''Australohalkieria'' (meaning "

south South is one of the cardinal directions or Points of the compass, compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pro ...

ern ''

Halkieria 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 smal ...

''") is an extinct genus of

halkieriid 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 smal ...

from

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

and

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

Description

''Australohalkieria superstes''

This species, named by Porter in 2004, is the most complete and abundant Australian halkieriid species. 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 that 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 Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...

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 A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Greek ...

. If halkieriids were early

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

s, the outer layers of the sclerites may have been similar to the

periostracum The periostracum ( ) is a thin, organic coating (or "skin") that is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs, it is primarily seen in snails and clams, i.e. in gastropods and ...

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 Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...

. All of the sclerites were tiny: the palmates 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 Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the s ...

, it is unlikely that preservational bias has produced an unrepresentative sample. Possible explanations for the small size of ''A. superstes'' sclerites include: the individuals 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 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''.

References

{{Taxonbar, from=Q55999637 Fossil taxa described in 2004 Fossil taxa described in 1990 Prehistoric molluscs Molluscs Fossils of Australia Fossils of Antarctica