Ctenorhabdotus capulus
| Ctenorhabdotus capulus | ||||||||
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| Temporal occurrence | ||||||||
| Middle Cambrian | ||||||||
| 515 to 505 million years | ||||||||
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| Systematics | ||||||||
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| Scientific name | ||||||||
| Ctenorhabdotus capulus | ||||||||
| Morris & Collins , 1996 |
Ctenorhabdotus capulus is an extinct species of the rib jellyfish (Ctenophora). It is known from the Canadian Burgess slate and comes from the geological period of the middle Cambrian .
The species was scientifically described in 1996 by the paleontologists Simon Conway Morris and Desmond H. Collins. Their generic name is derived from the Greek ctenos for "comb" and rhabdotus for "stripes" and refers to the special shape of the comb ribs typical of rib jellyfish; the epithet capulus for "capsule" indicates the organ of equilibrium at the end of the body facing away from the mouth. The genus and species are formally assigned to a family Ctenorhabdotidae.
features
The animals, which were up to seven centimeters in size, had a variable body shape, which for some individuals can be described as more cube-shaped, for others as more spherical. In the first case, the animals are approximately square in the side view, that is, both the mouth or oral side and the side facing away from the mouth or aboral side are flattened.
The most striking feature of the animals are the twenty-four comb ribs that run from the oral to the aboral end and unite there in eight groups of three. They are wider than the corresponding structures of the Cambrian species Xanioascus canadensis and also differ in that the middle row is shorter than the two outer ones. From the junction of three ribs, eight longitudinal strips continue, which unite in a ring around the end facing away from the mouth. Because they lack the characteristic comb rib cross bars that probably remains of the comb plate bearing pad cells represent, it is not in the longitudinal rows presumably comb ribs themselves. In modern comb jellies can be found similar position eyelashes tapes, run out of the vestibular system at the aboral end and the Passing on impulses to the comb ribs. Against such an interpretation, however, there is the low probability that such sensitive structures could have been preserved in fossil form; apart from that, the rows are too thick for such an interpretation compared to modern eyelash bands. Instead, the descriptions assume that both the longitudinal structures and the ring into which they open are parts of the internal canal system of the comb jellyfish, which is known as the gastrovascular system. This system, which, among other things, serves to distribute nutrients in the body, in modern species has a channel below the comb ribs, which is known as the meridional channel . The longitudinal structures of Ctenorhabdotus capulus could be analogous body parts.
In contrast to Xenioascus canadensis , in Ctenorhabdotus capulus a clear organ of equilibrium, the statocyst , can be identified on the side facing away from the mouth , which, as in modern species, is enclosed in a capsule formed by eyelashes that rises like a dome above the statocyst.
A wide mouth opening can be seen on the side of the mouth, which is surrounded by a flap-like, possibly muscular collar. However , there are no indications of tentacles .
Way of life and accompanying finds
Like the other Cambrian comb jellyfish, Ctenorhabdotus capulus was evidently an active swimmer and thus preyed on its food. This is also supported by the lack of passive trapping structures such as tentacles.
Along with the species was as accompanying Fund found a diverse fauna, which includes among other things sponges (Porifera) as Hazelia delicatula , priapulida (Priapulida) as Ottoia prolifica or Selkirkia columbia and Arthropods (Arthropoda) as Leanchoilia superlata , Canadaspis perfecta or trilobites -Art Peronopsis montis belonged to.
Site and age
All specimens come from the Burgess shale in the Canadian province of British Columbia , specifically the Stephen Formation quarries known as Raymond Quarry and Walcott Quarry . The holotype of the species, i.e. the specimen that defines its properties, was found by the American paleontologist Charles Walcott as early as the 1920s ; seventeen other specimens, which were established as paratypes , are known to this day. The fossils preserved in shale all date from the Middle Cambrian period, about 510 to 515 million years ago.
The holotype is now in the Royal Ontario Museum in Canada.
Systematic position
It is relatively certain that Ctenorhabdotus capulus belongs to the comb jellyfish, even if the suspicion was occasionally expressed that it could have been a species of cnidarians instead . Of the known Cambrian forms, however, the species is the one that morphologically most closely resembles the modern comb jellyfish. So, at least if the above-described interpretation of the fossils is correct, it already had an inner canal system with meriodional canals below the comb ribs, which ran through the body and supplied it with nutrients. The organ of equilibrium, the statocyst, presumably already had the same structure as in today's species of comb jellyfish.
The most striking difference can be seen in the number of comb ribs, which is eight in all modern species. Since two other Cambrian forms, Xanioascus canadensis and Fasciculus vesanus, had significantly more than these eight ridges, it can be said with great probability that the number of ridges stabilized at its current value only late. A possible scenario of how this process could have taken place is shown by a comparison with Xanioascus canadensis , a species that, like Ctenorhabdotus capulus, had twenty-four comb ribs, but which, as far as can be seen, all had the same length and did not extend towards the end turned away from the mouth united in groups of three. Although it is unlikely that Xanioascus canadensis a direct ancestor of ctenorhabdotus was demonstrating the comparison but, as the latter species could have arisen by the merger of three comb ribs at the aboral end of the animal from an ancestral species that Xanioascus canadensis at least in the comb ribs number and arrangement was similar. The selection pressure required for this could have been generated by better coordination and control options when moving. The shape of the modern species with their eight comb ribs would have come about either through the atrophy of the outer ribs of each triple or through the omission of the inner comb rib and merging of the two outer ones. At the latest in the Devonian , from which two species, Paleoctenophora brasseli and Archaeocydippida hunsrueckiana , are known, the current number of ridges was reached.
While these Devonian species already had tentacles, Ctenorhabdotus capulus, like other Cambrian species, was apparently tentacleless. It is uncertain whether this means that the species already belonged to today's Nuda , which is characterized by this very characteristic, or whether, as suggested by molecular genetic, embryological and morphological studies, the Nuda originated from secondary tentacle loss from tentacle-reinforced forms, the tentaculata .
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