Halkieriid
| Halkieria Temporal range:
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|---|---|
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| Halkieria evangelista from the Lower Cambrian Sirius Passet, North Greenland | |
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 |
| The Cambrian explosion |
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The halkieriids are a group of fossil organisms from the Lower to Middle
The group is sometimes equated to Sachitida, although as originally envisaged, this group includes the wiwaxiids
History of discovery
Armor plates called "
Description of the fossils
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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.[4] In both genera the 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 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°.[4]
Halkieria evangelista
The animals looked like
The shellplates and the sclerites were probably made of
The sole was soft and probably muscular. Since Halkieria was 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 a
Traces of a gut have been found in the rear halves of some fossils.
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-
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
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 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 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,
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.[4]
Australohalkieria parva
This species, whose name means "Small Southern Halkieria", was first described in 1990.[11] 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.[4]
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.[4]
Siphogonuchitids
Siphogonuchitids have two sclerite morphs as well as their shell(s), thus may have had a simpler scleritome than Halkieria and its ilk, concordant with the sclerites' simpler internal anatomy.[12]
The genera Siphogonuchites, Dabashanites, Lopochites, and Maikhanella all seem to represent components of the Siphogonuchites animal.[12] Sclerites of Drepanochites can be distinguished based on their aspect ratio.[12]
Maikhanella is shell formed of Siphogonuchites sclerites that are fused together with a calcified matrix. Juvenile shells appear not to incorporate sclerites.[13] The central cavity of the Siphogonuchites sclerite is simple, with no lateral chambers attached.[9]
Ninellids
The ninellids, typified by Ninella, 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
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 to
Lomasulcachites
Lomasulcachites is a further genus known from sclerites alone.[12]
Sachites
Sachites Meshkova 1969 is a genus that comprised spiny sclerites; many Sachites specimens are now referred to other halkieriid taxa.[15]
Although believed to be related to the halkieriids,
Sinosachites
| Sinosachites Temporal range:
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|---|---|
| Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Order: | †Chancelloriida |
| Family: | † Sachitidae
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| 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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Sinosachites is 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 in Australohalkieria; their greater number and arrangement as lateral rather than longitudinal bodies reflects the greater size of the Sinosachites sclerites, which measure about 1–2 mm in length.[9]
The sclerites are synonymous with Thambetolepis, which was originally described from Australia. Left-hand and right-hand sclerites exist, so the animal was bilaterally symmetrical; as in Halkieria, palmate, cultrate and siculate sclerite morphologies exist.[9]
Oikozetetes
Oikozetetes[18] is known only from two types of cap-shaped shell found in the Burgess Shale and dated to about 505 million 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
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| 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).
The lower Cambrian taxon Ocruranus (=Eohalobia) is putatively equivalent to the shells of Oikozetetes[19] and seemingly belonged to a halkieriid-type body,[21] although an intermediate valve suggests a Palaeoloricate-like body form.[22]
Occurrence
Halkieriids in context | ||
−560 — – −555 — – −550 — – −545 — – −540 — – −535 — – −530 — – −525 — – −520 — – −515 — – −510 — – −505 — | Trilobites | |
| ← | First Arthropod trace fossils |
| ← | Claimed bilaterian trace fossils |
| ← | and Echinoderms |
| ← |
| ← | Halkieriids |
| ← | Cambrian explosion, if sudden |
|
(last Palæozoic (first era of the Phanerozoic ) |
References for dates:
To be completed
The only reasonably complete specimens, of Halkieria evangelista, were found in the Sirius Passet lagerstätte in Greenland.[3] Fragments which are confidently classified as belonging to halkieriids have been found in China's Xinjiang province[23] and Australia's Georgina Basin,[4] while shells of a possible halkieriid have been found in Canada's Burgess Shale.[5] Halkieriid-like armor plates, called "sclerites" have been found in many other places as part of the small shelly fauna.[7]
The earliest known occurrences of Halkieriids
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 fossils[27][28][29] — paleontologists call this kind of uncertainty the Signor–Lipps effect.[30]
Species
Nearly all members of the genera Halkieria are known only from finds of isolated scaly sclerites:
- Halkieria alata Duan, 1984
- Halkieria amorpha Meshkova,1974
- Halkieria bisulcata Qian et Yin, 1984
- Halkieria costulata Meshkova, 1974
- Halkieria curvativa Mambetov in Missarzhevsky and Mambetov, 1981
- Halkieria deplanatiformis Mambetov in Missarzhevsky and Mambetov, 1981
- Halkieria desquamata Duan, 1984
- Halkieria directa Mostler, 1980
- Halkieria elonga Qian et Yin, 1984
- Halkieria equilateralis Qian et Yin, 1984
- Halkieria folliformis Duan, 1984
- Halkieria fordi Landing, 1991
- Halkieria hexagona Mostler, 1980
- Halkieria lata Mostler, 1980
- Halkieria longa Qian, 1977
- Halkieria longispinosa Mostler, 1980
- Halkieria maidipingensis Qian, 1977
- Halkieria mina Qian, Chen et Chen, 1979
- Halkieria mira Qian et Xiao, 1984
- Halkieria obliqua Poulsen, 1967
- Halkieria operculus Qian, 1984
- ?Halkieria pennata He, 1981 [=?Halkieria sthenobasis Jiang in Luo et al., 1982]
- Halkieria phylloidea He, 1981
- Halkieria praeinguis Jiang in Luo et al., 1982
- Halkieria projecta Bokova, 1985
- Halkieria sacciformis Meshkova, 1969
- Halkieria solida Mostler, 1980
- Halkieria sthenobasis Jiang in Luo et al., 1982
- Halkieria stonei Landing, 1989
- Halkieria symmetrica Poulsen, 1967
- Halkieria terastios Qian, Chen et Chen, 1979
- Halkieria uncostata Qian et Yin, 1984
- Halkieria undulata Wang, 1994
- Halkieria ventricosa Mostler, 1980
- Halkieria wangi Demidenko, 2010
- Halkieria zapfei Mostler, 1980[31]
At present, the structure of complete scleritome is known only for the single species named Halkieria evangelista from the Lower Cambrian of Greenland (Sirius Passet Formation).[6]
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
Relationship to Molluscs, Annelids and Brachiopods
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In 1995 Conway Morris and Peel presented a
- The siphogonotuchids, 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 halkieriid
- Halkieria evangelista, which Conway Morris had found in Greenland's 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.[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 that Halkieria was a
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Caron, Scheltema,
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.[39]
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, 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 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 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 of
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- This is the more likely, although it falls apart if the organisms' characteristics are changed even slightly:[32]
- Kimberella and Odontogriphus are early, primitive molluscs, without sclerites or any kind of mineralized armor.
