Halwaxiida

taxobox

image_caption="Halkieria"
regnum=Animalia
fossil_range=Early to Middle Cambrian
superphylum=Lophotrochozoa
phylum = "See text"

Halwaxiida or halwaxiids is a proposed clade loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lohpotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.

Definition and members

Butterfield's alternative.cite conference
first = Butterfield | last = N.J. | date = 2007-12-18 | year = 2007
title = Lophotrochozoan roots and stems
conference = Palaeontological Association Annual Meeting
conferenceurl = http://www.palass.org/modules.php?name=annual_meeting&page=1
booktitle = Programme with Abstracts
editor = Budd, G.E.; Streng, M.; Daley, A.C.; Willman, S.
location = Uppsala, Sweden | pages = 26-7]

"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.cite journal
author = Conway Morris, S. | year = 1985
title = The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada
journal=Philosophical Transactions of the Royal Society of London, Series B
volume = 307 | pages = 507–582 | doi=10.1098/rstb.1985.0005
url=http://www.jstor.org/pss/2396338 | accessdate=2008-08-04 ] 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. cite journal
title=Articulated halkieriids from the Lower Cambrian of north Greenland
author=Conway Morris, S., and Peel, J.S. | journal=Nature | volume=345 | pages=802–805
date=June 1990 | doi=10.1038/345802a0
url=http://www.nature.com/nature/journal/v345/n6278/abs/345802a0.html | accessdate=2008-07-31 A short but free account is given at cite web
url=http://www.stephenjaygould.org/library/naturalhistory_cambrian.html
title=Showdown on the Burgess Shale | accessdate=2008-07-31 ] 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 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.cite journal
author=Butterfield, N.J. | year=1990
title=A reassessment of the enigmatic Burgess Shale fossil "Wiwaxia corrugata" (Matthew) and its relationship to the polychaete "Canadia spinosa". Walcott
journal=Paleobiology | volume=16 | pages=287–303
url=http://www.jstor.org/stable/2400789 | accessdate=2008-08-05 ]

Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated "Wiwaxia" as an ancestor or "aunt" of the polychaetes. Hoever 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 aparatus 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. [ cite journal
title=Original Molluscan Radula: Comparisons Among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian Fossil "Wiwaxia corrugata"
author=Scheltema, A.H., Kerth, K., and Kuzirian, A.M.
journal=Journal of Morphology | volume=257 |pages=219–245 | date=2003 | doi= 10.1002/jmor.10121
url=http://www3.interscience.wiley.com/journal/104528759/abstract?CRETRY=1&SRETRY=0
accessdate=2008-08-05 ]

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 ojections against classification as a proto-annelid were the stronger..cite journal
title=A reevaluation of "Wiwaxia" and the polychaetes of the Burgess Shale
author=Eibye-Jacobsen, D. | journal=Lethaia | volume=37 | issue=3 | pages=317–335
month=September | year=2004 |doi=10.1080/00241160410002027
url=http://www.ingentaconnect.com/content/tandf/slet/2004/00000037/00000003/art00011;jsessionid=383hjtjp6370h.alexandra
accessdate = 2008-08-04 ]

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.cite journal
author = Butterfield, N.J. | year = 2006
title = Hooking some stem-group ‘‘worms’’: fossil lophotrochozoans in the Burgess Shale
journal = Bioessays | volume = 28 | issue = 12 | pages = 1161-6 | doi = 10.1002/bies.20507
url=http://www3.interscience.wiley.com/journal/113471993/abstract
accessdate=2008-08-06 ]

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:
# 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.
# 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 ma|515 and ma|510, 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. cite journal
author=Butterfield, N.J. | title=An Early Cambrian Radula
journal=Journal of Paleontology | date=May 2008 | volume=82 | issue=3 | pages=543-554
doi=10.1666/07-066.1
url=http://findarticles.com/p/articles/mi_qa3790/is_200805/ai_n25501673/pg_1?tag=artBody;col1
accessdate=2008-08-20]

Halkieriids and siphogonotuchids

Most of the debate has been about halkieriids' relationship to molluscs, annelids and brachiopods. However a paper published in 2008 argued that halkieriids were closely related to chancelloriids, which have been generally regarded as sponges.

