Halkieria

Taxobox
name = "Halkieria"
fossil_range = fossil range|533|511|Lower to Middle Cambrian


image_title = "Halkieria evangelista" from the Lower Cambrian Sirius Passet, North Greenland
regnum = Animalia
phylum = ?Mollusca
unranked_ordo = Halwaxiida
familia = Halkieriidae
familia_authority = Conway Morris and Peel 1990
genus = "Halkieria"
binomial = "Halkieria evangelista"
binomial_authority = Conway Morris and Peel 1990

"Halkieria" (IPA: IPA|hælˈkɪˌriʌ)Verify source|date=August 2008 is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is "Halkieria evangelista", from the North Greenland Sirius Passet Lagerstätte in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal "Nature". Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.

History of discovery

Armor plates called "sclerites" had long been known as elements of the small shelly fossils, and detailed analysis showed that some of these belonged to the same animal and how they fitted together. The first articulated specimens of "Halkieria evangelista", with all their hard parts together, were collected in 1989 from the Sirius Passet lagerstätte in Greenland, and were described in 1990 by Simon Conway Morris and John S. Peel. 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
] "H. evangelista" is used as a model for identifying and reconstructing as halkieriids other similar shells and sclerites.

Occurrence

Graphical timeline
help=off
title=Halkieriids in context
from=-560
to=-503
height=50
width=10

legend1=Neoproterozoic(last Eon_%28geology%29!æon of the Precambrian)
legend1-color=period color|neoproterozoic
bar1-to=-542
bar1-left=.0
bar1-right=.1
bar1-colour=period color|neoproterozoic
legend2=Palæozoic (first æon of the Phanerozoic)
legend2-colour=period color|palaeozoic
bar2-from=-542
bar2-left=0
bar2-right=.1
bar2-colour=period color|paleozoic

bar4-to=-542
bar4-left=.1
bar4-right=0.77
bar4-colour=period color|ediacaran
bar4-border-width=0
bar4-text=Ediacaran
bar4-nudge-down=6.7
note13=
C
a
m
b
r
i
a
n


note13-at=-505
note13-size=100%
note13-nudge-left=11.8
note13-remove-arrow=yes
bar3-from=-542
bar3-left=.1
bar3-right=.24
bar3-colour=period color|cambrian
bar3-border-width=0.1
bar5-left=0.252
bar5-right=0.26
bar5-colour=black
bar5-from=-542
bar6-from=-542
bar6-to=-534
bar6-text=Nemakit-
Daldynian

bar6-nudge-up=0.5
bar6-border-width=0.05
bar6-left=0.25
bar6-right=0.77
bar6-colour=period color|cambrian series 1
bar7-from=-534
bar7-to=-530
bar7-text=Tommotian
bar7-border-width=0.05
bar7-left=0.25
bar7-right=0.77
bar7-colour=period color|cambrian series 1
bar8-from=-530
bar8-to=-524
bar8-text=Atdabanian
bar8-border-width=0.05
bar8-left=0.25
bar8-right=0.77
bar8-colour=period color|cambrian series 2
bar9-from=-524
bar9-to=-518.5
bar9-text=Botomian
bar9-border-width=0.05
bar9-left=0.25
bar9-right=0.77
bar9-colour=period color|cambrian series 2
bar10-from=-518.5
bar10-to=-513
bar10-text=Toyonian
bar10-border-width=0.05
bar10-left=0.25
bar10-right=0.77
bar10-colour=period color|cambrian series 2
bar11-from=-513
bar11-to=-503
bar11-text=Middle
Cambrian

bar11-nudge-up=0.5
bar11-border-width=0.05
bar11-left=0.25
bar11-right=0.77
bar11-colour=period color|cambrian series 3

The only reasonably complete specimens, of "Halkieria evangelista", were found in the Sirius Passet lagerstätte in Greenland. Fragments which are confidently classified as belonging to halkieriids have been found in China's Xinjiang province and Australia's Georgina Basin, while shells of a possible halkieriid have been found in Canada's Burgess Shale. Halkieriid-like armor plates have been found in many other places as part of the small shelly fauna.

