Halkieriid
| Halkieria Temporal range:
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|---|---|
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| Halkieria evangelistafrom the Lower Cambrian Sirius Passet, North Greenland | |
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| Life restoration | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| (unranked): | Spiralia |
| Superphylum: | Lophotrochozoa |
| Phylum: | Mollusca |
| Family: | †Halkieriidae Poulsen, 1967 |
| Genus: | †Halkieria Poulsen, 1967 |
| Type species | |
| Halkieria obliqua Poulsen, 1967[1]
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| Species | |
| Part of a series on |
| TheCambrian explosion |
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Thehalkieriidsare a group of fossil organisms from the Lower to MiddleCambrian.Their eponymous genus isHalkieria/hælˈkɪəriə/,which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of thesmall shelly fossilassemblages. The best known species isHalkieria evangelista,from the North GreenlandSirius PassetLagerstätte,in which complete specimens were collected on an expedition in 1989. The fossils were described bySimon Conway Morrisand John Peel in a short paper in 1990 in the journalNature.Later a more thorough description was undertaken in 1995 in the journalPhilosophical Transactions of the Royal Society of Londonand wider evolutionary implications were posed.
The group is sometimes equated to Sachitida, although as originally envisaged, this group includes the wiwaxiids[2]and is thus equivalent to theHalwaxiida.
History of discovery
[edit]Armor plates called "sclerites"had long been known as elements of thesmall shelly fossils,and detailed analysis showed that some of these belonged to the same animal and how they fitted together. The first articulated specimens ofHalkieria evangelista,with all their hard parts together, were collected in 1989 from theSirius PassetlagerstätteinGreenland,and were described in 1990 bySimon Conway Morrisand John S. Peel.[3]H. evangelistais used as a model for identifying and reconstructing as halkieriids other similar shells and sclerites;[4][5]its epithetevangelistareflects its power to explain the Lower Cambrian fossil record.[6]
Description of the fossils
[edit]Features shared byHalkieriaandAustralohalkieria
[edit]
Only armor-likescleritesofAustralohalkieriahave been found, and much of the analysis assumes that these animals were similar toHalkieria.However the sclerites are so similar that this assumption looks fairly safe.[4]In bothgenerathe sclerites are of the type called "coelosclerites",[4]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.[7]Both genera also have sclerites of three different shapes: "palmate", flat and shaped rather like amapleleaf, 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 bothHalkieriaandAustralohalkieriathe 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°.[4]
Halkieria evangelista
[edit]
The animals looked likeslugsinchain mail- 1.5 centimetres (0.59 in) to 8 centimetres (3.1 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 thegrowth ringsof trees. The rest of the upper surface was covered with about 2,000scleritesthat overlapped each other like tiles and formed three zones with sclerites of different shapes:[8]"palmates", shaped rather likemapleleaves, 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. The sclerites bore a wide central cavity, and (at least in some specimens) finer lateral canals.[9]As the animals grew, the shell plates grew by adding material to the outer edges.[6]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. The sclerites seem to have grown by basal secretion.[9]There are traces of thin ribs between the sclerites and the skin.[10]
The shellplates and the sclerites were probably made ofcalcium carbonateoriginally;[6]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 whileHalkeieriafossils are three-dimensional like those oftrilobitesandhyoliths- in fact several specimens show curvature in the horizontal plane, which suggests that the muscles associated with the sclerites were still present at the time of burial[10]
The sole was soft and probably muscular. SinceHalkieriawas unsuited to swimming 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 apillbug,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 provided 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.[6]
Traces of a gut have been found in the rear halves of some fossils.[10]Parts of one specimen have been interpreted as aradula,[6]the toothedchitinoustongue that is the signature feature ofmolluscs,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.[10]
Australohalkieria superstes
[edit]The name of the most complete and abundant Australian find means "Southern Halkieria the Survivor" because it proves that halkieriids survived the end-Botomianextinction. 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. The internal cavity withinAustralohalkieriais more complicated than the simple tube inHalkieria;about half-way up the sclerite, the cylindrical tube splits into a pair of longitudinal canals, with the central canal flattening; the canals don't seem to be connected. The walls also have a different microscopic structure.[4]
