GEOL 331 Principles of Paleontology

Fall Semester 2008


Possible shell-less mollusks: Kimberlla of the Vendian and trace-fossil Climactichnites of the Cambrian

Possible Mollusca Phylogeny (combines some molecular and some morphological studies):


For a long time a hypothetical archimollusc (or HAM, basically a univalve unsegmented monoplacophoran with a single pair of gills, or an untorted gastropod) was suggested as the ancestral morphotype. However, because the outgroups are semi-segmented animals, it is more likely that aplacophorans and polyplacophorans represent the ancestral condition. HAM actually represents a good model for the most recent common ancestor of "Unnamed A" above.


Polyplacophora (chitons) (also called Amphineura)

Monoplacophora (long a gastropod subclass Tryblidiacea)

HAM is arguably the common ancestor of Diasoma + Cyrtosoma

Gastropoda (snails)

Scaphopoda (tusk shells)


Rostroconchia (rostroconchs)

Bivalvia aka Lamellibranchia aka Pelecyopoda (bivalves)

To Syllabus.

Last modified: 22 August 2008