Ammonoids and the Evolution of Shell Form

This lecture begins a survey of Bilateria, starting with Lophotrochozoa. Rather than perform a full survey, we will focus on one taxon - Ammonoidea that is significant as:

Adding a flavor of mystery is that they lack good living analogs.

Phylogenetic context - Lophotrochozoa: Cambrian (maybe Ediacaran) - Recent.

The large monophyletic group Lophotrochozoa is weakly supported by molecular evidence. Within it, mollusks and annelids are united by the synapomorphy of the trochophore larva. As we have seen from the Burgess Shale and Sirius Passet faunas, during the Early and Middle Cambrian, annelids and mollusks didn't really look that different.

Mollusca, in contrast, is diagnosed by many unambiguous synapomorphies, including:

Issues of the Mollusk Shell:

Major molluscan groups include:

Polyplacophora: Chitons. (Cambrian - Recent) Characterized by eight sequential valves (shells). Retain something like the Ediacaran mat-scratcher life style, grazing off of hard substrates.

Monoplacophora: (Fossil record Cambrian - Devonian but discovered living) Broadly similar to chitons but with one conical valve. Worth our attention because it is close to the ancestral morph from which more derived mollusks evolved.

Bivalvia: Clams, oysters, mussels, scallops, etc.

Gastropoda: Snails. (Cambrian - Recent)