The earliest known occurrences of these plates, called" sclerites", date from from the Early Cambrian period's Tommotian age. The mass extinction at the end of the Cambrian period's Botomian age was thought to have wiped out most of the small shellies, including the halkieriids, but in 2004 Halkieriid fossils classified as "Australohalkieria" were reported from Mid-Cambrian rocks of the Georgina Basin in Australia. It is not known why this clade would have survived while other halkieriid clades apparently did. cite journal
author=Porter, S.M. | date=May 2004 | accessdate=2008-08-01
url=http://findarticles.com/p/articles/mi_qa3790/is_200405/ai_n9377598/pg_1?tag=artBody;col1
title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia
journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590
doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2
year=2004
] It may be significant that the only archaeocyathans known to have survived the end-Botomian extinction also occur in Gondwana, the old super-continent that embraced South America, Africa, India, Australia and Antarctica. [ cite journal
author=Debrenne, F., Rozanov, A.Y., and Webers, G.E. | date=1984
title=Upper Cambrian Archaeocyatha from Antarctica | journal=Geological Magazine
volume=121 | pages=291–299
cited by cite journal
author=Porter, S.M. | date=May 2004 | accessdate=2008-08-01
url=http://findarticles.com/p/articles/mi_qa3790/is_200405/ai_n9377598/pg_1?tag=artBody;col1
title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia
journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590
doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2
year=2004
] [ cite journal
author=Wood, R.A., Evans, K.R., and Zhuravlev, A.Y. | date=1992
title=A new post-early Cambrian archaeocyath from Antarctica
journal=Geological Magazine | volume=129 | pages=491–495
cited by cite journal
author=Porter, S.M. | date=May 2004 | accessdate=2008-08-01
url=http://findarticles.com/p/articles/mi_qa3790/is_200405/ai_n9377598/pg_1?tag=artBody;col1
title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia
journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590
doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2
year=2004
]

Halkieriids and other small shelly fossils are typically, although not always, preserved in phosphate, which may or may not have been their original mineral composition. Preservation by a covering of phosphate only seems to have been common during the early Cambrian, becoming rarer with time as a result of increased disturbance of sea-floors by burrowing animals. Hence it is possible that halkieriids and other small shelly fossils were alive earlier than the earliest known fossils and later than the latest known fossilsThe Rise and Fall of the Ediacaran Biota|Dzik, J|The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition|405|414|30|http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf] cite journal
title=Closing the Phosphatization Window: Testing for the Influence of Taphonomic Megabias on the Pattern of Small Shelly Fossil Decline
author=Porter, S.M. | journal=Palaios | volume=19 | issue=2 | date=April 2004
url=http://www.bioone.org/perlserv/?request=get-document&doi=10.1669%2F0883-1351(2004)019%3C0178%3ACTPWTF%3E2.0.CO%3B2
accessdate=2008-07-30
year=2004
pages=178
doi=10.1669/0883-1351(2004)019<0178:CTPWTF>2.0.CO;2
url=http://www.bioone.org/perlserv/?request=get-document
unused_data=|10.1669/0883-1351(2004)019<0178:CTPWTF>2.0.CO;2
] cite journal
author = Dzik, J. | year = 1994
title = Evolution of ‘small shelly fossils’ assemblages of the early Paleozoic
journal = Acta Palaeontologica Polonica | volume = 39 | issue = 3 | pages = 27–313
url = http://www.paleo.pan.pl/people/Dzik/Dzik1994d.htm | accessdate=2008-08-01
] — paleontologists call this kind of uncertainty the Signor-Lipps effect. [ citation
author=Signor III, P. W. and Lipps, J. H.| date=1982
contribution=Sampling bias, gradual extinction patterns, and catastrophes in the fossil record
title=Geological implications of impacts of large asteroids and comets on the Earth
editor=Silver, L. T., and Schultz, P. H.
titile=Geological Society of America Special Publication | volume=190 | pages=291–296
]

Description of the fossils

Note that the smallest increments on the scale in the image are millimeters.

