Funnel Organism

Anus

Mantle cavity

Foot

■ Solenogastres 1 ' Caudofoveata J ' Polyplacophora i-Aplacophora i-Aplacophora

■ Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda

Palps

Enlarged gills

Head

Mantle cavity

Head

Mantle cavity

Siphons Mantle cavity

Mantle cavity

Siphuncle Chambers

Funnel

Funnel

Solenogastres Caudofoveata

Aplacopho

Aplacopho

Polyplacophora

Monoplacophora

Bivalvia

Scaphopoda

Gastropoda

Cephalopoda

Palps Enlarged gills

Head

Mantle cavity

Head

Mantle cavity

Siphons Mantle cavity

Funnel

Funnel

Aculifera

Mantle cavity

Siphons Mantle cavity

Mantle cavity

Siphuncle Chambers

Tentacles surrounding mouth

Conchifera

Mantle cavity

Siphons Mantle cavity

Mantle cavity

Siphuncle Chambers

Tentacles surrounding mouth

Testaria

Figure 13.1 Pseudocladograms of molluskan evolution: hypothetical archemollusk (HAM) evolution integrated with a cladistic-type framework. Model (a) demonstrates a split into the Aculifera and Conchifera, whereas (b) indicates a division into the Aplacophora and Testaria. (Based on Sigwart & Sutton 2007.)

Box 13.2 Kimberella and Odontogriphus join the mollusks

A modest-sized, disk-shaped fossil from the Late Precambrian, named Kimberella in 1959, has suffered mixed fortunes. First described from the Ediacaran rocks of Australia as a jellyfish and later a cubozoan, Mikhail Fedonkin and Ben Waggoner (1997) then reconstructed Kimberella as a bilaterally symmetric, benthic crawler with a non-mineralized, single shell, on the basis of new material from the White Sea, Russia. Kimberella is linked with a variety of trace fossils suggesting mobility and a feeding strategy that must have involved a radula. The body fossils and trace fossils place Kimberella near the base of the molluskan clade and suggest a deep origin for the phylum (Fig. 13.2), and for the bilateralians, significantly earlier than the Cambrian explosion. But who were its closest relatives? A new investigation by Jean-Bernard Caron and his colleagues (2006) offers some clues. They studied another enigmatic animal, Odontogriphus from the Burgess Shale. Odontogriphus had previously been allied with the brachiopods, bryozoans, phoronids and even early vertebrates. The new study shows that Odontogriphus possesses a radula, a broad foot and a stiffened dorsum, so placing it firmly within the mollusks, close to Kimberella, together with Wiwaxia (another enigmatic soft-bodied organism covered with possible scierites), which also possesses a radula, and another enigma, Halkieria (Box 13.3).

total-group Mollusca

Kimberella?

total-group Mollusca putative range extension stem-group Mollusca and known fossil range

I crown-group Mollusca and known fossil range

Annelida

Kimberella?

Cambrian substrate revolution: <

Mat-based ecology

Substrate iphus

Neomeniomorpha ' Polyplacophora Other crown-group Mollusca

Soft

Vertical burrowers

Cambrian substrate revolution: <

Mat-based ecology

Vertical burrowers

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