- Joined
- Nov 19, 2002
- Messages
- 3,031
John,
Thanks for the info about the Rhizocephalan. I know one shouldn’t be emotive about behaviour patterns but what a horrific creature! Hadn’t heard of that one before; much to my regret I read about it whilst eating a prawn sandwich. Bad idea.
Going back to the origins of the mollusca there have been a few interesting finds in recent years, notably a strange beast known as Kimberella that was described about five years ago and was discovered at the White Sea in the Arctic circle on the north coast of Russia and dates to about 550mya. This places it almost immediately before the Cambrian ‘explosion’ that gave rise to modern bauplans in a remarkably short space of time. (hotly debated, most estimates say about twenty million years). Kimberella is interesting as it is difficult to interpret but is thought to be an early mollusc. Physically it looks like a jellyfish without the tentacles but has bilateral symmetry and a tough (non-mineralised) but flexible shell. Some fossils apparently display a mollusc type foot and although no radula has yet been found associated with this animal, as far as I know, not all mollusc fossils do.
Trace marks and scrape marks in some late Precambrian sediments may well have been caused by a creature such as this or even its ancestor. Trace marks date from 565mya or even earlier so there must have been some ancestral creature that lived in a mollusc fashion scraping up algae at this even earlier date possibly as far back as 620mya.
Other early (probable) molluscs are evident in the Tommotian faunas from Siberia (about 530mya) which largely consists of many tiny shells, many of which resemble monoplacophora. There are many varieties of cone, spiral, horn-shape and tube evident though whether or not they were molluscs depends on how a mollusc is defined, I suppose. Try doing Google searches under Tommotia, Yochelcionella or Latouchella if interested. Following this is the sudden evolution of modern body plans during the Cambrian ‘explosion’ including the arthropods and although probable molluscs were evident (re: Wiwaxia above) the relationships of these creatures to each other animal groups has been the source of much debate and study recently. All very complex.
I still have not discovered a cephalopod earlier in date to Plectronoceras (Upper Cambrian). I attach a copy of its anatomy here. I had to copy it from a text book to avoid copyright problems duplicating it here. It is quite probable that Plectronoceras evolved from something very similar to Latouchella as they do look similar (from drawings in textbooks, that is!)
On a different note, I think that 10m long Nautiloid was called Cameroceras. Must look that one up.
Phil
(PS If I have made any mistakes above I apologise. I have never actually studied zoology or palaeontology and am working out of a few books at home. All very interesting, isn’t it!)
Thanks for the info about the Rhizocephalan. I know one shouldn’t be emotive about behaviour patterns but what a horrific creature! Hadn’t heard of that one before; much to my regret I read about it whilst eating a prawn sandwich. Bad idea.
Going back to the origins of the mollusca there have been a few interesting finds in recent years, notably a strange beast known as Kimberella that was described about five years ago and was discovered at the White Sea in the Arctic circle on the north coast of Russia and dates to about 550mya. This places it almost immediately before the Cambrian ‘explosion’ that gave rise to modern bauplans in a remarkably short space of time. (hotly debated, most estimates say about twenty million years). Kimberella is interesting as it is difficult to interpret but is thought to be an early mollusc. Physically it looks like a jellyfish without the tentacles but has bilateral symmetry and a tough (non-mineralised) but flexible shell. Some fossils apparently display a mollusc type foot and although no radula has yet been found associated with this animal, as far as I know, not all mollusc fossils do.
Trace marks and scrape marks in some late Precambrian sediments may well have been caused by a creature such as this or even its ancestor. Trace marks date from 565mya or even earlier so there must have been some ancestral creature that lived in a mollusc fashion scraping up algae at this even earlier date possibly as far back as 620mya.
Other early (probable) molluscs are evident in the Tommotian faunas from Siberia (about 530mya) which largely consists of many tiny shells, many of which resemble monoplacophora. There are many varieties of cone, spiral, horn-shape and tube evident though whether or not they were molluscs depends on how a mollusc is defined, I suppose. Try doing Google searches under Tommotia, Yochelcionella or Latouchella if interested. Following this is the sudden evolution of modern body plans during the Cambrian ‘explosion’ including the arthropods and although probable molluscs were evident (re: Wiwaxia above) the relationships of these creatures to each other animal groups has been the source of much debate and study recently. All very complex.
I still have not discovered a cephalopod earlier in date to Plectronoceras (Upper Cambrian). I attach a copy of its anatomy here. I had to copy it from a text book to avoid copyright problems duplicating it here. It is quite probable that Plectronoceras evolved from something very similar to Latouchella as they do look similar (from drawings in textbooks, that is!)
On a different note, I think that 10m long Nautiloid was called Cameroceras. Must look that one up.
Phil
(PS If I have made any mistakes above I apologise. I have never actually studied zoology or palaeontology and am working out of a few books at home. All very interesting, isn’t it!)