Halkeriid

The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is ''Halkieria'' , which 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.

The group is sometimes equated to Sachitida, although as originally envisaged, this group includes the wiwaxiids and is thus equivalent to the Halwaxiida.

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. A short but free account is given at ''H. evangelista'' is used as a model for identifying and reconstructing as halkieriids other similar shells and sclerites; its epithet ''evangelista'' reflects its power to explain the Lower Cambrian fossil record.

Description of the fossils

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. 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 - to 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 about 2,000 sclerites that overlapped 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. The sclerites bore a wide central cavity, and (at least in some specimens) finer lateral canals. As the animals grew, the shell plates grew by adding material to the outer edges. 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. 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 the
time of burial

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 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 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.

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.

''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. The internal cavity within ''Australohalkieria'' is more complicated than the simple tube in ''Halkieria''; 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.

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 palmate ones ranged from to in length, and the cultrates from to . The siculates fall into two groups: those with a shallow S-curve at the base, which range from to in length, and often have a slight twist at the base; and those with a 45° and 90° bend at the base and are to 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. 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''.

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.

The genera ''Siphogonuchites, Dabashanites, Lopochites, ''and ''Maikhanella ''all seem to represent components of the ''Siphogonuchites'' animal.
Sclerites of ''Drepanochites'' can be distinguished based on their aspect ratio.

''Maikhanella'' is shell formed of ''Siphogonuchites'' sclerites that are fused together with a calcified matrix. Juvenile shells appear not to incorporate sclerites.
The central cavity of the ''Siphogonuchites'' sclerite is simple, with no lateral chambers attached.

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.

''Hippopharangites''

''Hippopharangites'' 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?)
This genus is the closest in form to Chancelloriid sclerites, and is thus used to support the union of halkieriids and chancelloriids as Coeloscleritophora.

''Lomasulcachites''

''Lomasulcachites'' is a further genus known from sclerites alone.

''Sachites''

''Sachites'' Meshkova 1969 is a genus that comprised spiny sclerites; many ''Sachites'' specimens are now referred to other halkieriid taxa.

Although believed to be related to the halkieriids, a chancelloriid affinity has more recently been proposed.

''Sinosachites''

''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. 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.

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.

''Oikozetetes''

''Oikozetetes'' is known only from two types of cap-shaped shell found in the Burgess Shale and dated to about . The two types are thought to be the front and rear shells of a halkieriid.

They were probably calcareous while the organism was alive (although diagenesis sometimes replaces the original mineral with another, such as silica). It is thought to also have borne an armour coat consisting of biomineralised sclerites, like ''Halkieria''. These are never found in direct association with the shells, but there are many biostratinomic processes which could account for this fact.

The lower Cambrian taxon ''Ocruranus'' (=''Eohalobia'') is putatively equivalent to the shells of ''Oikozetetes'' and seemingly belonged to a halkieriid-type body, although an intermediate valve suggests a Palaeoloricate-like body form.

Occurrence

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 cited by and Australia's Georgina Basin, while shells of a possible halkieriid have been found in Canada's Burgess Shale. Halkieriid-like armor plates, called "sclerites" have been found in many other places as part of the small shelly fauna.

The earliest known occurrences of Halkieriids sclerites, classified as ''Halkieria longa'', date from the ''Purella antiqua Zone'' of the Upper Nemakit-Daldynian Stage in Siberia.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) 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 died. 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.

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 — paleontologists call this kind of uncertainty the Signor–Lipps effect.

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

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).

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

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.
*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.
*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.
*''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

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