Box Fossil annelids and their jaws

The annelids are segmented protostomes that are represented today by animals such as the earthworms and leaches. Recent species are important, widely distributed, benthic predators and occur from intertidal to abyssal depths. Modern molecular studies suggest they form a sister group to the mollusks and, in fact, share a number of morphological characters such as the possession of chaetae. In general the group has a fairly sparse fossil record, appearing fleetingly in Lagerst├Ątte deposits such as the Burgess Shale and Mazon Creek fauna. However many residues of acid-etched Paleozoic limestones contain scolecodonts (Fig. 13.28). These were the jaws of ancient annelids and are abundant and diverse at many horizons. They were similar to conodonts (see p. 429), forming multielement apparatuses with similar functions but were composed of collagen fibers and various minerals such as zinc. The group first appeared in the Lower Ordovician and diversified rapidly to become common in Upper Ordovician-Devonian carbonate facies. Scolecodonts were relatively rare after the Permian, but nevertheless have proved useful in biostratigraphic and thermal maturation studies.

Figure 13.28 Scolecodont morphology. Reconstruction of the polychaete jaw apparatus of the Ordovician Ramp hoprion Kielan-Jaworowska. (Courtesy of Olle Hints.)

Review questions

1 There has been some difficulty identifying the first mollusk. What are the key features of the phylum and how would they be recognized in the first mollusk?

2 Many taxa that form part of the Early Cambrian biota are undoubtedly mol-lusks. Which mollusk groups are already present in the small shelly fauna?

3 Theoretical morphospace is a useful tool to investigate shell morphology. Some groups are more constrained in their developmental opportunities than others. What advantages should univalved mol-lusks have over bivalved mollusks in a quest to generate extreme morphotypes?

4 Belemnites seem an unlikely group to test models for microevolution. What conditions should be met in such tests of micro-evolutionary hypotheses?

5 The Mesozoic marine revolution (or arms race) was a complex ecological event that set the agenda for marine life in the Modern evolutionary fauna. How did mollusks react to predation pressures?

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