Spineless Wonders of Evolution 195tube feet lengthens and straightens out (fig. 8.9). However, later research showed that Rowe’s
sampling was inadequate and he did no statistical studies at all. In 1954, Kenneth Kermack
(since updated by Nichols [1959], Ernst [1970], and Stokes [1977]) performed a more rigorous
analysis and found that, although the sequence was not a single smooth linear trend, there
is still a remarkable branching sequence of species that partially overlap in time (fig. 8.9),
from Micraster leskei to Micraster cortestudinarum (“tortoise heart”) to Micraster coranguinum
(“eel heart”), and a side branch leading to Micraster corbovis (“cow heart”) and Micraster gib-
bus. These fossils are no longer evidence of gradualism within a single unbranched lineage,
but there is still a good evolutionary sequence of transitional forms.
In the Jurassic beds of England to west of the White Cliffs, there are many additional
excellent fossil sequences. A classic one of these is the weird oyster known as Gryphaea.
Called “devil’s toenails” by the collectors, these creatures had one shell that was shaped like
a coiled saucer that lay flat on the bottom (concave side up), and the other shell was much
smaller and formed a lid on top (fig. 8.10). In a classic study, Trueman (1922) argued that
Gryphaea gradually became more and more tightly coiled until the coils actually impeded the
opening of the shell. In the thinking of the time, this was evolution run amok, developing to
the point of obsolescence and no longer under the control of natural selection. But later, more
careful studies by Philip (1962, 1967), Hallam (1968, 1982), Gould (1972), and Hallam and
Gould (1974) showed that Trueman had misinterpreted his data. There is a trend toward less
FIGURE 8.9. Evolutionary trends in the heart urchin Epiaster and Micraster from the Cretaceous chalk beds of the
White Cliffs of Dover. (From Nichols 1959; courtesy of the Royal Society of London)