that of the origin of clades. Molecular tests of rate constancy and divergence times can, in
principle, distinguish between these alternatives.
Problems with interpreting fossil evidence of the Cambrian
evolutionary ‘explosion’Duration of Early CambrianThe formalization of the base of the Cambrian at the base of the Placentian Stage in
Avalonia (=Nemakit Daldynian of Siberia; 545 Ma) has altered the perception of the
radiation in terms of stratigraphical nomenclature. First, new isotope ages have shown
that the Early Cambrian under this definition is considerably longer than was previously
believed (Landing et al. 1998). The earliest stage may be as much as 10 myr. Second, the
appearance of most of the major clades with which we are concerned is at the succeeding
Tommotian (or the next, Atdabanian) Stage. The fauna of the Nemakit Daldynian is a
relatively sparse array of enigmatic small shelly fossils, while the Placentian has yielded
mostly trace fossils. With regard to the phyla that concern us here, the earliest Cambrian
Stage should be added to the ‘fuse’ time. Hence the former use of the Precambrian-
Cambrian boundary as if it were synonymous with the time of ‘explosion’ requires a
certain modification. This is, however, a matter of definition and not of science. It should
be noted that one of the earliest trace fossils in the Placentian (Rusophycus avalonensis) is of
‘trilobite’ type, and assuredly made by a normal-sized schizoramous arthropod, whether
or not it was a ‘soft-bodied trilobite’. This sets the timing of this part of the arthropod
‘fuse’ at the (formal) base of the Cambrian.
Definitions of phylaCambrian fossil faunas include a variety of more or less plesiomorphic animals, often
exhibiting in addition peculiar autapomorphies (e.g. Edgecombe 1998). Not surprisingly,
they frequently offer challenges in cladistic analysis, as reflected in unresolved polytomies,
or shifting ‘basal’ positions (Wills and Fortey 2000). Budd and Jensen (2000) regard the
difficulties of attempting to include these problematic fossils in a classification based upon
Recent organisms (for which inexhaustible phylogenetic data are obtainable in principle)
as impractical and probably ultimately unsolvable. They prefer to define major animal
clades as crown-groups embraced by the most basal living representative and its
descendants. Thus, arthropodan Chelicerata would presumably include the common
ancestor of the primitive Limulus and all its sister taxa. Fossil taxa falling outside this
definition would find their place (or not) on the stem-group, without formal status. The
more usual view would attempt to place stem taxa within the total-group, whose
inclusiveness would be defined from the basal node separating Chelicerata from its sister
clade (for example, Crustacea). The former approach has the advantage that it should be
possible to define major groups with a battery of synapomorphies (say, n). Cambrian taxa
will tend to fall on the stem and, except in those cases where there are Early Cambrian
representatives lying inboard of living animals, this will displace phylum level taxa
upwards. In Budd and Jensen’s view there would be no ‘explosion of phyla’ in the Lower
46 RICHARD A.FORTEY ET AL.