Relationship to Molluscs, Annelids and Brachiopods

Halkieriid sclerites were known for a long time as elements of the small shelly fossils. The first articuluted specimen, with all their hard parts together, were collected in 1989 and were described briefly in 1990 by Conway Morris and Peel (1990). cite journal
title=Articulated halkieriids from the Lower Cambrian of north Greenland
author=Conway Morris, S., and Peel, J.S. | journal=Nature | volume=345 | pages=802–805
date=June 1990 | doi=10.1038/345802a0
url=http://www.nature.com/nature/journal/v345/n6278/abs/345802a0.html | accessdate=2008-07-31 A short but free account is given at cite web
url=http://www.stephenjaygould.org/library/naturalhistory_cambrian.html
title=Showdown on the Burgess Shale | accessdate=2008-07-31 ] The same authors produced a more detailed analysis in 1995, which divided the halkieriids 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.

In 2003 Cohen, Holmer and Luter supported the halkieriid-brachiopod relationship, suggesting that brachiopods may have arisen from a halkieriid lineage that developed a shorter body and larger shells, and then folded itself and finally grew a stalk out of what used to be the back. [ cite journal
author=Cohen, B. L. and Holmer, L. E. and Luter, C. | date=2003 |
title=The brachiopod fold: a neglected body plan hypothesis
url=http://eprints.gla.ac.uk/2920/01/Cohen_2920.pdf | accessdate=2008-08-07 | journal=Palaeontology
volume=46 | issue=1 | pages=59-65 | doi=10.1111/1475-4983.00287c ]

Vinther and Nielsen (2005) proposed instead that "Halkieria" was a crown group mollusc, in other words more similar to modern molluscs that to annelids, brachiopods or any intermediate groups. They argued that: "Halkieria"’s sclerites resembled those of the modern solenogaster aplacophoran shell-less molluscs, of some modern polyplacophoran molluscs, which have several shell plates, and of the Ordovician polyplacophoran "Echinochiton"; "Halkieria"’s shells are more similar to the shells of conchiferan molluscs, since shells of both of these groups show no trace of the canals and pores seen in polyplacophoran shell plates; the bristles of brachiopods and annelids are similar to each other but not to "Halkieria"’s sclerites. [ cite journal
url=http://www.jakobvinther.com/vintherandnielsen2005 | accessdate=2008-08-07
author=Vinther, J., and Nielsen, C.
title=The Early Cambrian Halkieria is a mollusc | journal=Zoologica Scripta | volume=34 | issue=1
date=January 2005 | pages=81–89]

However Conway Morris (2006) criticized Vinther and Nielsen's classification of "Halkieria" as a crown group mollusc, on the grounds that the growth of the spicules in the aplacophorans and polyplacophorans is not similar to the method of growth deduced for the complex halkieriid sclerites; in particular, he said, the hollow spines of various molluscs are not at all like the halkieriid sclerites with their complex internal channels. Conway Morris repeated his earlier conclusion that halkieriids were close to the ancestors of both molluscs and brachiopods. [cite journal
author=Conway Morris, S. | date=June 2006 | title=Darwin's dilemma: the realities of the Cambrian ‘explosion’
journal=Philososophical Transactions of the Royal Society: Biological Sciences | location=London
volume=361 | issue=1470
url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1578734 | accessdate = 2008-08-07 ]

In their descripton of the newly-discovered "Orthrozanclus" (2007), which has similarities to both "Wiwaxia" and the halkieriids, Conway Morris and Caron also took account of the siphogonuchitids, a group known only from isolated mineralized sclerites that resemble those of the halkieriids. They proposed the two "family trees" described above:
# The halkieriids formed part of a "sister" group to the molluscs. This hypothesis implies that the halwaxiids were not monophyletic, since the siphogonuchitids appear between "Wiwaxia" and the halkieriids. Nevertheless Conway Morris and Caron found that this phylogeny fitted the available data better, although it was not robust.
# The siphogonuchitids plus the halwaxiids formed a "sister" group to the annelids and brachopods, while the molluscs were "aunts" of all of these.

