Page 345 - Introduction to Paleobiology and The Fossil Record

332 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

the Lower Cambrian rocks of north Green- have often been called mollusks. The group
land has promoted new discussion on the ranges from the Cambrian to Permian with
identity of the earliest mollusks (Box 13.3). some of the 40 known genera reaching lengths
The halkieriid not only displays the articula- of 200 mm. Current studies assign the group
tion of a series of sclerites, or plates, com- to its own phylum, related to the mollusks
monly described in the past as discrete and the peanut worms, the Sipunculida.
organisms, but also two large mollusk-like
shells at the front and back of the worm-like
animal. The many, often bizarre but distinc- CLASS BIVALVIA
tive, early mollusks formed the basis for sub- Bivalves are among the commonest shelly
sequent radiation of the phylum particularly components of beach sands throughout the
during the Late Cambrian and Early Ordovi- world. Many taxa are farmed and harvested
cian. The shapes of these and other mollusk for human consumption, and pearls are a
shells have formed the basis numerical model- valuable by-product of bivalve growth. The
ing, demonstrating that fossil and living bivalves developed a spectacular variety of
shell shapes, and indeed many unknown in shell shapes and life strategies, during a history
nature, can be generated by computers (Box spanning the entire Phanerozoic, and all
13.4). are based on a simple bilaterally symmetric
The hyoliths – long, conical, calcareous exoskeleton. The fi rst bivalves were
shells with an operculum-covered aperture – marine shallow burrowers; epifaunal, deep

Box 13.4 Computer-simulated growth of mollusks

Most valves of any shelled organism can be modeled as a coil and, in fact, the ontogeny of living
Nautilus was known to approximate to a logarithmic spiral in the 18th century. David Raup

(University of Chicago), in an infl uential study, defined and computer-simulated the ontogeny of
shells on the basis of a few parameters: (i) the shape of the generating curve or axial ratio of the
ellipse; (ii) the rate of whorl expansion after one revolution (W); (iii) the position of the generating
curve with respect to the axis (D); and (iv) the whorl translation rate (T). Shells are generated by
translating a revolving generating curve along a fi xed axis (Fig. 13.4). For example, when T = 0,
shells lacking a vertical component such as bivalves and brachiopods, are simulated, whereas those
with a large value of T are typical of high-spired gastropods. Only a small variety of possible shell
shapes occur in nature. Raup’s (1966) original simulations were executed on a mainframe system.
Andrew Swan (1990) adapted the software for microcomputers and has simulated a wide variety of
shell shapes. More recent work has applied more complex techniques to simulate ammonite hetero-
morphs. Nevertheless only a relatively small percentage of the theoretically available morphospace
has actually been exploited by fossil and living mollusks. Clearly some fi elds map out functionally
and mechanically improbable morphologies – perhaps the aperture is too small for the living animal
to feed from within the shell, or the shape would not allow the animal to move; other fi elds have
yet to be tested in evolution. Raup’s morphospace is, however, non-orthogonal and it has been argued
that the mosaic of morphospace occupation is merely an artifact of presentation. Theoretical mor-
phospace has been explored for a range of other groups including bryozoans, echinoids, graptolites,

some fishes and some plants (Erwin 2007).

There have been many modifications of Raup’s original algorithm and a number of web
interfaces that can generate shell shapes; one of the simplest may be accessed via http://www.
blackwellpublishing.com/paleobiology/.