at a phylogenetic history antedating the first record of fossils. Connections at a still deeper
level (e.g. between phyla) are implicated. Nobody questions that the appearance of
mineralized tissue was a Cambrian phenomenon, and some of the discussion has centred
on arguments such as ‘can you have a brachiopod without a mineralized shell?’, but a
propos of arthropods, molluscs, and chordates the answer clearly is ‘yes, you can’. The
persistent paradox remains that fossils of these earlier animals stay obstinately
undiscovered. This in turn leads to two principal possibilities: (i) that the fossil record is
relatively complete and that it indicates greatly accelerated rates (presumably both
morphological and molecular) at about the Precambrian-Cambrian boundary (Budd and
Jensen 2000), or (ii) fossils of putative ‘ancestors’ remain undiscovered for taphonomic
reasons, or because they were soft bodied and of small size—and that there was, indeed, a
phylogenetic fuse (Figure 3.1). Under this scenario, rates do not have to be drastically
elevated, and the question of acquisition of shells and skeletons becomes decoupled from
Figure 3.1 Three models depicting the evolution of clade diversity among the Metazoa through time.
A, Traditional model. Metazoa originated simply and became more complex throughout the
Phanerozoic. B, Fortey et al. (1996). Evidence from phylogeny, palaeobiogeography, trace fossils
and molecular clocks imply extensive Precambrian cladogenesis prior to the appearance of the first
body fossils. C, Budd and Jensen (2000). Most Cambrian stem-lineage forms lacked some of the
defining features of modern phyla, suggesting that they were considerably less disparate than the
latter. Cladogenesis producing the stem-lineages of modern phyla probably occurred in the latest
Proterozoic but no earlier.
PHYLOGENETIC FUSES AND EVOLUTIONARY ‘EXPLOSIONS’ 45