there is no suggestion of small and cryptic ancestors, there is no suggestion either of a
‘phylogenetic fuse’.
The examples of evolutionary radiation where molecular ‘clock’ estimates indicate such
a prior history also tend to be those where the fossil examples show an apparently sudden
appearance of several clades. Worked examples include the radiations that occurred after
the extinction of the dinosaurs at the Cretaceous-Tertiary (K-T) boundary. The modern
birds undoubtedly proliferated in the Tertiary, and some of the major clades appeared
apparently suddenly. Molecular studies indicate that they descended from a common
ancestor well within the Cretaceous—a phylogenetic fuse is implied (Cooper and Penny
1997). On the contrary, some avian palaeontologists dispute this (Feduccia 1995), as the
record of such birds is unknown, and such a scenario is considered by them unnecessary.
The mammals can be interpreted analogously, although ‘fuses’ have been disputed by
mammologists (Alroy 1999). However, in this case every year brings the apparent
discovery of ancestors of clades within the Cretaceous. For example, Archibald et al.
(2001) have proposed rabbit/rodent relatives from the Cretaceous rocks of Uzbekistan,
while Hooker (2001) has described evidence that the apparently highly-derived bat clade
was already present pre-Tertiary. However, fossils of these animals remain rare, but it
does give more confidence in the fuse schema. Another good example of recently
discovered fossils challenging our acceptance of the origin of groups was the report of a
cirrate octopod from the Upper Carboniferous Mazon Creek fauna (Kluessendorf and
Doyle 2000). Prior to its discovery the earliest known octobrachid was from the Jurassic.
The origin of land plants is discussed elsewhere in this volume (Wellman, Chapter 7).
The appearance of plant body fossils in the Silurian suggests that, again, a ‘fuse’ of perhaps
50 million years is involved. In this case, however, there is evidence of the time involved
from the presence of air-dispersed cryptospore fossils in the absence of body fossils, which
occur back to the mid-Ordovician. However, there is an analogy with the mammalian case
in that some researchers were initially reluctant to accept a Silurian age for the
‘anomalously’ advanced plant Baragwanathia from Australia (Rickards 2000). The
acceptance of the ‘fuse’ removes the anomaly, and increases the expectation of discoveries
to come.
Cambrian evolutionary ‘explosion’The most contentious case of all is the so-called Cambrian evolutionary ‘explosion’ (see
reviews in Zhuravlev and Riding 2001) at about 540 Ma, which is the main focus of this
discussion. The questions involved were explored by Fortey et al. (1996) and have since
been articulated many times (e.g. Fortey et al. 1997; Conway Morris 2000). To
summarize briefly, the appearance of clades in the Cambrian is sudden. As research on early
faunas has continued, more and more putative Early Cambrian representatives of major
groups have appeared, for example vertebrates have been added to the list (Shu et al.
1999). Even at their first appearance some (e.g. trilobites and other arthropods) are both
taxonomically and biogeographically differentiated, implying an earlier history without a
fossil record. Trees of various groups (e.g. molluscs, arthropods, brachiopods) have now
been described, from palaeontological, morphological, and molecular bases, all of which
imply nodes that predate the appearance of various major groups (usually classes) and hint
44 RICHARD A.FORTEY ET AL.