic of characteristics exhibited by the chaetognaths is an indication of the ancestral character of this lineage. The paleontological evidence also supports this, because fossils dating back to more than 500 million years have been assigned to this group (Chen & Huang). Since inclusion in the Deuterostome group has been excluded, several hypotheses remain possible, including :

• The chaetognath lineage may have separated very early, before the Protostome-Deuterostome divergence. These animals would then be the group of choice for reconstituting the common ancestor of all the bilaterians (which, of course, does not imply that the chaetognaths would necessarily resemble this ancestor).
• The chaetognath lineage may have separated after the divergence of the Protostome branch. It would therefore occupy a basal position in the Protostome tree, outside of the two large groups of Lophotrochozoa (Mollusks, Annelids, Brachiopods, etc.) and the Ecdysozoa (Arthropods, Nematodes, etc.); alternatively they could be placed within one of these groups.