Halkieriidae
| Halkieriids | ||||||||||
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Halkieria evangelista from the Sirius Passet deposit in Lower Cambria in northern Greenland |
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| Temporal occurrence | ||||||||||
| lower Cambrian to upper Cambrian | ||||||||||
| 537 to 505 million years | ||||||||||
| Locations | ||||||||||
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Canada, Greenland, Siberia, China, Australia |
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| Systematics | ||||||||||
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| Scientific name | ||||||||||
| Halkieriidae | ||||||||||
| Poulsen , 1967 |
The family of Halkieriidae is an extinct group of animals during the Lower and Middle Cambrian lived. Their taxonomic position is not completely clear.
Initial description
The Halkieriidae family was first described scientifically in 1967 by Poulsen in the sub-Cambrian layers of Bornholm .
Occurrence
The only complete find of the taxon Halkieria evangelista so far was made in the Buen Formation in northern Greenland ( Sirius Passet Faunengemeinschaft ). Fossil remains that can almost certainly be assigned to the Halkieriids come from Sinkiang in China and from the Georgina Basin in Australia . Shell remnants from the Burgess Shale in Canada are already somewhat uncertain in their assignment to the Halkieriidae. So-called sclerites - small, armor-like plates of Halkieriidae - form part of the many places represented small shelly fauna of the Cambrian (547-520 million years BP ).
In Germany , Halkieriids were discovered in the Lower Cambrian of the Görlitz Synclinory. Other sites are the Antarctic , the United States of America , the southeast of Newfoundland , Denmark , England , France ( Montagne Noire ), Kazakhstan , Pakistan , Siberia and Mongolia .
discovery
In 1989 the first fully preserved fossil with all its hard parts was discovered in the Sirius Passet Fauna Community in northern Greenland. It was then described in 1990 by Simon Conway Morris and John S. Peel as the Halkieria evangelista . Since then it has served as a type fossil for the identification of shell remains or sclerites and their classification under the Halkieriidae. The taxon epithet evangelista is supposed to allude to this function of the Lower Cambrian fossil.
Sclerite
The protective plates known as sclerite (or in their entirety as scleritoma) have long been known as a significant part of the small -shelly fauna . Detailed studies have shown that a large number can be assigned to the same animal and their structural arrangement could also be determined. The sclerites should rather be called coelosclerites , because they had a cavity inside their mineralized shell that was originally filled with organic tissue. There are no signs of outward growth; the growth has evidently taken place from within. There are three basic types of sclerites:
- Palmate (palm-shaped) sclerites
- Cultrate (knife-shaped) sclerites
- Siculate (sickle-shaped) sclerites.
Palmate sclerites are the smallest of the sclerites, they are flat and look somewhat like a maple leaf. Cultrate sclerites are also flat and shaped like a knife edge. The siculate sclerites are the same size as the cultrate sclerites, but are spiky to sickle-shaped and give the impression of a compressed cylinder. Both palmate and cultrate sclerites have protruding spines, but are otherwise flat (with the exception of small, right-angled inversions along their base) and therefore lay quite close to the body of the animals. The siculate sclerites usually have no spines. But they stood at an angle of 45 to 90 ° from the outer skin of the animal.
During the growth of the animal, individual sclerites retained their original size and even formed a constant pattern among the cultrate sclerites. In the course of growth, old, too small sclerites may have been shed and replaced by new, larger ones. It is believed that the sclerites were secreted at their base starting from the animal's integument. There are thin ribs between the sclerites and the outer skin.
description
The 1.5 to 8 cm long, bilaterally symmetrical animals were flattened. Its underside had no sclerites, but the reinforcement on the upper side looked like chain mail - composed of a total of 2000 sclerites overlapping each other like roof tiles. Two circular areas at the front and rear of the animals were spared from this covering and were covered by shell plates with concentric growth rings. Palmate sclerites built up the central dorsal area between the two shell plates. Cultrate slerites flanked the palmate sclerites on either side and pointed towards the spine. Narrow siculate sclerites covered the outer margins.