InA. superstesthe 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 ofsmellby letting chemicals in the water penetrate the exposed skin. The phosphatic coating on sclerites ofA. supersteshas features that suggest they were originally covered by a thin organic skin. An outer organic layer has also been found on sclerites of thechancelloriids,sessile organisms that are thought to have looked rather likecacti.If halkieriids were earlymolluscs,the outer layers of the sclerites may have been similar to theperiostracumof some modern molluscs.[4]
The sclerites ofA. supersteshave right- and left-handed variants which are equally abundant, which suggests thatA. supersteswasbilaterally symmetrical.All of the sclerites were tiny: the palmate ones ranged from 250 micrometres (0.0098 in) to 650 micrometres (0.026 in) in length, and the cultrates from 300 micrometres (0.012 in) to 1,000 micrometres (0.039 in). The siculates fall into two groups: those with a shallow S-curve at the base, which range from 400 micrometres (0.016 in) to 1,000 micrometres (0.039 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 400 micrometres (0.016 in) to 500 micrometres (0.020 in) long.[4]
Scleritomes of Early Cambrian halkieriids have many more palmate and cultrate than siculate sclerites. On the other hand, siculate sclerites ofA. superstesare 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 inA. superstesthe 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. superstessclerites 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 ofA. superstessclerites 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.[4]
No shells that might be assigned to halkieriids have been found in the Georgina Basin. This does not prove thatAustralohalkierialacked shells, as shells ofHalkieriaare rarely found.[4]
Australohalkieria parva
[edit]This species, whose name means "Small Southern Halkieria", was first described in 1990.[11]LikeA. superstes,its sclerites have undivided longitudinal canals and a very similar structure to their walls wall, butA. parvahas sclerites whose central canals are not flattened.[4]
Other halkieriid fossils from Australia
[edit]The other sclerites from the Georgina Basin are different enough to be excluded fromAustralohalkieria superstes,but are not sufficiently abundant to provide enough detail for them to be classified. One type is very similar to those ofA.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 fromAustralohalkieriaand from the Early CambrianHalkieria.[4]
Siphogonuchitids
[edit]Siphogonuchitids have two sclerite morphs as well as their shell(s), thus may have had a simpler scleritome thanHalkieriaand its ilk, concordant with the sclerites' simpler internal anatomy.[12]
The generaSiphogonuchites, Dabashanites, Lopochites,andMaikhanellaall seem to represent components of theSiphogonuchitesanimal.[12] Sclerites ofDrepanochitescan be distinguished based on their aspect ratio.[12]
Maikhanellais shell formed ofSiphogonuchitessclerites that are fused together with a calcified matrix. Juvenile shells appear not to incorporate sclerites.[13] The central cavity of theSiphogonuchitessclerite is simple, with no lateral chambers attached.[9]
Ninellids
[edit]The ninellids, typified byNinella,are a Lower Cambrian group that had an even simpler scleritome, with only one sclerite type (although variation in the morphology of the sclerites is observed, and left- and right-sided sclerites exist). Their sclerites are hooked or scoop-like, and are very similar to halkieriid or siphonogunuchitid sclerites; they were hollow and calcareous and had a ridged upper surface.[12]
Hippopharangites
[edit]Hippopharangites[14]has sclerites with a broad central cavity and small pores opening through the shell wall, equivalent to the lateral chambers of other halkieriids (and the aesthete canals of Chitons?)[9] This genus is the closest in form toChancelloriidsclerites, and is thus used to support the union of halkieriids and chancelloriids as Coeloscleritophora.[12]
Lomasulcachites
[edit]Lomasulcachitesis a further genus known from sclerites alone.[12]
Sachites
[edit]SachitesMeshkova 1969 is a genus that comprised spiny sclerites; manySachitesspecimens are now referred to other halkieriid taxa.[15]
Although believed to be related to the halkieriids,[16]achancelloriidaffinity has more recently been proposed.[17]
Sinosachites
[edit]
| Sinosachites Temporal range:
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| Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Order: | †Chancelloriida |
| Family: | †Sachitidae |
| Genus: | †Sinosachites |
| Species: | †S. delicata
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| Binomial name | |
| †Sinosachites delicata Jell, 1981[16]
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| Synonyms | |
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(Genus)
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Sinosachitesis a genus of 'halkieriid' known only from sclerites; these have internal chambers that are sub-perpendicular to the central canal, to which they are connected by narrow channels.[9][16]The chambers are the same diameter, ~40 μm, as the longitudinal canals inAustralohalkieria;their greater number and arrangement as lateral rather than longitudinal bodies reflects the greater size of theSinosachitessclerites, which measure about 1–2 mm in length.[9]
The sclerites are synonymous withThambetolepis,which was originally described from Australia. Left-hand and right-hand sclerites exist, so the animal was bilaterally symmetrical; as inHalkieria,palmate, cultrate and siculate sclerite morphologies exist.[9]