Features shared by "Halkieria" and "Australohalkieria"

Only armor-like sclerites of "Australohalkieria" have been found, and much of the analysis assumes that these animals were similar to "Halkieria". However the sclerites are so similar that this assumption looks fairly safe. In both genera the sclerites are of the type called "coelosclerites", which have a mineralized shell around a space originally filled with organic tissue, and which show no evidence of growth by adding material round the outside.citation | contribution=Early skeletal fossils
author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M. | title=Neoproterozoic- Cambrian Biological Revolutions
year = 2004
journal = Palentological Society Papers
volume = 10
pages = 67–78
url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf | accessdate=2008-07-18
] Both genera also have sclerites of three different shapes: "palmate", flat and shaped rather like a maple leaf, which are generally the smallest; "cultrate", flat but shaped like knife blades; and "siculate", which are about the same size as the cultrates but are spine-shaped and like rather squashed cylinders. In both "Halkieria" and "Australohalkieria" the palmate and cultrate sclerites have prominent ribs, and are fairly flat except for 90° bends at the bases, which indicate that they fitted snugly against the animals' bodies. The siculates mostly lack ribs and appear to have projected away from the body at angles between about 45° and 90°.

"Halkieria evangelista"

The animals looked like slugs in chain mail - convert|1.5|cm|in to convert|8|cm|in long, bilaterally symmetric, flattened from top to bottom and unarmored on the bottom. Very near each end there is a shell plate with prominent growth lines rather like the growth rings of trees. The rest of the upper surface was covered with sclerites that overlappped each other like tiles and formed three zones with sclerites of different shapes: "palmates", shaped rather like maple leaves, ran along the center of the back between the shell plates; blade-shaped "cultrates" lay on either side of the palmates and pointing towards the middle of the upper surface; and slim, sickle-shaped "siculates" covered the outer edges. As the animals grew, the shell plates grew by adding material to the outer edges. cite journal
title=Articulated Halkieriids from the Lower Cambrian of North Greenland and their Role in Early Protostome Evolution
author=Conway Morris, S., and Peel, J. S.
journal=Philosophical Transactions of the Royal Society: Biological Sciences | volume=347
issue=1321 | pages=305–358 | doi=10.1098/rstb.1995.0029
url=http://journals.royalsociety.org/content/32l541667hj3071k/ | accessdate=2008-07-31
year=1995
] Individual sclerites stayed the same size; since the cultrate sclerites form a pattern that is constant in all fairly complete specimens, the old ones that were too small may have been shed and replaced by larger ones as the animals grew. There are traces of thin ribs between the sclerites and the skin.

The shellplates and the sclerites were probably made of calcium carbonate originally; it has been suggested on the basis of how they were preserved that they may have been wholly organic, but this is less likely since fossils of non-calcified organisms are usually thin films while "Halkeieria" fossils are three-dimensional like those of trilobites and hyoliths - in fact several specimens show curvature in the horizontal plane, which suggests that the muscles associated with the sclerites were still present at thetime of burial cite journal
title=The Early Cambrian "Halkieria" is a mollusc | author=Vinther, J., and Nielsen, C.
journal=Zoologica Scripta | volume=34 | issue=1 | pages=81−89 | date=2005
url=http://www.jakobvinther.com/vintherandnielsen2005 | accessdate=2008-07-31
doi=10.1111/j.1463-6409.2005.00177.x
]

The sole was soft and probably muscular. Since "Halkieria" was unsuited to swimmimg and had no obvious adaptations for burrowing, it must have lived on the sea-floor, "walking" by making its muscular sole ripple. The backward-projecting siculate sclerites may have improved its grip by preventing it from slipping backwards. Some specimens have been found partially rolled up, rather like a pillbug, and in this position the cultrate sclerites projected outwards, which probably deterred predators. It is difficult to determine the functions of the cap-shaped shells at either end of the animal, as the sclerites appear to have offered adequate protection. Scars on the inner surface of the front shell may indicate that it prodived an attachment for internal organs. In one specimen the rear shell appears to have rotated by about 45° before fossilization, which suggests there was a cavity underneath, which may have housed gills.

Traces of a gut have been found in the rear halves of some fossils. Parts of one specimen have been interpreted as a radula, the toothed chitinous tongue that is the signature feature of molluscs, but in this specimen the edge of the "scleritome", i.e. coat of sclerites, is folded and the putative radula could be a group of dislocated siculate sclerites.

