Page 317 - Introduction to Paleobiology and The Fossil Record

304 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 12.2 The brachiopod fold hypothesis and the search for stem-group brachiopods

Already by the Early Cambrian a range of diverse brachiopods populated nearshore environments.
But where can we find their ancestors and what sort of animals are we looking for? Many have

assumed that a prototype brachiopod probably arose in the Late Precambrian with a phosphatic
shell substance and an apparently simple Lingula-like morphology. But did it evolve from a burrow-
dwelling sessile organism or from a mobile, slug-like ancestor? A careful study of the early develop-
ment of the non-articulated brachiopod Neocrania by Claus Nielsen (University of Copenhagen) has
yielded a few, exciting clues. During ontogeny the embryo actually curls over at both ends (Fig.
12.3). The resulting embryo has the posterior end of the animal forming the dorsal surface (or valve)
and the anterior end, the ventral surface. This process, subsequently called the brachiopod fold
hypothesis (Cohen et al. 2003), provides an elegant model for how a brachiopod could have evolved
from a flat, possibly worm-like, animal with shells at its anterior and posterior ends. Care must be

taken in locating such possible ancestors. Halkieria, for example, has shells at its anterior and pos-
terior end but is a mollusk (see p. 331); however shells such as Micrina and Mickwitzia may have
belonged to a slug-like stem-group brachiopod. The mystery may be solved only when some excep-
tionally well-preserved fossil is found.

Posterior Anterior
Dorsal (brachial) valve

(a) Ventral (pedicle) valve

Anterior dorsal (brachial) valve
Plane of brachiopod fold

(b) Posterior dorsal (brachial) valve
Figure 12.3 (a) The traditional body plan with an upper dorsal and a lower ventral shell. (b) The
brachiopod fold hypothesis plan implies that the brachial valve is the anterior one and the pedicle
posterior – both were previously on the dorsal surface of the animal. (From Cohen et al. 2003.)

(a) (b)
Figure 12.4 Brachiopod larvae. (a) Ventral and (b) dorsal valves of the brachiopod Onniella. Black
arrows indicate the anterior extent of the larval shell. Scale bars, 200 μm. (From Freeman & Lundelius
2005.)