Xanioascus canadensis
| Xanioascus canadensis | ||||||||
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Xanioascus canadensis , reconstruction |
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| Temporal occurrence | ||||||||
| Middle Cambrian | ||||||||
| 515 to 505 million years | ||||||||
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| Systematics | ||||||||
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| Scientific name | ||||||||
| Xanioascus canadensis | ||||||||
| Conway Morris & Collins, 1996 |
Xanioascus canadensis is an extinct species from the group of the comb jellyfish , which is known from the geological period of the Middle Cambrian . It wasscientifically described for the first timein 1996 by the paleontologists Simon Conway Morris and Desmond H. Collins. The generic name is derived from the Greek askos for "hose" and xanion for "comb"; the epithet describes the origin of the fossils from Canada . Formally, the species is assigned to a family Xanioascidae.
construction
The relatively poorly preserved fossils suggest a stretchy, sacky body with longitudinal dimensions of between 21 and 75 millimeters. They are covered on the surface by 24 comb ribs, which presumably start from the poorly recognizable side of the mouth and converge at the end remote from the mouth. In contrast to the species Ctenorhabdotus capulus , also known from the Cambrian , the comb ribs do not unite in groups of three, but run separately from one another and are thus about the same length. In terms of their fine structure, they resemble the comb ribs of the species Fasciculus vesanus , another Cambrian comb jellyfish species.
Whether there was an organ of equilibrium ( statocyst ) at the end turned away from the mouth itself, as in modern forms , which served to synchronize the beat rhythm of the ridges and thus the orderly locomotion, cannot be clearly answered; if present, it was definitely a comparatively tiny structure. Two strip-shaped areas to the left and right of the possible statocyst can be interpreted as polar fields , completely ciliated areas of unknown function, which are also found in modern comb jellyfish. A dark ring runs around the potential statocyst, which was presumably part of the internal canal system that runs through the comb jellyfish. In the gastrovascular system of modern species, however, it has no equivalent; Ring canals occur here at most at the end of the mouth.
Unlike later forms, Xanioascus canadensis apparently did not yet have the tentacles that many species today use as prey-catching instruments.
Mysterious dark structures inside the fossils could represent food remains, reproductive organs, or even embryos; however, all of these interpretations are speculative.
Way of life and accompanying finds
Little is known about the way of life of Xanioascus canadensis . Descriptors Simon Conway Morris and Desmond H. Collins assume that the species was a relatively good swimmer and actively pursued its prey, unlike most species today. This would also fit the fact that the species lacks passive capture structures such as tentacles. The accompanying finds included specimens of the species Wiwaxia corrugata , which is assigned to the Halkieriids , presumed lineage representatives of the molluscs (Mollusca) as well as the annelids (Annelida) and arm pods (Brachiopoda); in addition, fossils of the genus Morania were found , which presumably represent colonies of cyanobacteria (Cyanobacteria).
Site and age
The holotype , the specimen defining the species, was found on July 6, 1982 by the Canadian paleontologist David Rudkin in the shale rock of the western slope of Mount Stephen , 1,500 meters southwest of the north shoulder, in the so-called West Stephen Formation, which in turn is the Burgess Shale is assigned. There are also seven paratypes , specimens that have been assigned to the same species by the descriptors. The age of the fossils is estimated at 512 million years, so that the finds can be assigned to the Middle Cambrian.
The holotype and paratype are now in the Royal Ontario Museum .
Systematic classification
Even if the classification of Xanioascus vesanus as a comb jellyfish species is not entirely undisputed, the first descriptors strongly argue in favor of such an assignment. In contrast to Fasciculus vesanus , another Cambrian comb jellyfish, the species shows significantly more "conventional" features such as the comb ribs, which are essentially of the same structure and length. Their number of 24, however, is three times as high as all modern forms.
Whether Xanioascus vesanus can be derived in a meaningful way from a species that, like Fasciculus vesanus, has several different body zones with regionally differentiated ridges or whether it has developed independently from evenly lashed forms, is in the dark. On the other hand, the transition to the modern rib jellyfish with their constant number of eight comb ribs can be better understood. This is because with Ctenorhabdotus capulus a transitional form is known which, regardless of whether it is actually a real descendant of Xanioascus vesanus , demonstrates a way in which the transformation to the modern species could have taken place. Also ctenorhabdotus 24 has comb ribs, but in contrast to those of Xanioascus vesanus converge towards the mouth facing away from (aboral) end into eight groups of three. A band-shaped structure continues from the junction of three comb ribs until all eight finally merge into a ring around the aboral pole. These ligaments are already interpreted as forerunners of the modern meridional canals, which are located as part of the inner canal system in modern comb jellyfish below the (there) eight comb ribs.
One trait that Xanioascus vesanus shares with other Cambrian forms is the lack of tentacles. From this it can be concluded that the modern tentacle-armored forms of the class Tentaculata did not develop until later, even if they are detected in the Devonian at the latest . Whether the modern tentacle-free species of the Nuda are derived directly from the Cambrian forms or are secondary from Tentaculata species, however, is not certain, even if morphological, embryological and molecular genetic characteristics speak for the latter.
Individual evidence
- ↑ Mena Schemm-Gregory , Ulrich Jansen (2007): " Wiwaxia - A puzzling fossil from the Burgess Shale". Nature and Museum , Volume 137, Issue 11/12. 2007.
literature
- SC Morris, DH Collins, Middle Cambrian ctenophores from the Stephen Formation, British Columbia, Canada , Philosophical Transactions of the Royal Society of London B, 351 , 1996, p. 279