Mullusca is both large and diverse. Molluscs comprise the familiar
animals commonly known for both their shells and fine seafood,
including snails, clams, octopus and squid.
development suggest a close ancestral relationship to other protostomes,
notably the Annelids, that includes segmented worms. Mulluscs
are believed to have appeared near the base of the Cambrian
and have left an extensive fossil record of several tens of thousands
of species. Science believes Molluscs fully participated in the
phenomena, with the appearance and disapperance of many forms,
many of which remain undiscovered in the fossil
Bivalves, and Cephalopods, the major groups of interest as fossils,
diversified and specialized into the Ordovician
to become ubiquitous throughout marine ecosystems. They adapted
with the new selective pressures as other marine life forms similrly
expanded, and they became both hunter of and hunted by new forms.
The fossil record attests to the bivalves adapting to fresh water
environments in the Devonian,
as well as the appearance of land-based Gastropods in the Carboniferous.
The ammonites particularly flourished
throughout the Mesozoic
before going extinct in the K-T event along with the dinosaurs.
deep sea wormlike animals
(those that carry shells)
Monoplacophora (note 1)
with discernable segmentation (example)
Extant, scallops, clams, oysters and mussels
tubular in form and rare in the fossil record
Gastropoda (note 2)
and most successful group of Mollusks
group containing ammonites,
nautiluses, squid, octopus
Rostroconchia (note 3)
enigmatic Class that went extinct in the Permian
Class thought extinct until 1952, they currently live deep in
ocean trenches. Body segmentation resembles primitive forms
of Phylum Annelida, suggesting a common ancestor with the annelids.
Tryblidiida could be considered a living fossil, while Stenothecoida
and Helcionelloida failed to survive the Cambrian.
This most prolific and most successful class of mollusks appeared
in the Upper Cambrian and today contains more than 60,000 known
living species species, of which snails and slugs are most commonly
known. The taxonomy comprises a myriad of subclasses, orders
and superfamilies, and seemingly undergoes constant revision.
a class of extinct mollusks dating from the early Cambrian to
the late Permian, initially thought to be bivalves, but were
later given their own class.