Halwaxiida - Wiwaxia and Odontogriphus

Wiwaxia and Odontogriphus

Further information: Wiwaxia and Odontogriphus

Ever since the debate started in 1990, the most intense part has centered round Wiwaxia. Until 2006 only one, poor-preserved fossil of Odontogriphus had been described; but in that year a description of many new, well-preserved specimens highlighted similarities between the two organisms, making the classification of Odontogriphus equally controversial.

Conway Morris (1985) originally dismissed the earliest classification of Wiwaxia as a polychaete worm, because he thought there was little structural similarity between a polychaete's scale-like elytra and Wiwaxia’s sclerites, and because the arrangement of the sclerites, with quite different numbers in each band, showed no sign of the regular segmentation that is a feature of polychaetes. Instead he thought Wiwaxia was very similar to shell-less aplacophoran molluscs, that it must have moved on a mollusc-like muscular foot, and that its feeding apparatus looked like a primitive form of the molluscan radula, a tooth-bearing chitinous "tongue". Hence he classified it as a "sister" of the molluscs. When he briefly described the first articulated specimens of Halkieria in 1990, Conway Morris wrote of "the halkieriid-wiwaxiid body plan" and that the halkieriids might be close relatives of molluscs.

"Blade" = Root Wiwaxia spine, seen from front and side. They had relatively soft bases and were rooted in pockets in the skin

= skin = aragonite = flesh Halkieriid sclerite structure

Shortly after this in 1990 Butterfield published his first paper on Wiwaxia. He argued that, since Wiwaxia’s sclerites appeared to be solid, they were not similar to the hollow sclerites of halkieriids. In fact he thought they were more similar to the chitinous bristles (setae) that project from the bodies of modern annelids and in some genera form leaf-like scales that cover the back like roof tiles - in composition, in detailed structure, in how they were attached to the body via follicle-like pockets in the skin, and in overall appearance. He also contended that Wiwaxia’s feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychaetes. He therefore classified Wiwaxia as a polychaete.

Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes. However they also argued that Wiwaxia was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller, more numerous and hollow. They wrote that in 1994 Butterfield had found Wiwaxia sclerites that were clearly hollow. They noted that one specimen of Wiwaxia showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that Wiwaxia’s feeding apparatus was much more like a molluscan radula. The cladogram they presented showed the halkieriids split into three groups: one as "aunts" of brachiopods, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore; the second group of halkieriids as "aunts" of both Wiwaxia and annelid worms; and the earliest halkieriids as "great aunts" of all of these.

Marine biologist Amélie H. Scheltema et al. (2003) argued that Wiwaxia’s feeding apparatus was very similar to the radulas of some modern shell-less aplacophoran molluscs, and that the sclerites of the two groups are very similar. They concluded that Wiwaxia was a member of a clade that includes molluscs.

Danish zoologist Danny Eibye-Jacobsen (2004) regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if Wiwaxia’s sclerites closely resembled bristles, which he doubted, this would not prove that Wiwaxia’s closest relative were annelids. He also pointed out that the very different numbers of sclerites in the various zones of Wiwaxia’s body do not correspond to any reasonable pattern of segmentation. Since in his opinion Wiwaxia lacked other clearly polychaete features, he regarded this as an argument against classifying Wiwaxia as a polychaete. In his opinion there were no strong grounds for classifying Wiwaxia as a proto-annelid or a proto-mollusc, although he thought the objections against classification as a proto-annelid were the stronger.

= Food = Radula = Odontophore "belt" = Muscles Snail radula at work.

Caron, Scheltema et al. (2006) interpreted Odontogriphus’s feeding apparatus as a forerunner of the molluscan radula, on the grounds that: the occasional and less distinct third tooth-row looked like evidence that the animals grew replacement tooth-rows at the rear of their mouths and shed worn-out ones from the front, as happens with molluscan radulae; the isolated pairs of tooth-rows they found, not associated with body fossils but in the same relative positions as in the more complete fossils, suggested they were mounted on a fairly tough surface, like the chitinous "belt" of a modern radula; they even found signs that discarded tooth-rows were sometimes eaten by the animals. They thought the teeth on the feeding apparatus of both Wiwaxia and Odontogriphus strongly resembled those of a modern group of molluscs, Neomeniomorpha. On the other hand they thought Wiwaxia bore little resemblance to polychaetes as it showed no signs of segmentation, appendages in front of the mouth, or "legs" – all of which are typical polychaete features. Hence they classified both Odontogriphus, and Wiwaxia with its "nearly identical" feeding apparatus, as primitive relatives of molluscs.

