Helcionellids
| Helcionellids | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Pelagiella atlantoides |
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| Temporal occurrence | ||||||||||||
| Lower Cambrian to Middle Cambrian | ||||||||||||
| 542 to 501 million years | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Helcionelloida | ||||||||||||
| Peel , 1991 | ||||||||||||
| Orders | ||||||||||||
The only fossil known Helcionellidae (Helcionelloida) are a class of mollusks (Mollusca) and are counted among the sub-tribe of the shell molluscs (Conchifera). They have endogastrically curved, cup- to planispirally rolled up housings. At the rear end of the housing there is a sine or a point at which the shell edge is more or less bent upwards; in derivative forms, this sinus can be closed at the bottom and form a snorkel. The stratigraphic range is from Lower to Middle Cambrian . They were spread all over the world.
construction
The Helcionellids come very close to the basic plan of the shell molluscs (Conchifera). The body was presumably (as in the original shell mollusc) divided into head, foot and visceral sac. The foot was probably designed as a creeper in most forms. The Helcionellids are oriented so that the snorkel or a marginal sinus lies behind. The housing and the viscera were thus rolled up endogastrically. The gills were thus with some probability in the back of the mantle cavity. The water flow could have entered the front and could have left the mantle cavity through the snorkel or the posterior sinus. Thus, the Helcionelloida are oriented exactly the other way around than the Einplatter (Monoplacophora), whose cases are exogastrically rolled up or curved. That was also the reason for these fossils to split off from the monoplacophora to which they were originally placed.
Way of life
The Helcionellids probably lived in shallow marine areas of the sea during the Lower to Middle Cambrian. Presumably they penetrated in the Lower Cambrian as far as the reef-near areas of the Archaeocyathid reefs.
literature
- Peel, John, S. 1991. Functional morphology, evolution and systematics of Early Palaeozoic univalved molluscs. Grønlands Geologiske Undersøgelse Bulletin 161: 1-116, Copenhagen.