Oikozetetes
[edit]Oikozetetes[18]is known only from two types of cap-shaped shell found in theBurgess Shaleand dated to about505million years ago.The two types are thought to be the front and rear shells of a halkieriid.[5]
| Oikozetetes Temporal range:
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| Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | incertae sedis |
| Genus: | †Oikozetetes |
| Species: | †O. seilacheri
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| Binomial name | |
| †Oikozetetes seilacheri Conway Morris 1995[20]
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They were probably calcareous while the organism was alive (although diagenesis sometimes replaces the original mineral with another, such as silica).[19]It is thought to also have borne an armour coat consisting of biomineralised sclerites, likeHalkieria.These are never found in direct association with the shells, but there are manybiostratinomicprocesses which could account for this fact.[19]
The lower Cambrian taxonOcruranus(=Eohalobia) is putatively equivalent to the shells ofOikozetetes[19]and seemingly belonged to a halkieriid-type body,[21]although an intermediate valve suggests a Palaeoloricate-like body form.[22]
Occurrence
[edit]Halkieriids in context | ||||||||||||||||||||||||
−560 — – −555 — – −550 — – −545 — – −540 — – −535 — – −530 — – −525 — – −520 — – −515 — – −510 — – −505 — | Middle Cambrian |
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Axis scale: millions of years ago.
References for dates: To be completed | ||||||||||||||||||||||||
The only reasonably complete specimens, ofHalkieria evangelista,were found in theSirius PassetlagerstätteinGreenland.[3]Fragments which are confidently classified as belonging to halkieriids have been found inChina'sXin gian gprovince[23]andAustralia's Georgina Basin,[4]while shells of a possible halkieriid have been found inCanada'sBurgess Shale.[5]Halkieriid-like armor plates, called "sclerites"have been found in many other places as part of thesmall shelly fauna.[7]
The earliest known occurrences of Halkieriidssclerites,classified asHalkieria longa,date from thePurella antiqua Zoneof the Upper Nemakit-Daldynian Stage in Siberia.[24]Themass extinctionat the end of the Cambrian period'sBotomianage was thought to have wiped out most of the small shellies, including the halkieriids, but in 2004 Halkieriid fossils classified asAustralohalkieriawere reported from Mid-Cambrian rocks of the Georgina Basin inAustralia.It is not known why thiscladewould have survived while other halkieriid clades apparently died.[4]It may be significant that the onlyarchaeocyathansknown to have survived the end-Botomian extinction also occur inGondwana,the old super-continent that embracedSouth America,Africa,India,AustraliaandAntarctica.[25][26][4]
Halkieriids and other small shelly fossils are typically, although not always, preserved inphosphate,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 increaseddisturbance 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 fossils[27][28][29]—paleontologistscall this kind of uncertainty theSignor–Lipps effect.[30]
Species
[edit]Nearly all members of the generaHalkieriaare known only from finds of isolated scaly sclerites:
- Halkieria alataDuan, 1984
- Halkieria amorphaMeshkova,1974
- Halkieria bisulcataQian et Yin, 1984
- Halkieria costulataMeshkova, 1974
- Halkieria curvativaMambetov in Missarzhevsky and Mambetov, 1981
- Halkieria deplanatiformisMambetov in Missarzhevsky and Mambetov, 1981
- Halkieria desquamataDuan, 1984
- Halkieria directaMostler, 1980
- Halkieria elongaQian et Yin, 1984
- Halkieria equilateralisQian et Yin, 1984
- Halkieria folliformisDuan, 1984
- Halkieria fordiLanding, 1991
- Halkieria hexagonaMostler, 1980
- Halkieria lataMostler, 1980
- Halkieria longaQian, 1977
- Halkieria longispinosaMostler, 1980
- Halkieria maidipingensisQian, 1977
- Halkieria minaQian, Chen et Chen, 1979
- Halkieria miraQian et Xiao, 1984
- Halkieria obliquaPoulsen, 1967
- Halkieria operculusQian, 1984
- ?Halkieria pennataHe, 1981[=?Halkieria sthenobasisJiang in Luo et al., 1982]
- Halkieria phylloideaHe, 1981
- Halkieria praeinguisJiang in Luo et al., 1982
- Halkieria projectaBokova, 1985
- Halkieria sacciformisMeshkova, 1969
- Halkieria solidaMostler, 1980
- Halkieria sthenobasisJiang in Luo et al., 1982
- Halkieria stoneiLanding, 1989
- Halkieria symmetricaPoulsen, 1967
- Halkieria terastiosQian, Chen et Chen, 1979
- Halkieria uncostataQian et Yin, 1984
- Halkieria undulataWang, 1994
- Halkieria ventricosaMostler, 1980
- Halkieria wangiDemidenko, 2010
- Halkieria zapfeiMostler, 1980[31]
At present, the structure of complete scleritome is known only for the single species namedHalkieria evangelistafrom the Lower Cambrian of Greenland (Sirius Passet Formation).[6]
Phylogenetic position ofHalkieria
[edit]The evolutionary relationships of the halkieriids are a complex topic which is still being debated. Most of this debate is about their relationship toWiwaxiaand to the three majorlophotrochozoanphyla—molluscs,annelidsandbrachiopods.The question of their relationship to an apparently much more primitiveCambriangroup, thechancelloriidsis also significant and may raise some difficult questions.
