|332 HETEROCHRONY AND PAEDOMORPHOSIS
one. In other species (E. rathbunae and E. emeritus), males may be only one-tenth the length of females.
As in most cases of dwarfism (Gould, 1971), these males more closely resemble geometrically scaled-down adults than larvae.5 But they all retain several important larval characters, left behind so to speak by their precocious maturation. Efford writes:
They are certainly precociously mature although this maturity is achieved when they have a juvenile adult form rather than a larval form. They do, however, retain certain larval characters. Commonly they are very small and soft and in this resemble the megalopae rather than the large adults; they also show a general simplicity of the appendages associated with their small size. This is demonstrated particularly by the antennae which are simple and do not have the regularly arranged, closely packed setal net of the large animals. In both E. talpoida and E. emeritus the small males retain the stumps of the pleopods normally only present in the zoeae, megalopae and the females. (1967, p. 89)
I doubt that any primary selective significance can be attributed to these paedomorphic features. If Ghiselin is right, the determinant of progenesis must be precocious maturation (and perhaps small size for more efficient transport by currents)—"first come, first service," to cite Ghiselin's aphorism. The partial retention of larval features is a developmental consequence of selection for a life-history strategy involving precocious maturation. These features are not inadaptive; they are probably not even "neutral"; but I would be surprised if selective pressures for their evolution played any important role in the evolution of dwarf males.
Progenesis as an Adaptive Response
Although previous attention has been focused on larval morphology, progenesis involves two other events of potential evolutionary import: precocious maturation and small size. I have been arguing, in this section, for the primary significance of precocious maturation as an ecological strategy. Other examples indicate that selection is principally for small size, and again, that partially larval morphology may be a passive consequence of this primary need.
Surlyk (1974) has studied the progenesis of Aemula inusitata, a brachiopod from the very fine grained Cretaceous chalk deposits of Northwestern Europe. It is very small (maximum length of 7 mm), short lived (as deduced from growth lines), and paedomorphic (the lophophore remains in the schizolophous state so characteristic of early ontogeny in complex forms). Its progenesis cannot easily be at-