From Three to Two

(I've been waiting three and a half years to use Neil's icon.)

The mysterious anabaritids of the Lower Cambrian have been referred to on this site before. In the earlier, somewhat brief post, I referred to their triradial structure and uncertain, though probably coelenterate-grade, relationships. In the time since that post appeared, the anabaritids have been the subject of a review by Kouchinsky et al. (2009) that brought together a lot of the previously scattered information on these animals.

The image of an anabaritid in the previous post showed the best known species, Anabarites trisulcatus. However, this was not the only species in the group. The image just above, from Kouchinsky et al. (2009), shows another species, Anabarites biplicatus, recorded from the Siberian Platform. This species differs from A. trisulcatus in that it started out life triradial (albeit with the internal dividing ridges between the lobes only weak), but as it grew it lost its triradiality and became more bilateral (cross-section below from Kouchinsky et al.):

Some of my readers may remember that Cambrian problematica were something of a cause célèbre during the mid-90s when a lot of journals and magazines ran features on them (probably inspired to a certain degree by Stephen Jay Gould's somewhat dreadful book Wonderful Life). In the more academic corners of this pageant, the triradiality of anabaritids (as well as some other early animals such as Tribrachidium) garnered them a certain degree of attention. It was suggested by some that they might represent a unique animal lineage that was eventually superseded by our own bilateral dynasty. However, the changing symmetry of Anabarites biplicatus serves as a reminder that we should not be too hasty to assign great significance to such features. Indeed, in the modern fauna, nematodes are partially triradial (they have a triradial head structure, with one upper and two lower lips around the mouth). Though the affinities of anabaritids are somewhat debatable, the most popular scenario is that they are related to cnidarians: their tubes bear a certain resemblance to the polyps of some medusozoans. Cnidarians also exhibit a wide variety of symmetries, such as the tetraradial organisation of scyphozoans and the hexaradial organisation of hexacorals. Without preserved soft tissue to inform us what the organisation of the inhabitant animal may have been, it is difficult to say just how much weight the triradiality of the anabaritid tube should be given.

REFERENCE

Kouchinsky, A., S. Bengtson, W. Feng, R. Kutygin & A. Val'kov. 2009. The Lower Cambrian fossil anabaritids: affinities, occurrences and systematics. Journal of Systematic Palaeontology 7 (3): 241-298.