Evolution What the Fossils Say and Why it Matters

196 Evolution? The Fossils Say YES!

tightly curved and thinner, more dish-like shells that are much wider than long (fig. 8.10),
but it happens in a number of different species lineages through the Jurassic, and much of
the change in shape is primarily due to growth to a larger size.
Many other examples from the mollusks could be added to this list. For example (Rodda
and Fisher 1964), the marine snail Athleta is very common in the Eocene (from 55 to 34 million

FIGURE 8.10. Evolution within the Jurassic oyster Gryphaea. (A) A series of Gryphaea shells, showing their
decreasingly tight coiling through time. (Photo by the author) (B) The sequence of less tightly coiled shells
found in the Jurassic beds of southern England. (After Hallam 1968: fig. 26; courtesy of the Royal Society of
London)

(A)

Spinatum

Margaritatus

Davoei

Ibex

Jamesoni

Raricostatum

Oxynotum

Obtusum

Turneri

Semicostatum

Bucklandi

Angulata G. arcuata obliquata

G. gigantea

G. mocullochii

G. arcuata incurva

Shape

preserved

Coilin

g decreases by paedomorphosis

(B)