Relationship to chancelloriids

Porter (2008) revived an early 1980s idea that the sclerites of "Halkieria" are extremely similar to those of chancelloriids. These were sessile, bag-like, radially-symmetric organisms with an opening at the top.

Since their fossils show no signs of a gut or other organs, chancelloriids were originally classified as some kind of sponge. Butterfield and Nicholas (1996) argued that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites is similar to that of fibers of spongin, a collagen protein, in modern keratose (horny) demosponges. [ cite journal
author=Butterfield, N. J., and C. J. Nicholas | date=1996
title=Burgess Shale-type preservation of both non-mineralizing and "shelly" Cambrian organisms from the Mackenzie Mountains, northwestern Canada
journal=Journal of Paleontology | volume=70 | pages=893-899] However Janussen, Steiner and Zhu (2002) denied that chancellorids were sponges on the grounds that: chancellorids' hollow sclerites, probably made of aragonite, were quite different from the solid, silica-based spines of demosponges; sponges have simple skins that are only one cell thick, while chancellorids' thicker, more complex skins resemble those of more complex animals. In their opinion chancellorids were at least members of the Epitheliazoa, a grade more complex than the Porifera, to which sponges belong. cite journal
author=Janussen, D., Steiner, M., and Zhu, M-Y.
title=New Well-preserved Scleritomes of Chancelloridae from the Early Cambrian Yuanshan Formation (Chengjiang, China) and the Middle Cambrian Wheeler Shale (Utah, USA) and paleobiological implications
journal=Journal of Paleontology | date=July 2002 | volume=76 |issue=4 | pages=596-606
doi=10.1666/0022-3360(2002)076<0596:NWPSOC>2.0.CO;2
url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/76/4/596 | accessdate=2008-08-04 Free full text without images at cite web | url=http://findarticles.com/p/articles/mi_qa3790/is_200207/ai_n9134583/pg_1?tag=artBody;col1 | title=(as above) | accessdate=2008-08-04 ]

Porter (2008) found that the sclerites of halkieriids and chancelloriids resemble each other at all levels: both have an internal "pulp cavity" and a thin external organic layer; the walls are made of the same material, aragonite; the arrangement of the aragonite fibers is in each is the same, running mainly from base to tip but with each being closer to the suface at the end nearest the tip. It is extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related. This dilemma may be resolved in various ways: cite journal
title=Skeletal microstructure indicates Chancelloriids and Halkieriids are closely related
author=Porter, S.M | journal=Palaeontology | volume=51 | issue=4 | date=2008 | pages=865–879
doi=10.1111/j.1475-4983.2008.00792.x
url=http://www.geol.ucsb.edu/faculty/porter/Papers/Coeloscleritophora.pdf | accessdate=2008-08-07 ]
* One possibility is that chancelloriids evolved from bilaterian ancestors but then adopted a sessile lifestyle and rapidly lost all unnecessary features. However the gut and other internal organs have not been lost in other bilaterians that lost their external bilateral symmetry.
* On the other hand perhaps chancelloriids are similar to the organisms from which bilaterians evolved, and the earliest bilaterians had similar sclerites. However there are no fossils of such sclerites before ma|542, while "Kimberella" from ma|555 was almost certainly a bilaterian,cite journal
author = Fedonkin, M.A. | coauthors = Waggoner, B.M. | year = 1997
title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
journal = Nature | volume = 388 | issue = 6645 | pages = 868
doi = 10.1038/42242] but shows no evidence of sclerites.
* One solution to this dilemma may be that preservation of small shelly fossils by coatings of phosphate was common only for a relatively short time, during the Early Cambrian, and that coelosclerite-bearing organisms were alive several million years before and after the time of phosphatic preservation.
* Alternatively, perhaps the common ancestor of both chancelloriids and halkieriids had very similar but unmineralized coelosclerites, and some intermediate groups independently incorporated aragonite into these very similar structures. [ citation
author=Bengtson, S. | contribution=Mineralized skeletons and early animal evolution | page=pp. 101–124
editor=Briggs, D.E.G. | title=Evolving form and function: fossils and development |
publisher=Peabody Museum of Natural History, Yale University | location=New Haven, CT | pages=288 ]

Notes and references

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