Figure 13.2 The early mollusks (a) Kimberella, (b) Odontogriphus and (c) phylogeny and stratigraphic ranges of early mollusks mapped onto some ecological changes. N-D, Nemakit-Daldynian; T, Tommotian; A, Atdabanian; B/T, Botomian. (a, courtesy of Ben Waggoner; b, c, courtesy of ten-Bernard Caron.)
Box 13.3 Halkieria: from stem-group brachiopod to new class of mollusk
Halkieria was first described on the basis of disarticulated shells from the Cambrian rocks of the Danish island of Bornholm. But the discovery in the 1980s of articulated specimens from the Early Cambrian Sirius Passet fauna from North Greenland (see p. 386) generated huge excitement. The animal was in fact an elongate, worm-like creature with two mollusk-like shells at the front and the back separated by an armor of sclerites between (Fig. 13.3), quite bizarre and quite different from previous interpretations of the animal. Initial attempts to place it together with the mollusks were superseded by its placement as a stem-group brachiopod; reasonable enough because both shells are very similar to the dorsal and ventral valves of some non-articulated brachiopods. However, to become a brachiopod, Halkieria would have had to lose its foot, develop a lophophore as a feeding organ and convert its sclerites to chaetae. Jakob Vinther and Claus Nielsen (University of Copenhagen) in 2004 dissected the fossil in detail and compared it with a range of living mollusks. There was a simpler solution. Halkieria is in fact a mollusk, possessing most of the features that define the phylum, but a number of characters (such as the shells at the anterior and posterior of the animal) have formed the basis for a new class of mollusk, the Diplacophora.
the Lower Cambrian rocks of north Greenland has promoted new discussion on the identity of the earliest mollusks (Box 13.3). The halkieriid not only displays the articulation of a series of sclerites, or plates, commonly described in the past as discrete organisms, but also two large mollusk-like shells at the front and back of the worm-like animal. The many, often bizarre but distinctive, early mollusks formed the basis for subsequent radiation of the phylum particularly during the Late Cambrian and Early Ordovi-cian. The shapes of these and other mollusk shells have formed the basis numerical modeling, demonstrating that fossil and living shell shapes, and indeed many unknown in nature, can be generated by computers (Box 13.4).
The hyoliths - long, conical, calcareous shells with an operculum-covered aperture -
have often been called mollusks. The group ranges from the Cambrian to Permian with some of the 40 known genera reaching lengths of 200 mm. Current studies assign the group to its own phylum, related to the mollusks and the peanut worms, the Sipunculida.
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