Page 343 - Introduction to Paleobiology and The Fossil Record

330 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.2 Kimberella and Odontogriphus join the mollusks

A modest-sized, disk-shaped fossil from the Late Precambrian, named Kimberella in 1959, has suf-
fered mixed fortunes. First described from the Ediacaran rocks of Australia as a jellyfi sh and later
a cubozoan, Mikhail Fedonkin and Ben Waggoner (1997) then reconstructed Kimberella as a bilater-
ally symmetric, benthic crawler with a non-mineralized, single shell, on the basis of new material
from the White Sea, Russia. Kimberella is linked with a variety of trace fossils suggesting mobility
and a feeding strategy that must have involved a radula. The body fossils and trace fossils place
Kimberella near the base of the molluskan clade and suggest a deep origin for the phylum (Fig. 13.2),

and for the bilateralians, significantly earlier than the Cambrian explosion. But who were its closest
relatives? A new investigation by Jean-Bernard Caron and his colleagues (2006) offers some clues.
They studied another enigmatic animal, Odontogriphus from the Burgess Shale. Odontogriphus had
previously been allied with the brachiopods, bryozoans, phoronids and even early vertebrates. The
new study shows that Odontogriphus possesses a radula, a broad foot and a stiffened dorsum, so
placing it firmly within the mollusks, close to Kimberella, together with Wiwaxia (another enigmatic

soft-bodied organism covered with possible scierites), which also possesses a radula, and another
enigma, Halkieria (Box 13.3).

(a) (b)
Annelida
total-group Mollusca
Kimberella?
putative range extension Odontogriphus
stem-group Mollusca Wiwaxia
and known fossil range Halkieriid
crown-group Mollusca ??? Neomeniomorpha
and known fossil range Polyplacophora
Other crown-group
Mollusca
Firm Substrate Soft
Cambrian substrate revolution: Mat-based ecology
Vertical burrowers
N-D T A B/T Mid Late
Ediacaran Early Ordovician
Cambrian
555 542 501 488 Time (Ma)
(c)
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.)