862 THE STRUCTURE OF EVOLUTIONARY THEORY
all fossil mammal lineages... which have large enough sample sizes and recent
systematic revision" (p. 258) for the seven million year period (37-30 million years
ago) across the Eocene-Oligocene transition (Chadronian to Whitneyan North
American land mammal "ages"). The protocol also includes the two other factors—
good geographic spread and temporal resolution—most essential for proper studies
of punctuated equilibrium, but all too rarely realized: "This study considers
geographic variation over a wide area (from western Montana and North Dakota in
the north and west, to Colorado in the south), with very fine-scale
chronostratigraphic control from magnetic stratigraphy and^40 Ar/^39 Ar dating" (p.
258). Finally, and fortunately, this interval includes a major global climatic change
(with no disruption of continuity in sedimentation), thus permitting researchers to
study how such an external input influences rates of speciation and styles of
phyletic change.
Prothero and Heaton found near exclusivity for punctuated equilibrium in the
177 well-documented mammalian species of this fauna. "Most species are static for
2 - 4 million years on average, and some persist much longer" (p. 257). "Only three
examples of gradualism can be documented in the entire fauna, and these are
mostly size changes" (p. 257). The details of these three cases also illustrate the
exceptional status of gradualism, even at the smaller scale of their own taxonomic
context:
- The lagomorph Falaeolagus undergoes reduction in size of upper molars,
accompanied by loss of their roots, during the early Orellan, but maintains stasis
for much longer invervals both above and below: "Chadronian falaeolagus shows
about 2 m.y. of stasis, followed by gradual reduction in size and development of
rootless upper molars during the early Orellan. From Orella B onward, several
species of Falaeolagus are present, and except for slight changes, they are static
for several million years" (p. 273). - The artiodactyl Leptomeryx experiences "subtle, gradual change in a
number of characters" (p. 263) in the transition from L. speciosus to L. evansi, but
both species show stasis throughout most of their substantial history—so we do not
here witness the "classic" continuous anagenesis that supposedly makes the
definition of species so arbitrary in temporal sequences: "While the transition from
L. speciosus to L. evansi is not stratigraphically instantaneous, it occurs in a
relatively short time compared to the long durations of both species." - The oreodont Merycoidodon does seem to undergo extensive and gradual
dwarfing (30 percent size reduction) over a one million year interval in the early
Orellan. I accept this case as a good example of extended gradualism (see Fig. 9- 26
taken from Prothero and Heaton, 1996, p. 262), but also note that the trend occurs
within a common genus, including several species otherwise showing predominant
stasis—and that the dwarfing trend only involved the labile character of size,
without concomitant changes of shape, a common finding among exceptional cases
of gradualism in faunas dominated by punctuated equilibrium (see previous
discussion of Jurassic bivalves on page 855).
Such exhaustive and unprejudiced tabulations can give us insight into the