Other "Halkieria" species

The sclerites of "Halkieria mira" have multiple lateral canals. cite journal
author=Conway Morris, S., and Chapman, A.J | date=1997
title=Lower Cambrian halkieriids and other coeloscleritophorans from Aksu-Wushi, Xinjiang, China
journal=Journal of Paleontology | volume=71 | pages=6–22
cited by cite journal
author=Porter, S.M. | date=May 2004 | accessdate=2008-08-01
url=http://findarticles.com/p/articles/mi_qa3790/is_200405/ai_n9377598/pg_1?tag=artBody;col1
title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia
journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590
doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2
year=2004
]

anchor|Australohalkieria spp"Australohalkieria superstes"

The name of the most complete and abundant Australian find means "Southern Halkieria the Survivor", because it proves that halkieriids survived the end-Botomian extinction. The sclerites assigned to this species are convex on the upper surface and concave on the lower. They may also curve within their own plane, and they overlap so that the concave side of each is partly covered by the convex side of the next one. Unlike those of "Halkieria", sclerites from the genus "Australohalkieria" have a pair of longitudinal internal canals as well as the central canal. Their walls also have a different microscopic structure.

In "A. superstes" the central canals of sclerites are flattened on their upper surfaces, and this produces a depression on the upper surface of the tip. The surface of this depression is not mineralized, which suggests the depression may have helped the animals' sense of smell by letting chemicals in the water penetrate the exposed skin. The phosphatic coating on sclerites of "A. superstes" has features that suggest they were originally covered by a thin organic skin. An outer organic layer has also been found on sclerites of the chancelloriids, sessile organisms that are thought to have looked rather like cacti. If halkieriids were early molluscs, the outer layers of the sclerites may have been similar to the periostracum of some modern molluscs.

The sclerites of "A. superstes" have right- and left-handed variants which are equally abundant, which suggests that "A. superstes" was bilaterally symmetrical. All of the sclerites were tiny: the palmates ones ranged from convert|250|µm|in to convert|650|µm|in in length, and the cultrates from convert|300|µm|in to convert|1000|µm|in. The siculates fall into two groups: those with a shallow S-curve at the base, which range from convert|400|µm|in to convert|1000|µm|in in length, and often have a slight twist at the base; and those with a 45° and 90° bend at the base and are convert|400|µm|in to convert|500|µm|in long.

Scleritomes of Early Cambrian halkieriids have many more palmate and cultrate than siculate sclerites. On the other hand, siculate sclerites of "A. superstes" are more abundant than either cultrate or palmate sclerites; in fact palmate sclerites are rare. Possibly some process after death removed many of the palmates and some of the cultrates, but it is more likely that in "A. superstes" the part of the scleritome, or "coat of mail", closest to the sea-bed was larger relative to the lateral and dorsal zones further up and towards the center. "A. superstes" sclerites are also about one-third the size of Early Cambrian halkieriid sclerites. Since the Georgina assemblage includes larger fossils and most Early Cambrian halkieriids are preserved by the same method, phosphatization, it is unlikely that preservational bias has produced an unrepresentative sample. Possible explanations for the small size of "A. superstes" sclerites include: the individual(s) represented in the Georgina assemblage were juveniles; their scleritomes were composed of many more sclerites than those of Early Cambrian halkieriids; or the species itself was relatively small.

No shells that might be assigned to halkieriids have been found in the Georgina Basin. This does not prove that "Australohalkieria" lacked shells, as shells of "Halkieria" are rarely found.

"Australohalkieria parva"

This species, whose name means "Small Southern Halkieria", was first described in 1990. [ cite journal
author=Bengtson, S., Conway Morris, S., Cooper, B.J., Jell, P.A., and Runnegar, B.N. | date=1990
title=Early Cambrian fossils from South Australia
journal=Memoirs of the Association of Australian Palaeontologists | volume=9 | pages=1–364
cited by cite journal
author=Porter, S.M. | date=May 2004 | accessdate=2008-08-01
url=http://findarticles.com/p/articles/mi_qa3790/is_200405/ai_n9377598/pg_1?tag=artBody;col1
title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia
journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590
doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2
year=2004
] Like "A. superstes", its sclerites have undivided longitudinal canals and a very similar structure to their walls wall, but "A. parva" has sclerites whose central canals are not flattened.

Other halkieriid fossils from Australia

The other sclerites from the Georgina Basin are different enough to be excluded from "Australohalkieria superstes", but are not sufficiently abundant to provide enough detail for them to be classified. One type is very similar to those of "A.superstes", even having a two-pronged tip, but the middle canal is not flattened. The other has a flattened central canal and no longitudinal canals, and may represent an additional Middle Cambrian halkieriid genus, distinct from "Australohalkieria" and from the Early Cambrian "Halkieria".

