Page 344 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 331

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superseded by its placement as a stem-group brachiopod; reasonable enough because both shells are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 Copenha-
gen) 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 defi ne 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.

Figure 13.3 The mollusk Halkieria from Sirius Passet (natural size).