- Wiwaxia, the siphogonotuchids, Orthrozanclus and Halkieria 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.[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:
- Kimberella and Odontogriphus are early, primitive lophotrochozoans.
- The siphogonotuchids, Halkieria, 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.[32]
The network of internal cavities within sclerites of the halkieriid Sinosachites have been likened to the aesthete canals in polyplacophora, strengthening the case for a molluscan affinity.[9]
Relationship to chancelloriids
Porter (2008) revived an early 1980s idea that the sclerites of Halkieria are extremely similar to those of
Since their fossils show no signs of a gut or other organs, they were originally classified as some kind of
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 from kinorhynchs.[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 before 542 million years ago, while Kimberella from 555 million years ago was almost certainly a bilaterian,[45] but shows no evidence of sclerites.[41]
- One solution to this 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 542 million years ago and 521 million years ago, but only one between 555 million years ago and 542 million 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
Notes
- ^ Chr. Poulsen: Fossils from the Lower Cambrian of Bornholm. In: Det Kongelige Danske Videnskabernes Selskab – Matematisk-fysiske Meddelelser, Vol. 36, No. 2, 48 S. + 9 Tafeln, 1967.
- .
- ^ S2CID 4324153. A short but free account is given at "Showdown on the Burgess Shale". Archived from the originalon 2010-12-10. Retrieved 2008-07-31.
- ^ S2CID 131557288.
- ^ S2CID 132943124.
- ^ .
- ^ S2CID 3639924.
- ISBN 978-0-632-05149-6. Retrieved 12 Nov 2009.
- ^ S2CID 84163005.
- ^ S2CID 84493997. Archived from the originalon 2008-08-20. Retrieved 2008-07-31.
- S2CID 131557288.
- ^ S2CID 131199965.
- .
- S2CID 9885948.
- S2CID 130881046.
- ^ .
- S2CID 130024613.
- ^ "Oikozetetes seilacheri". Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011. Archived from the original on 2020-11-12. Retrieved 2023-01-21.
- ^ .
- S2CID 132943124.
- .
- .
- S2CID 131557288.
- ^ M.A. Semikhatov (2008). "The Upper Precambrian." In: "State of level of scrutiny of Precambrian and Phanerozoic stratigraphy of the Russia. The goals of the further studies." Decisions of the Interdepartmental Stratigraphical Committee and its constant Commissions 38. St.-Petersburg: VSEGEI. pp. 15-27. (in Russian)
- S2CID 131557288.
- S2CID 131557288.
- OCLC 156823511
- S2CID 33128487. Retrieved 2008-07-30.
- ^ Dzik, J. (1994). "Evolution of 'small shelly fossils' assemblages of the early Paleozoic". Acta Palaeontologica Polonica. 39 (3): 27–313. Retrieved 2008-08-01.
- ^ Signor III, P. W.; Lipps, J. H. (1982). "Geological implications of impacts of large asteroids and comets on the Earth; Sampling bias, gradual extinction patterns, and catastrophes in the fossil record". In Silver, L. T.; chultz, P. H. (eds.). Geological Society of America Special Publication. Vol. 190. pp. 291–296.
- S2CID 86632689.
- ^ S2CID 22556453. Retrieved 2008-08-07.
- S2CID 88100863.
- S2CID 4428441.
- . Retrieved 2008-08-07.
- S2CID 84493997. Archived from the originalon 2008-08-20. Retrieved 2008-08-07.
- ^ S2CID 4431853.
- ISBN 9780128143124.
- PMID 16754615.
- S2CID 29130876. Archived from the originalon 2011-08-13. Retrieved 2008-08-06.
- ^ . Retrieved 2008-08-07.
- S2CID 133427906.
- S2CID 129127213. Retrieved 2008-08-04.
- ISBN 978-0-9812885-1-2.
- S2CID 4395089.
- ^ Bengtson, S. "Mineralized skeletons and early animal evolution". In Briggs, D.E.G. (ed.). Evolving form and function: fossils and development. New Haven, CT: Peabody Museum of Natural History, Yale University. p. 288.
External links
- Palaeos' article on Halkieria [1] & H. evangelista [2]
- Pharyngula entry Archived 2007-03-04 at the Wayback Machine on Orthrozanclus reburrus