Other genera

"Thambetolepis", found in Australia in 1981, may be the same as "Sinosachites", found in China in 1980 — if so both finds would be classified as "Sinosachites" since that was described first. [ cite journal
title=Early Cambrian Small Shelly Fossil "Sinosachites" from Southwest China | author=Yue, Z.
journal=Acta Palaeontologica Sinica | date=2004
url=http://translate.google.co.uk/translate?hl=en&sl=zh-CN&u=http://scholar.ilib.cn/Abstract.aspx%3FA%3Dgswxb200402002&sa=X&oi=translate&resnum=8&ct=result&prev=/search%3Fq%3Dthambetolepis%26hl%3Den%26sa%3DG
accessdate=2008-08-06
(original is in Chinese)
]

"Oikozetetes" is known only from two types of cap-shaped shell found in the Burgess Shale and dated to about ma|505. The two types are thought to be the front and rear shells of a halkieriid. cite journal
author=Conway Morris, S. | date=1994
title=Enigmatic shells, possibly halkieriid, from the Middle Cambrian Burgess Shale, BritishColumbia
journal=Neues Jahrbuch Geologie Paläontologie | volune=195 | pages=319–331
]

Phylogenetic position of "Halkieria"

The evolutionary relationships of the halkieriids are a complex topic which is still being debated. Most of this debate is about their relationship to "Wiwaxia" and to the three major lophotrochozoan phyla — molluscs, annelids and brachiopods. The question of their relationship to an apparently much more primitive Cambrian group, the chancelloriids is also significant and may raise some difficult questions.

Relationship to Molluscs, Annelids and Brachiopods

clade
1=clade
1=Siphogonotuchida
2=clade
1=MOLLUSCA
(crown group)
2=clade
1="Siberian halkieriid"
2=clade
1=clade
1=clade
1=clade
1=ANNELIDA
(crown group)
2="Canadia"

2="Wiwaxia"

2="Thambetolepis"
(halkieriid)

2=clade
1="Halkieria evangelista"
2=BRACHIOPODA
(crown group)
Cladogram: Conway Morris & Peel (1995)
In 1995 Conway Morris and Peel presented a cladogram based both on the fossils' features and on early 1990s research in molecular phylogeny, which is the application of cladistic analysis to DNA and RNA:
*The siphogonotuchids, a group found in Earliest Cambrian rocks, were the "sister" group to all the rest. These are known only from isolated fragments. cite journal
author=Conway Morris, S., and Caron, J.-B.,
url=http://www.sciencemag.org/cgi/content/abstract/315/5816/1255 | accessdate=2008-08-07
title=Halwaxiids and the Early Evolution of the Lophotrochozoans | journal=Science
date=March 2007 | volume=315 | issue=5816 | pages=1255-1258 | doi=10.1126/science.1137187
]
*The earliest halkieriids were a "sister" group to the molluscs, in other words descendants of a fairly closely-related commmon ancestor. This relationship, they said, was supported by the muscular foot that most researchers assumed halkieriids had.
*Another halkieriid genus, "Thambetolepis" / "Sinosachites", was a "great aunt" of annelids and "Wiwaxia" was an "aunt" of annelids. Their claim of a close relationship between halkieriids and "Wiwaxia" was based on both groups' having sclerites divided into three concentric zones. The close relationship of "Wiwaxia" to annelids was based on the similarities Butterfield (1990) found between "Wiwaxia"’s sclerites and the bristles of polychaete annelids. "Canadia" is a Burgess Shale fossil that is widely agreed to be 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
]
*"Halkieria evangelista", which Conway Morris had found in Greenland's Sirius Passet lagerstätte, was a "sister" group" to brachiopods, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore. Brachiopods have bristles that are similar to those of annelids and hence to "Wiwaxia"’s sclerites, and hence to halkieriid sclerites.

