Chapter 9
The limitations of the fossil record and the
dating of the origin of the Bilateria
Graham E.Budd and Sören Jensen
ABSTRACT
The origin of the bilaterian phyla is one of the classic areas of conflict between
molecular dates and the fossil record. Here we examine possible sources of bias in
the fossil record, and conclude that on several grounds, not least broad
stratigraphic and phlyogenetic congruence, it is hard to see the Cambrian fossil
record as representing anything but a broadly ‘real-time’ event. Refinements of
molecular estimates of the dating of bilaterian origins have increasingly cut the gap
between fossil and molecular estimates, and a rapprochement must be hoped for in
the next few years. Further refinements of our understanding of this event must
increasingly focus on better phylogenies, and a consideration of the ecology and
biogeography of the event.IntroductionThe apparent conflict between molecular and fossil evidence for the origin of the Bilateria
is one that is increasingly being treated in a more mature way than the protagonists simply
gainsaying the data of their opponents. The pressure that has been brought to bear on both
sides has led to a critical examination of both types of data. Whilst the fossil record has
been criticized in this process, estimates of the degree of uncertainty introduced by its
undeniable imperfections have been much more difficult to obtain. In other words, the
validity of the logical step from ‘the fossil record is flawed’ to ‘the fossil record can tell us
nothing about timing of the origin of animals’ needs to be carefully scrutinized. The
purpose of this present chapter is to lay out briefly the various reasons for thinking the
fossil record of the Cambrian and terminal Proterozoic might be considerably
misrepresentative of the true timing of the events it is purported to depict, together with
attempts that have been made to circumvent them, and to examine them critically.
Finally, we offer some further ways in which the record could be tested, and outline some
of the latest evidence suggesting that the fossil and molecular conflict might soon be
resolved.