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:
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:
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)
Scaphopoda: Tusk-shells. (Ordivician - Recent)
Cephalopoda: Cephalopods. (Cambrian - Recent)
What do these differences mean?
Septa: There is a trend toward increasing spetal complexity over ammonoid evolution. Paleontologists recognize three grades of complexity, although these grade into one another:
Goniatitic: Speta describe a series of tight curves.
Ceratitic: Speta describe a series of tight curves with a single series of smaller curves superimposed at apices.
Diversity of shell form: Ammonoids not only encompass wide ranges of the growth parameters indicated for mollusks above, they achieve novel morphologies by altering these parameters ontogenetically:
Planktonic: A common morph is strongly coiled with a small aperture. Since swimming is by recoil locomotion and the mantle cavity in this form is small compared to the shell, swimming must have been weak. On the other hand, the surface area of the camerae is large compared with the animal's mass.
Benthic: In this morph the aperture is medium-sized but the shell is heavy and non-streamlined, suggesting a benthic life habit.
Nektic: In this morph the aperture and living chamber are large compared to the shell, which is streamlined and laterally compressed. In some fossils, gut contents include pieces of other ammonites, suggesting active macropredation and fast swimming.
Heteromorphic: By altering growth parameters ontogenetically, some ammonoids achieved complex shapes while conforming to the basic molluscan growth strategies. In these, the center of mass is often suspended far below the center of bouyancy, suggesting that the creatures specialized in rapid movement up and down in the water column, but not in active swimming. It has been suggested that they and other non-active swimming ammonoids might have been pelagic plankton eaters rather than macropredators.
So what happened? Ammonoids were very diverse during the Mesozoic, but were extinguished at the end of the Cretaceous (along with the non-avian dinosaurs, marine reptiles, etc.) Nautilids, although not diverse in the Mesozoic, managed to sail through the K-T extinction. Speculation on the difference focuses on reproductive strategy. It is known that the K-T extinction in the marine realm was particularly hard on planktonic organisms. Nautilids have large eggs that develop directly into adults and do not become part of the plankton. Ammonoids may have been different, with planktonic larvae, leaving them more vulnerable to the events at the end of the Cretaceous.