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
]

clade
1=clade
1=ANNELIDA
2=clade
1=? "Kimberella"
2=clade
1="Odontogriphus"
2=clade
1=clade
1=clade
1="Wiwaxia"
2=clade
1=Halkieriids
2=MOLLUSCA
(crown group)
Cladogram: Caron, Scheltema, "et al" (2006)
Caron, Scheltema, Schander and Rudkin (2006) also interpreted "Halkieria" as a crown group mollusc, with "Wiwaxia" and "Odontogriphus" as stem group molluscs, in other words "sister" and "aunt" of the crown group molluscs. Their main reason for regarding "Halkieria" as crown group molluscs is that both possessed armor mineralized with calcium carbonate. They treated "Wiwaxia" and "Odontogriphus" as stem group molluscs because in their opinion both possessed the distinctive molluscan radula, a chitonous toothed "tongue".cite journal
author = Caron, J.B. | coauthors = Scheltema, A., Schander, C., and Rudkin, D. | year = 2006 | date=2006-07-13
title = A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale
journal = Nature | volume = 442 | issue = 7099 | pages = 159–163 | issn = | doi = 10.1038/nature04894
url = http://www.nature.com/nature/journal/v442/n7099/pdf/nature04894.pdf
accessdate = 2008-08-07
]

Also in 2006, Conway Morris criticized Vinther and Nielsen's (2005) 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
]

Butterfield (2006) accepted that "Wiwaxia" and "Odontogriphus" were closely related, but argued that they were stem-group polychaetes rather than stem-group molluscs. In his opinion the feeding apparatus of these organisms, with consisted of two or at most four rows of teeth, could not perform perform the functions of the "belt-like" molluscan radula with their numerous tooh-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.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?CRETRY=1&SRETRY=0
accessdate=2008-08-06
]

Conway Morris and Caron (2007) published the first description of "Orthrozanclus reburrus". This resembled the halkieriids in having concentric bands of sclerites, although only two and not mineralized; and "one" shell at what was presumed to be the front and which was similar in shape to "Halkieria"’s front shell. It also had long spines rather like those of "Wiwaxia". Conway Morris and Caron regarded this creature as evidence that the "halwaxiids" were a valid taxon and were monophyletic, in other words shared a common ancestor with each other and with no other organism. They published "two" cladograms, representing alternative hypotheses about the evolution of the lophotrochozoa, the lineage that includes molluscs, annelids and brachiopods:

Two alternative cladograms: Conway Morris and Caron (2007)

# This is the more likely, although it falls apart if the organisms' characteristics are changed even slightly:
#*"Kimberella" and "Odontogriphus" are early, primitive molluscs, without sclerites or any kind of mineralized armor.
#*"Wiwaxia", the siphogonotuchids, "Orthrozanclus" and "Halkeria" from a side-branch of the mollusc family tree, which diverged 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 the scleritome was simpler; halkieriids then develop more complex scleritomes, while 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 becme unmineralized and in "Wiwaxia" the shells were lost.

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, they 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
autohr=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) opposed this view, arguing that: spongin does not appear in all Porifera, but may be a defining feature of the demosponges; the silica-based spines of demosponges are solid, while chancellorid sclerites are hollow and filled with soft tissues connected to the rest of the animal at the bases of the sclerites; chancellorid sclerites were probably made of aragonite, which is not found in demosponges; sponges have loosely bound-together skins called pinacoderms, which are only one cell thick, while the skins of chancellorids were much thicker and shows signs of connective structures called belt desmosomes. In their opinion the presence of belt desmosomes made chancellorids members of the Epitheliazoa, the next higher taxon above the Porifera, to which sponges belong. They thought it was difficult to say whether chancellorids were members of the Eumetazoa, "true animals" whose tissues are organized into Germ layers: chancellorids' lack of internal organs would seem to exclude them from the Eumetazoa; but possibly chancellorids descended from Eumetazoans that lost these features after becoming sessile filter-feeders. 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 ]

The coelosclerites ("hollow 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, such as echinoderms, priapulids, and kinorhynchs.
* On the other hand perhaps chancelloriids are similar to the organisms from which bilaterians evolved. That would imply that the earliest bilaterians had similar coelosclerites. 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. In fact there are over 25 cases of phosphatic preservation between ma|542 and ma|521, but only one between ma|555 and ma|542.
* 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
]

ee also

* Coeloscleritophoran

Notes

External links

*Palaeos' article on "Halkieria" [http://www.palaeos.org/Halkieria] & "H. evangelista" [http://www.palaeos.org/Halkieria_evangelista]
*Pharyngula [http://scienceblogs.com/pharyngula/2007/03/orthozanclus.php entry] on "Orthrozanclus reburrus"


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