754 THE STRUCTURE OF EVOLUTIONARY THEORY
of phyletic gradualism happening throughout a species at any one moment in time.
Species A never changes into species B everywhere simultaneously and
gradually."
When we consider trilobites, the exemplars of Paleozoic invertebrate
"complexity," Robison (1975, p. 220) concluded from extensive study of Middle
Cambrian agnostid trilobites in Western North America: "I have found a
conspicuous lack of intergradation in species-specific characters, and I have also
found little or no change in these characters throughout the observed stratigraphic
ranges of most species."
Fortey (1985) spent many years studying a particularly favorable sequence for
fine-scale temporal resolution from the early Ordovician of Spitzbergen. He
examined 111 trilobite and 56 graptoloid species, finding a predominance of
punctuated equilibrium in both groups—with gradualism in "less than 10 percent
of the total" for trilobites, and, for graptoloid species, with punctuational origins
"at least four times as important as gradualistic ones" (1985, p. 27). Fortey's case
becomes particularly convincing because he could calibrate punctuational
sequences against rarer cases of gradualism in the same strata—and therefore be
confident that punctuations do not merely represent the missing strata of
conventional gradualistic rates. In a later paper, Fortey, who is, by the way, no
partisan of punctuated equilibrium, reaches the following general conclusion, and
also affirms our point about respect for the age-old knowledge of biostratigraphic
practitioners: "Many invertebrate paleontologists would agree that the fossil record
of species of their groups is dominated by lack of change—by stasis—and that
where phylogenies have been worked out then the evidence direct from the rocks
shows punctuated lineages in a majority of cases. For reasons I have explained, it
is likely that stratigraphic paleontologists would always have maintained such a
view, but the difference is that now this would be accepted by paleobiologists as
well" (1988, p. 13).
Moving to a different arthropod group from another time, Coope's famous
studies of Late Cenozoic fossil beetles (summarized in Coope, 1979) provide one
of our best cases for dominance of the punctuational mode. Unusually good
preservation greatly increases the power of this example. Coope discusses his best
case (for beetles extracted from the carcasses of woolly rhinos in the western
Ukraine), but then extends his argument to most examples:
Here the complete beetles were preserved down to the tarsal and antennal
joints; when the elytra were raised, the wings could be unfolded and
mounted; and parasitic mites, both larvae and adults, were found
underneath the wings. Although this was quite exceptional preservation, it
is common to find intact abdomens from which the genitalia can be dis-
sected; the frequently transparent integument often reveals detailed
structures of the internal sclerites. Preservation is frequently adequate to
enable details of the microstructure of the surface of the hairs and scales to
be examined with scanning electron microscopy (1979, p. 248).