Evolution__3rd_Edition

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Molluskan larval types can be
inferred in fossils
Figure 23.7
Larval shell form is correlated
with type of development in
mollusks. The species in (a) and
(b) are modern gastropods,
and in (c) and (d) are late
Cretaceous fossil bivalves.
Note the relative sizes of the
regions labeled PdI and PdII
(prodissoconch I and II).
(a) Rissoa guerini, which is
known to have a planktonic
larva (size bar = 50 µm);
(b) Barleeia rubra, which is
known to develop directly
without a stage in the plankton
(size bar = 50 µm); (c) Uddenia
texana, which had small PdI
and large PdII regions like
(a) and is inferred to have had
planktonic development
(size bar = 20 µm); and
(d) Vetericardiella crenalirata,
which had large PdI and small
PdII regions like (b) and is
inferred to have had direct
development (size bar =
20 µm). D, dissoconch.
Reprinted, by permission of the
publishers, from Jablonski &
Lutz (1983).
CHAPTER 23 / Extinction and Radiation 661
metamorphoses into an adult snail. With direct development, the eggs and young
grow up near or (to begin with) inside the parental snail. Various ecological trends
are known among modern forms, such as that planktonic development is commoner
among shallow- than deep-water species, and commoner among tropical species than
polar species. These results suggest that the mode of development in a species is an
adaptation to the local ecological conditions.
The relation between larval type and speciation and extinction rates can be studied in
fossil gastropods. Larval types in fossils are inferred by analogy with modern species.
These kinds of inference were pioneered in the work of Thorson, and several criteria
have now been used. Figure 23.7 shows one, which uses the size of regions in the larval
shell. Modern species with planktonic development typically have small, yolk-poor eggs;
in the larval shell, a region called prodissoconch I tends to be small and another region
called prodissoconch II is larger (Figure 23.7a). Species with direct development have
the reverse condition (Figure 23.7b). These morphological regions can be distinguished
in fossil larval shells by scanning electron microscopy. We can reasonably assume that
shell form is correlated with development type in the same way as modern forms.