| Australohalkieria Temporal range: Early Middle Cambrian, | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| (unranked): | Spiralia |
| Superphylum: | Lophotrochozoa |
| Phylum: | Mollusca |
| Family: | † Halkieriidae |
| Genus: | † Australohalkieria Porter, 2004 |
| Type species | |
| Australohalkieria parva Bengtson et al., 1990 | |
| Otherpecies | |
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| Synonyms | |
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Australohalkieria (meaning "southern Halkieria ") is an extinct genus of halkieriid from Australia and Antarctica.
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. [1]
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. [1]
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 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. [1]
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 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. [1]
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. [1]
This species was first described in 1990. [2] 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. [1]
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.
A sclerite is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly to the hardened parts of arthropod exoskeletons and the internal spicules of invertebrates such as certain sponges and soft corals. In paleontology, a scleritome is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.
Wiwaxia is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. Wiwaxia fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from early Cambrian and middle Cambrian fossil deposits across the globe. The living animal would have measured up to 5 cm (2 inch) when fully grown, although a range of juvenile specimens are known, the smallest being 2 millimetres (0.079 in) long.
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 Coelosclerithophorans are a polyphyletic group of organisms bearing hollow sclerites made of aragonite, and with a supposedly distinctive microstructure.
Orthrozanclus is a genus of sea creatures known from two species, O. reburrus from the Middle Cambrian Burgess shale and O. elongata from Early Cambrian Maotianshan Shales. Animals in this genus were one to two centimeters long, with spikes protruding from their armored bodies. The placement of this genus into a specific family is not universally accepted.
Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.
The Chancelloriids are an extinct family of superficially sponge-like animals common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism. Other specimens appear to be more complete and to represent sessile, radially symmetrical hollow bag-like organisms with a soft skin armored with star-shaped calcareous sclerites from which radiate sharp spines.
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.
Brachiopods, phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major categories are traditionally recognized, articulate and inarticulate brachiopods. The word "articulate" is used to describe the tooth-and-groove structures of the valve-hinge which is present in the articulate group, and absent from the inarticulate group. This is the leading diagnostic skeletal feature, by which the two main groups can be readily distinguished as fossils. Articulate brachiopods have toothed hinges and simple, vertically-oriented opening and closing muscles. Conversely, inarticulate brachiopods have weak, untoothed hinges and a more complex system of vertical and oblique (diagonal) muscles used to keep the two valves aligned. In many brachiopods, a stalk-like pedicle projects from an opening near the hinge of one of the valves, known as the pedicle or ventral valve. The pedicle, when present, keeps the animal anchored to the seabed but clear of sediment which would obstruct the opening.
Volborthella is an animal of uncertain classification, whose fossils pre-date 530 million years ago. It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain. It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have occurred by chance. The Ordovician scleritome-bearing Curviconophorus, as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature. The related Campitius was originally suggested to be part of a radula rather than a scleritome, but is now considered a synonym of Volborthella.
Tommotiids are an extinct group of Cambrian invertebrates thought to be early lophophorates.
The origin of the brachiopods is uncertain; they either arose from reduction of a multi-plated tubular organism, or from the folding of a slug-like organism with a protective shell on either end. Since their Cambrian origin, the phylum rose to a Palaeozoic dominance, but dwindled during the Mesozoic.
Propebela rathbuni is a species of sea snail, a marine gastropod mollusk in the family Mangeliidae.
Sachites is an extinct genus of halkeriid that is only known from fossilised spiny sclerites; many Sachites specimens are now referred to as other halkieriid taxa. Although believed to be related to the halkieriids, a chancelloriid affinity has more recently been proposed.
Tegulaherpia is a genus of pholidoskepian solenogasters, shell-less, worm-like, marine mollusks. Its sclerites are flattened and resemble the sclerites of Halkieria.
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.
Thambetolepis is a dubious genus of sachitid halkieriid from the Cambrian. The genus Sinosachites may have been the same as Thambetolepis.
Tannuolina is a genus of tommotiid, belonging to the brachiopod stem lineage.
Micrina is an extinct genus of tommotiids with affinities to brachiopods.