Chancelloriidae

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Chancelloriidae
Temporal occurrence
lower Cambrian to upper Cambrian
542 to 490 million years
Locations

Canada, United States, Greenland, China

Systematics
Empire : Animalia
Trunk : incertae sedis
Class : Coeloscleritophora
Order : Chancelloriida
Family : Chancelloriidae
Scientific name of the  order
Chancelloriida
Walcott , 1920
Scientific name of the  family
Chancelloriidae
Walcott , 1920
Chancelloria eros from the Central Cambrian Wheeler Shale in Utah

The Chancelloriidae (seldom also written Chancelloridae ) are an extinct family of the animal kingdom (some later authors also ranked them as an order), which appear for the first time in sediments of the Lower Cambrian and were widespread until the early Upper Cambrian. Their taxonomic classification is difficult, but they could be moved in the vicinity of sponges . There are also similarities with the Halkieriidae in the shape of the skeletal needles.

First description and occurrence

Chancelloriidae were first found and described in the Burgess Shale in Canada in 1920 by Charles Doolittle Walcott , who then classified them as very primitive sponges. Walcott assigned all of his finds to a single, broad species, which he named Chancelloria eros .

Chancelloriidae were then found in many places, most of the finds consisted of the isolated, characteristically shaped skeletal needles. But body fossils have also been found several times elsewhere in so-called conservation deposits, for example in the Central Cambrian Wheeler and Marjum Formation in Utah, USA, the Kaili Formation in Guizhou, China, the Mount Cap Formation in the Northwest Territories, Canada, the Early Cambrian Chengjiang Faunal Community in Yunnan, China and Sekwi Formation in Canada.

The oldest fossils come from an SSF fauna community of the Anabarites trisulcatus zone of the lower Nemakit Daldynian in Siberia and its analogue, the Protohertzina anabarica zone from the lower level of the Meishuchuum in China. From the fossil record it can be seen that the Chancelloriidae rapidly declined in their biodiversity in the Upper Cambrian and were probably completely extinct by the end of the Cambrian.

Fossil Conservation

Many of the fossils of the Chancelloriidae consist only of sclerite needles and other fragments, so that a clear assignment to certain taxa is often very difficult. Nevertheless, quite complete finds are known that reveal sessile, pocket-shaped organisms whose soft skin is reinforced with star-shaped calcareous clerites with sharp spines protruding from them. The assignment of taxa described according to isolated sclerites and according to body fossils to one another is difficult, since the form of the individual sclerites in the body fossils is more difficult to recognize. The exact number and delimitation of the taxa is therefore unclear today.

description

The Chancelloriidae had a radially symmetrical, sac-like body with a mouth opening at the upper end of the body, but no internal organs in the large, central cavity. The mouth opening was surrounded by a ring of slightly enlarged sclerite. The outer integument was probably relatively sturdy and leathery. There are no indications of inflow openings (oscula), as is typical for sponges. The animals were, as far as recognizable, attached to hard substrates of the seabed or other organisms, especially sponges and other Chancelloriidae, and were fixed (sessile), some investigators also speculate about a possible anchoring in soft substrates consolidated by microbial mats, as is typical for Precambrian sediments . Towards the point of attachment, the animals were somewhat narrowed like a stalk. Individual taxa differ in their size and their external shape. For example, Chancellaria eros was a slender, 4 to 6 centimeter long, tapering cone with a maximum diameter of 1.5 to 2 centimeters. Allonnia junyani, however, was disk-like to cylindrical in shape with a diameter of 6 to 7 centimeters and a maximum length of 20 centimeters.

Most fossil finds are made up of a collection of hard mineralized parts, the sclerites. The totality of the hard parts of a single individual is called a scleritoma. Individual taxa have only a few sclerites for their determination and some finds could therefore not yet be assigned to any species or genus .

Individual sclerites are formed from a star-shaped base from which a spine protrudes at a right angle; three- or six-pointed, complexly connected sclerites are also common. The sclerites have cavities inside, which is why many sclerites have only survived due to their phosphate filling. It is believed that these cavities in living animals were filled by tissue that secreted the hard parts. It is unclear what kind of substance this was, as the walls had either been replaced or converted to another crystalline form. It was probably an unstable material such as the calcium carbonate aragonite . Some of the sclerites give the impression that they have been skinned; but others were covered by skin, some only partially. According to the fossil record, the most likely position appears to be resting on the skin; the sclerites would not have sat as reinforcing skeletal elements inside, but, like the spines of a cactus, protruding outwards.