The two shell plates and the sclerites were probably originally composed of calcium carbonate . Due to their state of preservation, a purely organic composition was also considered, but this is to be regarded as less likely, since fossils of non-calcified organisms usually only consist of thin films. However, like trilobites and hyolites, the Halkieriid fossils show a three-dimensional structure (viewed in cross-section in the form of a cut-off cone). In some finds a curvature was even found in the horizontal plane; this suggests that muscle tissue was still on the sclerites during the course of the sediment burial of the fossil.
The undersides of the animals were soft and probably muscular. Since Halkieriidae could not swim freely and were not adapted to a burrowing way of life, they must have lived on the sea floor, where they probably moved by means of contractions of the muscular soles of their feet. The backward-facing siculate sclerites strengthened the grip and prevented slipping back. Fossils rolled up like Armadillidiidae have been found, the cultrate sclerites of which pointed outwards and thus possibly deterred possible predators. The function of the lid-like shell plates at both ends poses problems. They do not seem to have had a protective function, since the sclerites were sufficient for this. Scars on the inside of the anterior plate indicate attachment points for internal organs. One fossil record shows a 45 ° rotation of the posterior plate prior to fossilization. Perhaps there was a gilled orifice below the back panel.
Signs of viscera were found in the posterior half of some fossils. If it is found, a radula is even suspected - the chitinous tongue of the molluscs. The alleged radula could just as well have been just misplaced siculate sclerites of the everted scleritoma.
Taxonomy
The taxonomic relationships of the Halkieriidae are a complex topic and remain controversial. The central point of these discussions is their position towards the taxon Wiwaxia and the relationship to the three main tribes of the Lophotrochozoa - mollusks , annelids and brachiopods . Their position in relation to the Chancelloriidae - a rather primitive group of animals of the Cambrian - is also of great importance and involves difficult questions.
Taxa
The following genera are classified under the Halkeriidae:
- Halkieria : all continents
- Australohalkieria : Georgina Basin, Australia
- Hippopharangites
- Lomasulcachites
- Oikozetetes
- Sachites
- Sinosachites (or Thambetolepis )
Possible family relationships also exist to the families of the Siphogonuchitidae and the Ninellidae .
Individual evidence
- ^ Chr. Poulsen: Fossils from the Lower Cambrian of Bornholm. In: Det Kongelige Danske Videnskabernes Selskab - Matematisk-fysiske Meddelelser , Vol. 36, No. 2, 48 pp. + 9 plates, 1967. (digitized version)
- ^ A b Conway Morris, S. and Peel, JS: Articulated halkieriids from the Lower Cambrian of north Greenland . In: Nature . tape 345 (6278) , 1990, pp. 802-805 , doi : 10.1038 / 345802a0 .
- ^ Geyer, G. and Elicki, O .: Lower Cambrian trilobites from the Görlitz Synclinorium (Germany) - review and new results . In: Paleontological Journal . tape 69 (1/2) , 1995, pp. 87-119 .
- ↑ Elicki, Olaf: First report of Halkieria and eigmatic globular fossils from the central european Marianian (Lower Cambrian, Görlitz Syncline, Germany) . In: Revista Española de Paleontología . n ° extr. Homenaje al Prof. Gonzalo Vidal, 1998, p. 51-64 .
- ^ Porter, SM: Halkieriids in Middle Cambrian phosphatic limestones from Australia . In: Journal of Paleontology . tape 78 , 2004, p. 574-590 , doi : 10.1666 / 0022-3360 (2004) 0782.0.CO; 2 .
- ^ Conway Morris, S. and Peel, JS: Articulated Halkieriids from the Lower Cambrian of North Greenland and their Role in Early Protostome Evolution . In: Philosophical Transactions of the Royal Society B . tape 347 (1321) , 1995, pp. 305-358 , doi : 10.1098 / rstb.1995.0029 .