Relationship to Molluscs, Annelids and Brachiopods
[edit]
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In 1995 Conway Morris and Peel presented acladogrambased both on the fossils' features and on early 1990s research inmolecular phylogeny,which is the application of cladistic analysis toDNAandRNA:[6]
- Thesiphogonotuchids,a group found in Earliest Cambrian rocks, were the "sister" group to all the rest.[6]These are known only from isolated fragments.[32]
- The earliest halkieriids were a "sister" group to the molluscs, in other words descendants of a fairly closely related common ancestor. This relationship, they said, was supported by the muscular foot that most researchers assumed halkieriids had.[6]
- Another halkieriidgenus,Thambetolepis/Sinosachites,was a "great aunt" of annelids andWiwaxiawas an "aunt" of annelids. Their claim of a close relationship between halkieriids andWiwaxiawas based on both groups' having sclerites divided into threeconcentriczones. The close relationship ofWiwaxiato annelids was based on the similarities Butterfield (1990) found betweenWiwaxia'ssclerites and the bristles ofpolychaeteannelids.Canadiais aBurgess Shalefossil that is widely agreed to be a polychaete.[6][33]
- Halkieria evangelista,which Conway Morris had found in Greenland'sSirius Passetlagerstätte,was a "sister" group "tobrachiopods,animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, thelophophore.Brachiopods have bristles that are similar to those of annelids and hence toWiwaxia'ssclerites, and hence to halkieriid sclerites.[6]A brachiopod affinity seemed plausible because brachiopods pass through a larval phase that resembles a halkieriid, and some isolated fossil shells thought to belong to halkieriids had a brachiopod-like microstructure.[34]
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.[35]
Vinther and Nielsen (2005) proposed instead thatHalkieriawas acrown groupmollusc, in other words more similar to modern molluscs that to annelids, brachiopods or any intermediate groups. They argued that:Halkieria'ssclerites resembled those of the modernsolenogasteraplacophoranshell-less molluscs (seeScheltema, A. H.; Ivanov, D. L. (2002). "An aplacophoran postlarva with iterated dorsal groups of spicules and skeletal similarities to Paleozoic fossils".Invertebrate Biology.121:1–10.doi:10.1111/j.1744-7410.2002.tb00124.x.), of some modernpolyplacophoranmolluscs, which have several shell plates, and of theOrdovicianpolyplacophoranEchinochiton;Halkieria'sshells are more similar to the shells ofconchiferanmolluscs, 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 toHalkieria'ssclerites.[36]
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Caron, Scheltema,Schanderand Rudkin (2006) also interpretedHalkieriaas acrown groupmollusc, withWiwaxiaandOdontogriphusas stem group molluscs,[38]in other words "sister" and "aunt" of the crown group molluscs. Their main reason for regardingHalkieriaas crown group molluscs is that both possessed armor mineralized withcalcium carbonate.They treatedWiwaxiaandOdontogriphusas stem group molluscs because in their opinion both possessed the distinctive molluscanradula,achitinoustoothed "tongue".[37]
Also in 2006, Conway Morris criticized Vinther and Nielsen's (2005) classification ofHalkieriaas 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.[39]
Butterfield (2006) accepted thatWiwaxiaandOdontogriphuswere closely related, but argued that they were stem-group polychaetes rather than stem-group molluscs. In his opinion the feeding apparatus of these organisms, which consisted of two or at most four rows of teeth, could not perform the functions of the "belt-like" molluscan radula with their numerous tooth-rows; the different tooth-rows in bothWiwaxiaandOdontogriphustooth-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 ofOdontogriphusfossils as evidence of external segmentation, since the lines are evenly spaced and run exactly atright anglesto the long axis of the body. As in his earlier papers, Butterfield emphasized the similarities of internal structure betweenWiwaxia'ssclerites and the bristles of polychaetes, and the fact that polychaetes are the only modern organisms in which some of the bristles form a covering over the back.[40]