Way of life

The Chancelloriidae probably lived in the silt on the sea floor, as their sclerites increase in size upwards and their approaches had thickenings that are interpreted as anchors. Chancelloriidae are often found attached to other organisms or shell remains. There is no information about their diet. The absence of any intestinal fillings or remains of prey speaks against a predatory diet. The relatively compact construction with a single mouth opening, without finely branched appendages or an internal canal system, is atypical for a filter feeder.

Since the sclerites sat up externally and were not entangled, they could not exercise any support function. And since the organism was also firmly anchored to the sea floor, the sclerites could not have been useful for locomotion by train. Their sole purpose should therefore have been the defense against predators, roughly comparable to the spines of modern cacti.

Taxonomic position

In paleontology, the correct taxonomic position of the Chancelloriidae, which turns out to be very difficult and controversial, is indispensable, as they play an important role in the development of multicellular organisms . Their original assignment by Walcott to the sponges is entirely plausible because of their sedentary lifestyle and their simple construction plan. Walcott's assumption was first questioned by Bengtson and colleagues because of the differences between Chancelloriidae sclerites and sponge needles. In addition, the opinion was expressed that Chancelloriidae were further developed and descended from far more complex ancestors. For example, their skin shows a lot more structures than the sponges. This discrepancy led to the assumption that the Chancelloriidae were related to the Halkeriidae, which were so important for the Cambrian explosion - snail-like organisms protected by "chain mail". The sclerites of these two groups of animals are very similar down to the microscopic detail and therefore speak for a common development. However, this contradicts their completely different construction plan.

The sclerite problem was reopened in 1996 by Butterfield and Nicholas. They came to the conclusion that the Chancelloriidae are very close to the sponges, since the fine structure of their sclerite resembles spongin fibers . Spongin is a protein made from collagen that is found in modern keratotic horn silica sponges ( demo sponges ) such as Darwinella .

Janussen, Steiner and Zhu contradicted this in 2002. They argued that spongin does not have to occur in all sponges, it is only crucial for demo sponges. The silica-based needles of demospongia are secreted by specialized cells surrounding them - the sclerocytes . Demospongia needles do not contain any aragonite, which is only found in Sclerospongia and is suspected to be in the Chancelloriidae. Sclerospongia are encased in massive shield skeletons made of aragonite or calcite.

For their part, Janussen and colleagues went into the skin issue and referred to the fact that the pinacoderm in sponges is only loosely connected and only one cell layer thick, whereas Chancelloriidae have a much thicker skin layer and belt desmosomes - connecting structures. In their opinion, the Chancelloriidae therefore belong to the Epitheliazoa standing above the Porifera (sponges) . A classification of the Chancelloriidae under the Eumetazoa (real animals with several dermias) they see as difficult because they have neither sensory organs, muscles nor a digestive system. However, they do not rule out the possibility that the Chancelloriidae have lost these characteristics again in the course of evolution due to their sedentary, filtering way of life.

Porter (2008) pointed to the extraordinary similarity of the sclerites of Chancelloriidae and Halkeriidae - snail-like, bilaterally symmetrical, chain-mailed animals that lived in the Lower and Central Cambrian. The hollow coelosclerites are comparable in all respects in both groups of animals - they are covered by a thin organic outer layer and their interior consists of a cavity that is connected to the rest of the body via a narrow channel. Their walls are made of the same material, aragonite. The arrangement of the aragonite needles is also the same in both groups of animals - they run from the base to the tip and approach each other towards the tip. Porter thinks it is very unlikely that two unrelated groups of animals would develop such similar sclerites independently. Porter explains the dilemma of the huge differences in other body characteristics as follows:

  • It is quite possible that the Chancelloriidae developed from bilaterally symmetrical precursors, but then changed to a sedentary way of life and very quickly lost all unnecessary features. For other bilateral animals , which also gave up their bilateral symmetry, such as echinoderms (Echinodermata), priapulida (Priapulida) and kinorhyncha (Kinorhyncha), but the digestive tract and other internal organs have been preserved.
  • On the other hand, the Chancelloriidae show a similarity with precursor organisms of the Bilateria. The earliest Bilateria must therefore also have had Coelosklerites. In the fossil record, however, there is no evidence of sclerites that are older than 542 million years. The Kimberella , dated 555 million years ago, has no signs of sclerite, but was almost certainly a bilaterally symmetrical animal.
  • One way out of the dilemma may be based on the fact that SSF fauna communities were fossilized only for a relatively short period of time during the Lower Cambrian in phosphate conservation, but organisms with coelosclerites possibly lived several million years before and afterwards. In fact, 25 taxa with phosphate conservation are known in the period 542 to 521 million years BP , but only one is known in the period 555 to 542 million years BP.
  • It could also be that the ancestors of both the Chancelloriidae and the Halkieriidae originally had very similar, non-mineralized coelosclerites and only incorporated aragonite independently of one another into the similar structures at a later point in time.

Those paleontologists who, because of the structure of the sclerites, favor a group of the Chancelloriidae, the Halkieriidae (and other groups), regard them as taxons in the class of a class they call Coeloscleritophora. Others consider the groups summarized here as not belonging together, a taxon Coeloscleritophora would then not be justified. The riddle of the evolutionary assignment of the Chancelloriidae thus remains unsolved.

Taxa

The following taxa are classified under the Chancelloriidae family:

swell

  • Stefan Bengtson and Desmond Collins: Chancelloriids of the Cambrian Burgess Shale . In: Palaeontologia Electronica 18.1.6A . 2015, p. 1-67 . on-line
  • Randell, RD, Lieberman, BS, Hasiotis, ST and Pope, MC: New Chancelloriids from the Early Cambrian Sekwi Formation with a comment on Chancelloriid affinities . In: Journal of Paleontology . 2005.

Individual evidence

  1. ^ Walcott, CD: Cambrian geology and paleontology IV: 6 — Middle Cambrian Spongiae . In: Smithsonian Miscellaneous Collections . tape 67 , 1920, pp. 261-364 .
  2. ↑ Janussen, D., Steiner, M. and Zhu, MY .: New Well-preserved Scleritomes of Chancelloridae from the Early Cambrian Yuanshan Formation (Chengjiang, China) and the Middle Cambrian Wheeler Shale (Utah, USA) and paleobiological implications . In: Journal of Paleontology . tape 76 (4) , 2002, pp. 596-606 , doi : 10.1666 / 0022-3360 (2002) 076 <0596: NWPSOC> 2.0.CO; 2 .
  3. ↑ Porter, SM: Skeletal microstructure indicates Chancelloriids and Halkieriids are closely related . In: Palaeontology . tape 51 (4) , 2008, pp. 865-879 , doi : 10.1111 / j.1475-4983.2008.00792.x .
  4. ↑ Bengtson, S .: Origins and early evolution of predation . In: Kowalewski, M. and Kelley, PH The fossil record of predation (Eds.): The Paleontological Society Papers . tape 8 . The Paleontological Society, 2002, pp. 289-317 .
  5. ↑ Bengtson, S. and Missarzhevsky, VV: Coeloscleritophora — a major group of enigmatic Cambrian metazoans . In: US Geological Survey Open-file Report . 1981, p. 81-743 .
  6. ^ Butterfield, NJ and Nicholas, CJ: Burgess Shale-Type Preservation of Both Non-Mineralizing and 'Shelly' Cambrian Organisms from the Mackenzie Mountains, Northwestern Canada . In: Journal of Paleontology . tape 70 (6) , 1996, pp. 893-899 , doi : 10.2307 / 1306492 .
  7. ↑ Fedonkin, MA and Wagoner, BM: The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism . In: Nature . tape 388 (6645) , 1997, pp. 868 , doi : 10.1038 / 42242 .
  8. ↑ Bengtson, S .: Mineralized skeletons and early animal evolution . Ed .: Briggs, DEG Evolving form and function: fossils and development. Peabody Museum of Natural History, Yale University, New Haven, CT, S. 288 .
  9. ↑ Bengtson, Stefan and Collins, Desmond: Burgess Shale Chancelloriids - A Prickly Problem . In: Smith, Martin R., O'Brien, Lorna J. and Caron, Jean-Bernard (Eds.): Abstract Volume. International Conference on the Cambrian Explosion (Walcott 2009) . Toronto, Ontario, Canada: The Burgess Shale Consortium 2009, ISBN 978-0-9812885-1-2 .