Zoologia

CHAPTER 1

The earliest fossil record of the
animals and its significance
Graham E. Budd

The fossil record of the earliest animals has been
enlivened in recent years by a series of spectacular
discoveries, including embryos, from the Ediacaran
to the Cambrian, but many issues, not least of dating and interpretation, remain controversial. In
particular, aspects of the taphonomy of theearliest fossils require careful consideration before pronouncements about their affinities. Nevertheless, a
reasonable case can be now made for the extension
of the fossil record of at least basal animals (sponges
and perhaps cnidarians) to a period of time sig nificantly before the beginning of the Cambrian. The
Cambrian explosion itself still seems to represent
the arrival of thebilaterians, and many new fossils in recent years have added sig nificant data
on the origin of the three major bilaterian clades.
Why animals appear so late in the fossil record is
still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause
remains unproven and may even involve a degree
of circularity.

1.1 Introduction
The ‘Cambrian explosion’ is a popular termthat
refers to the period of profound evolutionary and
environmental change that took place at the opening of the Phanerozoic some 540 m illion years ago
(Ma). Although this set of events is multifaceted, it
is associated primarily with the origin of animals
in the fossil record. For over 150 years, an argument has raged about the reality of this event. Is it
a genuine evolutionary event,or merely a sudden
manifestation in the fossil record of evolutionary

processes that took place long before? Even if the
fossil record of that time is accurately recording
the unfolding of events in real time, the question
of why the events took place then—and what the
potential trigger was—has continued to be problematic. The Cambrian explosion itself has been
much discussed (Gould, 1989;Conway Morris,
1998a, 2003a; K noll and Carroll, 1999; Budd and
Jensen, 2000; K noll, 2003). Here I want to focus on
three issues: the age of the earliest animal fossils,
the continuing debate about their affinities, and
finally, a critical examination of the most popular candidate for ‘triggering’ the explosion; the
concentration of atmospheric oxygen.
Geologists as long ago as WilliamBuckland
(1784 –1856) realized that a dramatic step change in
the fossil record occurred at the base of what we
now call the Cambrian. The apparent appearance
in the fossil record of many animal groups with
few or no antecedents caused Charles Darwin great
trouble—indeed he devoted a substantial chapter
of the Origin to this problem. Further insights were
provided by the remarkable amount ofwork on
North American faunas by Charles D. Walcott, who
proposed that an interval of time, or the ‘Lipalian’,
was not represented in the fossil record and/or
did not preserve fossils and that the forms ancestral to the Cambrian taxa evolved during this time.
However, the intense modern interest in the subject
was probably sparked by the work of Whittington
and colleagues in theirredescriptions of the
Burgess Shale (see below), together with Stephen
Jay Gould’s popular account of this work, Wonderful
Life, published in 1989. In recent years, the attention
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ANIMAL EVOLUTION

paid to the youngest part of the Precambrian has
led to the erection of the formal Ediacaran Period
of c. 630–542 Ma (Knoll et al., 2006), an interval that
has been intensely scrutinized forits bearing on
the origin of the animals.

of how Cambrian taxa should be classified; and
(3) various dating problems.

1.2 Fossil evidence for the origin of
animals: the state of play

The processes that convert a living organism into a
mineralized or organically preserved fossil are far
from being fully understood; nevertheless, at least
some understanding of them is essential if...