Butterfield (2006) accepted that Wiwaxia and Odontogriphus were closely related but wrote, "…it certainly does not require that they be shoehorned into the same lineage. …the seemingly trivial distinction between these two taxa is exactly what is expected at the divergence points leading from a last common ancestor to extant phyla." In his opinion the feeding apparatus of these organisms, with consisted of two or at most four rows of teeth, could not perform the functions of the "belt-like" molluscan radula with its numerous tooth-rows; the different tooth-rows in both Wiwaxia and Odontogriphus tooth-rows also have noticeably different shapes, while those of molluscan radulae are produced one after the other by the same group of "factory" cells and therefore are almost identical. He also regarded lines running across the middle region of Odontogriphus fossils as evidence of external segmentation, since the lines are evenly spaced and run exactly at right angles to the long axis of the body. As in his earlier papers, Butterfield emphasized the similarities of internal structure between Wiwaxia’s sclerites and the bristles of polychaetes, and the fact that polychates are the only modern organisms in which some of the bristles form a covering over the back. He therefore concluded that that both Wiwaxia and Odontogriphus were stem-group polychaetes rather than stem-group molluscs.

In 2007 Conway Morris and Caron described a new fossil, Orthrozanclus, which had a mineralized shell like that of halkieriids, and unmineralized sclerites and long spines like those of Wiwaxia – and, like both of these, a soft underside which they interpreted as a muscular foot, and a similar arrangement of the sclerites into three concentric bands. Some of Orthrozanclus’s sclerites appear to have been hollow, as the specimen includes what look like internal castings. They took these features as evidence that the halkieriids, Wiwaxia and Orthrozanclus were very closely related and formed the group "halwaxiids". However the simplest "family tree" faces an obstacle: the siphogonuchitids appear in earlier rocks and had mineralized sclerites. Hence Conway Morris and Caron found it necessary to consider two more complex family trees, concluding that "Hypothesis 1" fitted the available data better, but was sensitive to minor changes in the characteristics used:

1. Conway Morris and Caron considered this hypothesis the more likely, although it is not robust:
   • Kimberella and Odontogriphus were early, primitive molluscs, without sclerites or any kind of mineralized armor.
   • Wiwaxia, the siphogonotuchids, Orthrozanclus and Halkeria form a side-branch of the mollusc family tree, and diverged from it in that order. This would mean that: Wiwaxia was the first of them to have sclerites, which were unmineralized; the siphogonotuchids were the first to have mineralized sclerites, although their scleritomes were simpler; halkieriids then developed more complex scleritomes, and a shell at each end; in Orthrozanclus the scleritome became unmineralized again and the rear shell vanished or became so small that it has not been seen in fossils. This hypothesis faces the difficulty that siphogonotuchids appear in earlier rocks and have simpler scleritomes than the other three groups.
   • The annelids and brachiopods evolved from the other main branch of the family tree, which did not include the molluscs.
2. The alternative view is:
   • Kimberella and Odontogriphus are early, primitive lophotrochozoans.
   • The siphogonotuchids, Halkeria, Orthrozanclus and Wiwaxia form a group that is closer to the shared ancestor of annelids and brachiopods than it is to the molluscs. The siphogonotuchids are the first of the group to become distinctive, with two types of mineralized sclerites and a "shell" made of fused sclerites. Halkieriids had three types of sclerites and two one-piece shells. In Orthrozanclus the sclerites became unmineralized and in Wiwaxia the shells were lost.

In 2008 Butterfield described a set of micro-fossils dated to between 515 million years ago and 510 million years ago, found in the Mahto Formation in Alberta's Jasper National Park – this fossil bed is 5 – 10 M years older than the Burgess Shale in which the only known specimens of Odontogriphus and Wiwaxia were found. Some groupings of these micro-fossils showed a consistent arrangement that he interpreted as an "articulated apparatus" with tens of closely spaced tooth rows, apparently mounted on an organic base, and with noticeable signs of wear in the rows at one end. The rows were not quite identical, but he noted that some modern aplacophoran molluscs show similar variations. He concluded that the "articulated apparatus" was a genuine molluscan radula, most similar to those of modern aplacophorans or gastropods. He then commented on the contrast between this apparatus and the two or rarely three widely spaced and more heterogeneous tooth-rows found in fossils of Odontogriphus and Wiwaxia, and concluded that Odontogriphus and Wiwaxia were unlikely to be molluscs. He also noted that Wiwaxia lacked some polychaete features which he would expect to be easily preserved in fossils, and therefore classified Wiwaxia as a stem-group annelid, in other words an evolutionary "aunt" of modern annelids.