Fasciculus vesanus
| Fasciculus vesanus | ||||||||
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Fasciculus vesanus , artistic 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 | ||||||||
| Fasciculus vesanus | ||||||||
| Simonetta & Delle Cave , 1978 |
Fasciculus vesanus is an extinct species of the comb jellyfish (Ctenophora), which is known from the Canadian Burgess schist andcan be classifiedin the geological period of the Middle Cambrian . It was scientifically described in 1978 by the paleontologists Antonia M. Simonetta and Laura Delle Cave and, in its combination of two sets of long and short comb ribs, has no equivalent in the modern comb jellyfish. Formally, the species isassigned toa family Fasciculidae .
features
Fasciculus vesanus has a diameter of 10.3 centimeters and approximately the shape of a sphere; the body symmetry could have been bilateral (two-sided symmetrical) as well as biradial (with two mutually perpendicular planes of symmetry).
The most noticeable features of the species are two different sets of comb ribs, the locomotion organs of the comb jellyfish. The traces that have been preserved are strips of transverse (transversal) bars, which presumably do not correspond to the comb platelets themselves, which were probably lost during the fossilization, but rather to the "cushion cells" underneath, in which the comb platelets were anchored.
The ridges appear in six different zones, which can be divided into two central and four marginal zones.
- In the two central zones, which are opposite one another, eight long comb ribs can be seen, which start from the end remote from the mouth and run parallel to one another towards the end of the mouth. They are all roughly the same length and eventually taper to a point.
- In the four marginal zones, which are to the left and right of the central zones, when the side of the mouth is oriented upwards, there are sixteen short comb ribs, which are longer, the greater their distance from the center line of the central zone. The short comb ribs also taper towards the end. A thread-like structure that apparently ran in the middle below the crest ribs can possibly be interpreted as a nerve cord ( axon ).
This results in a total of 16 long and 64 short comb ribs; their total number was therefore 80, ten times as many as in all modern comb jellyfish that have only 8 comb ribs.
Between the central and the adjoining marginal zones there are characteristic lobed “objects” that have a segmented, scaly appearance and increase in width towards the side of the mouth. Possibly these suspected organs also existed within the central zone; there, however, they are difficult to see because of the long ridges in the foreground. They are also much narrower than the structures further out, perhaps because they are seen from the side in the preserved fossil. Depending on whether the objects are the same or not, this leads to two to four “organs” whose function, however, is completely in the dark. The paleontologists Simon Conway Morris and Desmond H. Collis speculate that it may have been tentacles or their approaches; however, such identification is not guaranteed. There is also no further evidence of the possible existence of tentacles.
Way of life
Since only a single specimen has survived, few conclusions can be drawn about the animals' way of life. However, it is likely that the species was an active swimmer. It would fit the image of a species actively pursuing its prey, if the fasciculus vesanus, like the other Cambrian comb jellyfish known to date, did not yet have passive trapping devices such as tentacles. However, this can only be said with probability when the identity and function of the internal flap organs has been clarified.
Site and age
The only known specimen of the species was discovered on August 19, 1917 by the American paleontologist Charles Walcott in the Burgess slate and is now a holotype in the National Museum of Natural History of the Smithsonian Institution in Washington, DC, USA It has an approximate one Age from 510 to 515 million years and thus falls in the Middle Cambrian period .
Systematic classification
The affiliation of the Fasciculus vesanus to the comb jellyfish is not entirely undisputed. Some researchers have suggested that it may have been a species of cnidarians instead . The paleontologists Simon Conway Morris and Desmond H. Collins have decidedly rejected this interpretation.
Still, it cannot be denied that the vesaneal fasciculus is atypical in several ways when comparing the species with modern forms. In addition to the lobed organs, which have no obvious equivalent in modern comb jellyfish, the large number of a total of 80 comb ribs, ten times the modern value, is striking. In addition, there is the curious division of the rib jellyfish body into central and marginal zones, the associated comb ribs are clearly differentiated from one another.
It is therefore unclear whether the Fasciculus vesanus was an early side branch of the comb jellyfish or whether a similar form could have been the parent species of later comb jellyfish. However, there are also examples from other animal groups that show that certain anatomical features were only stabilized in terms of evolutionary history after a long “experimental phase”, such as the number of fingers in terrestrial vertebrates (Tetrapoda).
A hypothetical scenario derives the rib jellyfish from an ancestor with a baggy body shape whose surface was still flagellated evenly. Specialization for the purpose of better coordination of locomotion led to a condensation of the flagella in some places and a thinning in others, so that over time numerous bands of flagella resulted, which finally fused to comb ribs. Fasciculus vesanus could therefore have been a representative of this early stage. Later, the number of rows of ridges was further reduced until there were forms like Xanioascus canadensis , which have 24 evenly structured ridges. However, a concrete derivation of this type from the vesanus fasciculus is not without problems; in particular, it is unclear whether the comb ribs of the central or the marginal zone disappeared or merged with one another and if so, in which way or in which order this happened exactly.
Another feature of the Fasciculus vesanus , which is shared by the other Cambrian species, is the lack of any tentacles. Although the classical division of the comb jellyfish provides for a fundamental division into tentacle-reinforced ( Tentaculata ) and tentacle-free forms ( Nuda ), morphological, embryological and molecular genetic studies have increasingly questioned this classification and instead assume that the modern tentacle-less forms of Derive tentaculata rib jellyfish. In contrast , the findings that can be gained from the structure of the Fasciculus vesanus suggest that the earliest comb jellyfish did not yet have tentacles. This apparent conflict can, however, be resolved by assuming that the tentacles were lost secondarily in the modern Nuda forms, so that they are not direct descendants of the tentacle-free Cambrian species such as the Fasciculus vesanus .
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