Conway Morris and Caron (2007) published the first description ofOrthrozanclusreburrus.This resembled the halkieriids in having concentric bands of sclerites, although only two and not mineralized; andoneshell at what was presumed to be the front and which was similar in shape toHalkieria'sfront shell. It also had long spines rather like those ofWiwaxia.Conway Morris and Caron regarded this creature as evidence that the "halwaxiids" were a validtaxonand weremonophyletic,in other words shared a common ancestor with each other and with no other organism. They publishedtwocladograms, representing alternative hypotheses about the evolution of thelophotrochozoa,the lineage that includes molluscs, annelids and brachiopods:[32]
- This is the more likely, although it falls apart if the organisms' characteristics are changed even slightly:[32]
- KimberellaandOdontogriphusare early, primitive molluscs, without sclerites or any kind of mineralized armor.
- Wiwaxia,the siphogonotuchids,OrthrozanclusandHalkieriafrom a side-branch of the mollusc family tree, which diverged in that order. This would mean that:Wiwaxiawas 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 inOrthrozanclusthe 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.[32]
- The annelids and brachiopods evolved from the other main branch of the family tree, which did not include the molluscs.
- The alternative view is:
- KimberellaandOdontogriphusare early, primitive lophotrochozoans.
- The siphogonotuchids,Halkieria,OrthrozanclusandWiwaxiaform 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, withtwotypes of mineralized sclerites and a "shell" made of fused sclerites. Halkieriids had three types of sclerites and two one-piece shells. InOrthrozanclusthe sclerites became unmineralized and inWiwaxiathe shells were lost.[32]
The network of internal cavities within sclerites of the halkieriidSinosachiteshave been likened to the aesthete canals in polyplacophora, strengthening the case for a molluscan affinity.[9]
Relationship to chancelloriids
[edit]Porter (2008) revived an early 1980s idea that the sclerites ofHalkieriaare extremely similar to those ofchancelloriids.These were sessile, bag-like,radially symmetricorganisms with an opening at the top.[41]
Since their fossils show no signs of a gut or other organs, they were originally classified as some kind ofsponge.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 ofspongin,acollagenprotein,in modern keratose (horny)demosponges.[42]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; thesilica-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 ofaragonite,which is not found in demosponges; sponges have loosely bound-together skins calledpinacoderms,which are only one cell thick, while the skins of chancellorids were much thicker and shows signs of connective structures called beltdesmosomes.In their opinion the presence of belt desmosomes made chancellorids members of theEpitheliazoa,the next highertaxonabove thePorifera,to which sponges belong. They thought it was difficult to say whether chancellorids were members of theEumetazoa,"true animals" whose tissues are organized intoGerm 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 becomingsessilefilter-feeders.[43]There are intriguing hints that the Ediacaran genusAusiamay represent a halkieriid ancestor with strong similarity to the chancelloriids.[44]
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 surface 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:[41]
- One possibility is that chancelloriids evolved frombilaterianancestors 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 asechinoderms,priapulids,andkinorhynchs.[41]
- 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 before542million years ago,whileKimberellafrom555million years agowas almost certainly a bilaterian,[45]but shows no evidence of sclerites.[41]
- One solution to thisdilemmamay be that preservation ofsmall shelly fossilsby coatings ofphosphatewas 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 between542million years agoand521million years ago,but only one between555million years agoand542million years ago.[41]
- 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.[41][46]
See also
[edit]Notes
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External links
[edit]- Palaeos' article onHalkieria[1]&H. evangelista[2]
- PharyngulaentryArchived2007-03-04 at theWayback MachineonOrthrozanclusreburrus