Historical Constraints and the Evolution of Development 1043
10 - 4. Evidence of neoteny for the same sequence of G. arcuata to G. gigantea as measured by
increasing juvenilization of form in the increasing length to height ratio—implying that
descendant adults become less coiled and therefore more like the flatter juvenile shells. Our
analysis of absolute ages for each shell allowed us to specify this case of paedomorphosis as
neotenic. From Gould, 2000c.establishes a long-enduring, iteratively-evolved pattern demanding functional
explanation. The adaptive value of coiling in Gryphaea has long been ascribed with
much evidence in support (see Hallam, 1968, p. 119) and little dissent among experts,
to the animal's need to keep the shell commisure above the muddy substrate, lest the
shell become entombed or clogged, leading to the animal's death.
Coiling represents an excellent morphological means—presumably the best
available given the limitations of bivalved molluscan design—for continually raising
the commisure above the substrate as the shell grows. But coiling, particularly if
intensified in a relatively narrow shell, also entails the negative consequence of
increasing instability, for a narrow object, shaped like the rocker of a hobby horse,
can easily be tipped over from its presumed upright life position (see Fig. 10-5, with
the plane of bilateral symmetry orthogonal to the substrate). In fact, in highly coiled
Gryphaea, a shell tipped over onto its side (Fig. 10-6) lies in a position of greater
stability than a shell in this presumed, and only viable, life orientation. Some early
German paleobiologists, after discovering this fact from hydrodynamic experiments,
actually postulated that Gryphaea might have lived in such a side-down position. But
Hallam (1968) and others argue convincingly that a shell on its side would soon
become clogged with mud, rendering the animal unable to feed. Moreover, once the
heavy shell is tipped, the animal cannot right itself—so quick death would seem to
follow as an inevitable consequence of such displacement from the bilateral living
position.
We therefore assume that stabilization of a shell that must coil to rise above a
muddy substrate represents a fundamental functional problem for gryphaeate oysters.
(Indeed, the most strongly coiled Gryphaea incurva, the ancestral state of the Jurassic
sequence, developed an especially thickened lower valve, presumably to gain
stability by ballasting such a non-optimal form.)